ADAPTATION AND EVOLUTION
The concept of biological evolution is one of the most important ideas ever generated by the application of scientific methods to the natural world. The evolution of all the organisms that live on Earth today from ancestors that lived in the past is at the core of genetics, biochemistry, neurobiology, physiology, ecology, and other biological disciplines. It helps to explain the emergence of new infectious diseases, the development of antibiotic resistance in bacteria, the agricultural relationships among wild and domestic plants and animals, the composition of Earth's atmosphere, the molecular machinery of the cell, the similarities between human beings and other primates, and countless other features of the biological and physical world. As the great geneticist and evolutionist Theodosius Dobzhansky wrote in 1973, "Nothing in biology makes sense except in the light of evolution."
Progress in science consists of the development of better explanations for the causes of natural phenomena. Scientists never can be sure that a given explanation is complete and final. Some of the hypotheses advanced by scientists turn out to be incorrect when tested by further observations or experiments. Yet many scientific explanations have been so thoroughly tested and confirmed that they are held with great confidence.
The theory of evolution is one of these well-established explanations. An enormous amount of scientific investigation since the mid-19th century has converted early ideas about evolution proposed by Darwin and others into a strong and well-supported theory. Today, evolution is an extremely active field of research, with an abundance of new discoveries that are continually increasing our understanding of how evolution occurs.
In Biology 11, we will consider the science that supports the theory of evolution, focusing on three categories of scientific evidence:
1) Evidence for the origins of the universe, Earth, and life
2) Evidence for biological evolution, including findings from paleontology, comparative anatomy, biogeography, embryology, and molecular biology
3) Evidence for human evolution
The Origin of the Universe, Earth, and Life
The term "evolution" usually refers to the biological evolution of living things. But the processes by which planets, stars, galaxies, and the universe form and change over time are also types of "evolution." In all of these cases there is change over time, although the processes involved are quite different.
In the late 1920s the American astronomer Edwin Hubble made a very interesting and important discovery. Hubble made observations that he interpreted as showing that distant stars and galaxies are receding from Earth in every direction. Moreover, the velocities of recession increase in proportion with distance, a discovery that has been confirmed by numerous and repeated measurements since Hubble's time. The implication of these findings is that the universe is expanding.
Hubble's hypothesis of an expanding universe leads to certain deductions. One is that the universe was more condensed at a previous time. From this deduction came the suggestion that all the currently observed matter and energy in the universe were initially condensed in a very small and infinitely hot mass. A huge explosion, known as the Big Bang, then sent matter and energy expanding in all directions.
This Big Bang hypothesis led to more testable deductions. One such deduction was that the temperature in deep space today should be several degrees above absolute zero. Observations showed this deduction to be correct. In fact, the Cosmic Microwave Background Explorer (COBE) satellite launched in 1991 confirmed that the background radiation field has exactly the spectrum predicted by a Big Bang origin for the universe.
As the universe expanded, according to current scientific understanding, matter collected into clouds that began to condense and rotate, forming the forerunners of galaxies. Within galaxies, including our own Milky Way galaxy, changes in pressure caused gas and dust to form distinct clouds. In some of these clouds, where there was sufficient mass and the right forces, gravitational attraction caused the cloud to collapse. If the mass of material in the cloud was sufficiently compressed, nuclear reactions began and a star was born.
Some proportion of stars, including our sun, formed in the middle of a flattened spinning disk of material. In the case of our sun, the gas and dust within this disk collided and aggregated into small grains, and the grains formed into larger bodies called planetesimals ("very small planets"), some of which reached diameters of several hundred kilometers. In successive stages these planetesimals coalesced into the nine planets and their numerous satellites. The rocky planets, including Earth, were near the sun, and the gaseous planets were in more distant orbits.
The ages of the universe, our galaxy, the solar system, and Earth can be estimated using modern scientific methods. The age of the universe can be derived from the observed relationship between the velocities of and the distances separating the galaxies. The velocities of distant galaxies can be measured very accurately, but the measurement of distances is more uncertain. Over the past few decades, measurements of the Hubble expansion have led to estimated ages for the universe of between 7 billion and 20 billion years, with the most recent and best measurements within the range of 10 billion to 15 billion years.
The age of the Milky Way galaxy has been calculated in two ways. One involves studying the observed stages of evolution of different-sized stars in globular clusters. Globular clusters occur in a faint halo surrounding the center of the Galaxy, with each cluster containing from a hundred thousand to a million stars. The very low amounts of elements heavier than hydrogen and helium in these stars indicate that they must have formed early in the history of the Galaxy, before large amounts of heavy elements were created inside the initial generations of stars and later distributed into the interstellar medium through supernova explosions (the Big Bang itself created primarily hydrogen and helium atoms). Estimates of the ages of the stars in globular clusters fall within the range of 11 billion to 16 billion years.
A second method for estimating the age of our galaxy is based on the present abundances of several long-lived radioactive elements in the solar system. Their abundances are set by their rates of production and distribution through exploding supernovas. According to these calculations, the age of our galaxy is between 9 billion and 16 billion years. Thus, both ways of estimating the age of the Milky Way galaxy agree with each other, and they also are consistent with the independently derived estimate for the age of the universe.
Radioactive elements occurring naturally in rocks and minerals also provide a means of estimating the age of the solar system and Earth. Several of these elements decay with half lives between 700 million and more than 100 billion years (the half life of an element is the time it takes for half of the element to decay radioactively into another element). Using these time-keepers, it is calculated that meteorites, which are fragments of asteroids, formed between 4.53 billion and 4.58 billion years ago (asteroids are small "planetoids" that revolve around the sun and are remnants of the solar nebula that gave rise to the sun and planets). The same radioactive time-keepers applied to the three oldest lunar samples returned to Earth by the Apollo astronauts yield ages between 4.4 billion and 4.5 billion years, providing minimum estimates for the time since the formation of the moon.
The oldest known rocks on Earth occur in northwestern Canada (3.96 billion years), but well-studied rocks nearly as old are also found in other parts of the world. In Western Australia, zircon crystals encased within younger rocks have ages as old as 4.3 billion years, making these tiny crystals the oldest materials so far found on Earth.
The best estimates of Earth's age are obtained by calculating the time required for development of the observed lead isotopes in Earth's oldest lead ores. These estimates yield 4.54 billion years as the age of Earth and of meteorites, and hence of the solar system.
The origins of life cannot be dated as precisely, but there is evidence that bacteria-like organisms lived on Earth 3.5 billion years ago, and they may have existed even earlier, when the first solid crust formed, almost 4 billion years ago. These early organisms must have been simpler than the organisms living today. Furthermore, before the earliest organisms there must have been structures that one would not call "alive" but that are now components of living things. Today, all living organisms store and transmit hereditary information using two kinds of molecules: DNA and RNA. Each of these molecules is in turn composed of four kinds of subunits known as nucleotides. The sequences of nucleotides in particular lengths of DNA or RNA, known as genes, direct the construction of molecules known as proteins, which in turn catalyze biochemical reactions, provide structural components for organisms, and perform many of the other functions on which life depends. Proteins consist of chains of subunits known as amino acids. The sequence of nucleotides in DNA and RNA therefore determines the sequence of amino acids in proteins; this is a central mechanism in all of biology.
Experiments conducted under conditions intended to resemble those present on primitive Earth have resulted in the production of some of the chemical components of proteins, DNA, and RNA. Some of these molecules also have been detected in meteorites from outer space and in interstellar space by astronomers using radiotelescopes. Scientists have concluded that the "building blocks of life" could have been available early in Earth's history.
An important new research avenue has opened with the discovery that certain molecules made of RNA, called ribozymes, can act as catalysts in modern cells. It previously had been thought that only proteins could serve as the catalysts required to carry out specific biochemical functions. Thus, in the early prebiotic world, RNA molecules could have been "autocatalytic"--that is, they could have replicated themselves well before there were any protein catalysts (called enzymes). Laboratory experiments demonstrate that replicating autocatalytic RNA molecules undergo spontaneous changes and that the variants of RNA molecules with the greatest autocatalytic activity come to prevail in their environments. Some scientists favor the hypothesis that there was an early "RNA world," and they are testing models that lead from RNA to the synthesis of simple DNA and protein molecules. These assemblages of molecules eventually could have become packaged within membranes, thus making up "protocells"--early versions of very simple cells.
For those who are studying the origin of life, the question is no longer whether life could have originated by chemical processes involving nonbiological components. The question instead has become which of many pathways might have been followed to produce the first cells.
Will we ever be able to identify the path of chemical evolution that succeeded in initiating life on Earth? Scientists are designing experiments and speculating about how early Earth could have provided a hospitable site for the segregation of molecules in units that might have been the first living systems. The recent speculation includes the possibility that the first living cells might have arisen on Mars, seeding Earth via the many meteorites that are known to travel from Mars to our planet.
Of course, even if a living cell were to be made in the laboratory, it would not prove that nature followed the same pathway billions of years ago. But it is the job of science to provide plausible natural explanations for natural phenomena. The study of the origin of life is a very active research area in which important progress is being made, although the consensus among scientists is that none of the current hypotheses has thus far been confirmed. The history of science shows that seemingly intractable problems like this one may become amenable to solution later, as a result of advances in theory, instrumentation, or the discovery of new facts.
Evidence Supporting Biological Evolution
A long path leads from the origins of primitive "life," which existed at least 3.5 billion years ago, to the profusion and diversity of life that exists today. This path is best understood as a product of evolution.
You will recall from the introductory unit that neither the term nor the idea of biological evolution began with Charles Darwin and his foremost work, On the Origin of Species by Means of Natural Selection (1859). Many scholars from the ancient Greek philosophers on had inferred that similar species were descended from a common ancestor. The word "evolution" first appeared in the English language in 1647 in a nonbiological connection, and it became widely used in English for all sorts of progressions from simpler beginnings. The term Darwin most often used to refer to biological evolution was "descent with modification," which remains a good brief definition of the process today.
Darwin proposed that evolution could be explained by the differential survival of organisms following their naturally occurring variation--a process he termed "natural selection." According to this view, the offspring of organisms differ from one another and from their parents in ways that are heritable--that is, they can pass on the differences genetically to their own offspring. Furthermore, organisms in nature typically produce more offspring than can survive and reproduce given the constraints of food, space, and other environmental resources. If a particular off spring has traits that give it an advantage in a particular environment, that organism will be more likely to survive and pass on those traits. As differences accumulate over generations, populations of organisms diverge from their ancestors.
Darwin's original hypothesis has undergone extensive modification and expansion, but the central concepts stand firm. Studies in genetics and molecular biology--fields unknown in Darwin's time--have explained the occurrence of the hereditary variations that are essential to natural selection. Genetic variations result from changes, or mutations, in the nucleotide sequence of DNA, the molecule that genes are made from. Such changes in DNA now can be detected and described with great precision. Genetic mutations arise by chance. They may or may not equip the organism with better means for surviving in its environment. But if a gene variant improves adaptation to the environment (for example, by allowing an organism to make better use of an available nutrient, or to escape predators more effectively--such as through stronger legs or disguising coloration), the organisms carrying that gene are more likely to survive and reproduce than those without it. Over time, their descendants will tend to increase,
changing the average characteristics of the population. Although the genetic variation on which natural selection works is based on random or chance elements, natural selection itself produces "adaptive" change--the very opposite of chance.
Scientists also have gained an understanding of the processes by which new species originate. A new species is one in which the individuals cannot mate and produce viable descendants with individuals of a preexisting species. The split of one species into two often starts because a group of individuals becomes geographically separated from the rest. This is particularly apparent in distant remote islands, such as the Galápagos and the Hawaiian archipelago, whose great distance from the Americas and Asia means that
arriving colonizers will have little or no opportunity to mate with individuals remaining on those continents. Mountains, rivers, lakes, and other natural barriers also account for geographic separation between populations that once belonged to the same species.
Once isolated, geographically separated groups of individuals become genetically differentiated as a consequence of mutation and other processes, including natural selection. The origin of a species is often a gradual process, so that at first the reproductive isolation between separated groups of organisms is only partial, but it eventually becomes complete. Scientists pay special attention to these intermediate situations, because they help to reconstruct the details of the process and to identify particular genes or sets of genes that account for the reproductive isolation between species.
A particularly compelling example of speciation involves the 13 species of finches studied by Darwin on the Galápagos Islands, now known as Darwin's finches. The ancestors of these finches appear to have emigrated from the South American mainland to the Galápagos. Today the different species of finches on the island have distinct habitats, diets, and behaviors, but the mechanisms involved in speciation continue to operate. A research group led by Peter and Rosemary Grant of Princeton University has shown that a single year of drought on the islands can drive evolutionary changes in the finches. Drought diminishes supplies of easily cracked nuts but permits the survival of plants that produce larger, tougher nuts. Droughts thus favor birds with strong, wide beaks that can break these tougher seeds, producing populations of birds with these traits. The Grants have estimated that if droughts occur about once every 10 years on the islands, a new species of finch might arise in only about 200 years.
The following sections consider several aspects of biological evolution in greater detail, looking at paleontology, comparative anatomy, biogeography, embryology, and molecular biology for further evidence supporting evolution.
The Fossil Record
Although it was Darwin, above all others, who first marshaled convincing evidence for biological evolution, earlier scholars had recognized that
organisms on Earth had changed systematically over long periods of time. For example, in 1799 an engineer named William Smith reported that, in undisrupted layers of rock, fossils occurred in a definite sequential order, with more modern-appearing ones closer to the top. Because bottom layers of rock logically were laid down earlier and thus are older than top layers, the sequence of fossils also could be given a chronology from oldest to
youngest. His findings were confirmed and extended in the 1830s by the paleontologist William Lonsdale, who recognized that fossil remains of organisms from lower strata were more primitive than the ones above. Today, many thousands of ancient rock deposits have been identified that show corresponding successions of fossil organisms.
Thus, the general sequence of fossils had already been recognized before Darwin conceived of descent with modification. But the paleontologists and geologists before Darwin used the sequence of fossils in rocks not as proof of biological evolution, but as a basis for working out the original sequence of rock strata that had been structurally disturbed by earthquakes and other forces.
In Darwin's time, paleontology was still a rudimentary science. Large parts of the geological succession of stratified rocks were unknown or inadequately studied. Darwin, therefore, worried about the rarity of intermediate forms between some major groups of organisms.
Today, many of the gaps in the paleontological record have been filled by the research of paleontologists. Hundreds of thousands of fossil organisms, found in well-dated rock sequences, represent successions of forms through time and manifest many evolutionary transitions. As mentioned earlier, microbial life of the simplest type was already in existence 3.5 billion years ago. The oldest evidence of more complex organisms (that is, eucaryotic cells, which are more complex than bacteria) has been discovered in fossils sealed in rocks approximately 2 billion years old. Multicellular organisms, which are the familiar fungi, plants, and animals, have been found only in younger geological strata. The following list presents the order in which increasingly complex forms of life appeared:
Life Form / Millions of Years Since First Known Appearance (Approximate)
Microbial (procaryotic cells) 3,500 Mammals 200
Complex (eucaryotic cells) 2,000 Nonhuman primates 60
First multicellular animals 670 Earliest apes 25
Shell-bearing animals 540 Australopithecine ancestors of
Vertebrates (simple fishes) 490 humans 4
Amphibians 350 Modern humans 0.15 (150,000
Reptiles 310 years)
So many intermediate forms have been discovered between fish and amphibians, between amphibians and reptiles, between reptiles and mammals, and along the primate lines of descent that it often is difficult to identify categorically when the transition occurs from one to another particular species. Actually, nearly all fossils can be regarded as intermediates in some sense; they are life forms that come between the forms that preceded them and those that followed.
The fossil record thus provides consistent evidence of systematic change through time--of descent with modification. From this huge body of evidence, it can be predicted that no reversals will be found in future paleontological studies. That is, amphibians will not appear before fishes, nor mammals before reptiles, and no complex life will occur in the geological record before the oldest eucaryotic cells. This prediction has been upheld by the evidence that has accumulated until now: no reversals have been found.
Inferences about common descent derived from paleontology are reinforced by comparative anatomy. For example, the skeletons of humans, mice, and bats are strikingly similar, despite the different ways of life of these animals and the diversity of environments in which they flourish. The correspondence of these animals, bone by bone, can be observed in every part of the body, including the limbs; yet a person writes, a mouse runs, and a bat flies with structures built of bones that are different in detail but similar in general structure and relation to each other.
Scientists call such structures homologies and have concluded that they are best explained by common descent. Comparative anatomists investigate such homologies, not only in bone structure but also in other parts of the body, working out relationships from degrees of similarity. Their conclusions provide important inferences about the details of evolutionary history, inferences that can be tested by comparisons with the sequence of
ancestral forms in the paleontological record.
The mammalian ear and jaw are instances in which paleontology and comparative anatomy combine to show common ancestry through transitional stages. The lower jaws of mammals contain only one bone, whereas those of reptiles have several. The other bones in the reptile jaw are homologous with bones now found in the mammalian ear. Paleontologists have discovered intermediate forms of mammal-like reptiles (Therapsida) with a double jaw joint--one composed of the bones that persist in mammalian jaws, the other consisting of bones that eventually became the hammer and anvil of the mammalian ear.
The Distribution of Species
Biogeography also has contributed evidence for descent from common ancestors. The diversity of life is stupendous. Approximately 250,000 species of living plants, 100,000 species of fungi, and one million species of animals have been described and named, each occupying its own peculiar ecological setting or niche; and the census is far from complete. Some species, such as human beings and our companion the dog, can live under a wide range of environments. Others are amazingly specialized. One species of a fungus (Laboulbenia) grows exclusively on the rear portion of the covering wings of a single species of beetle (Aphaenops cronei) found only in some caves of southern France. The larvae of the fly Drosophila carcinophila can develop only in specialized grooves beneath the flaps of the third pair of oral appendages of a land crab that is found only on certain Caribbean islands.
How can we make intelligible the colossal diversity of living beings and the existence of such extraordinary, seemingly whimsical creatures as the fungus, beetle, and fly described above? And why are island groups like the Galápagos so often inhabited by forms similar to those on the nearest mainland but belonging to different species? Evolutionary theory explains that biological diversity results from the descendants of local or migrant predecessors becoming adapted to their diverse environments. This explanation can be tested by examining present species and local fossils to see whether they have similar structures, which would indicate how one is derived from the other. Also, there should be evidence that species without an established local ancestry had migrated into the locality.
Wherever such tests have been carried out, these conditions have been confirmed. A good example is provided by the mammalian populations of North and South America, where strikingly different native organisms evolved in isolation until the emergence of the isthmus of Panama approximately 3 million years ago. Thereafter, the armadillo, porcupine, and opossum--mammals of South American origin--migrated north, along with many other species of plants and animals, while the mountain lion and other North American species made their way across the isthmus to the south.
The evidence that Darwin found for the influence of geographical distribution on the evolution of organisms has become stronger with advancing knowledge. For example, approximately 2,000 species of flies belonging to the genus Drosophila are now found throughout the world. About one-quarter of them live only in Hawaii.More than a thousand species of snails and other land mollusks also are found only in Hawaii. The biological explanation for the multiplicity of related species in remote localities is that such great diversity is a consequence of their evolution from a few common ancestors that colonized an isolated environment. The Hawaiian Islands are far from any mainland or other islands, and on the basis of geological evidence they never have been attached to other lands. Thus, the few colonizers that reached the Hawaiian Islands found many available ecological niches, where they could, over numerous generations, undergo evolutionary change and diversification. No mammals other than one bat species lived in the Hawaiian Islands when the first human settlers arrived; similarly, many other kinds of plants and animals were absent.
The Hawaiian Islands are not less hospitable than other parts of the world for the absent species. For example, pigs and goats have multiplied in the wild in Hawaii, and other domestic animals also thrive there. The scientific explanation for the absence of many kinds of organisms, and the great multiplication of a few kinds, is that many sorts of organisms never reached the islands, because of their geographic isolation. Those
that did reach the islands diversified over time because of the absence of related organisms that would compete for resources.
Similarities During Development
Embryology, the study of biological development from the time of conception, is another source of independent evidence for common descent. Barnacles, for instance, are sedentary crustaceans with little apparent similarity to such other crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through a free-swimming larval stage in which they look like other crustacean larvae. The similarity of larval stages supports the conclusion that all crustaceans have homologous parts and a common ancestry.
Similarly, a wide variety of organisms from fruit flies to worms to mice to humans have very similar sequences of genes that are active early in development. These genes influence body segmentation or orientation in all these diverse groups. The presence of such similar genes doing similar things across such a wide range of organisms is best explained by their having been present in a very early common ancestor of all of these groups.
New Evidence from Molecular Biology
The unifying principle of common descent that emerges from all the foregoing lines of evidence is being reinforced by the discoveries of modern biochemistry and molecular biology.
The code used to translate nucleotide sequences into amino acid sequences is essentially the same in all organisms. Moreover, proteins in all organisms are invariably composed of the same set of 20 amino acids. This unity of composition and function is a powerful argument in favor of the common descent of the most diverse organisms.
In 1959, scientists at Cambridge University in the United Kingdom determined the three-dimensional structures of two proteins that are found in almost every multicelled animal: hemoglobin and myoglobin. Hemoglobin is the protein that carries oxygen in the blood. Myoglobin receives oxygen from hemoglobin and stores it in the tissues until needed. These were the first three-dimensional protein structures to be solved, and they yielded some key insights. Myoglobin has a single chain of 153 amino acids wrapped around a group of iron and other atoms (called "heme") to which oxygen binds. Hemoglobin, in contrast, is made of up four chains: two identical chains consisting of 141 amino acids, and two other identical chains consisting of 146 amino acids. However, each chain has a heme exactly like that of myoglobin, and each of the four chains in the hemoglobin molecule is folded exactly like myoglobin. It was immediately
obvious in 1959 that the two molecules are very closely related.
During the next two decades, myoglobin and hemoglobin sequences were determined for dozens of mammals, birds, reptiles, amphibians, fish, worms, and molluscs. All of these sequences were so obviously related that they could be compared with confidence with the three-dimensional structures of two selected standards--whale myoglobin and horse hemoglobin. Even more significantly, the differences between sequences from different organisms could be used to construct a family tree of hemoglobin and myoglobin variation among organisms. This tree agreed completely with observations derived from paleontology and anatomy about the common descent of the corresponding organisms.
Similar family histories have been obtained from the three-dimensional structures and amino acid sequences of other proteins, such as cytochrome c (a protein engaged in energy transfer) and the digestive proteins trypsin and chymotrypsin. The examination of molecular structure offers a new and extremely powerful tool for studying evolutionary relationships. The quantity of information is potentially huge--as large as the thousands of different proteins contained in living organisms, and limited only by the time and resources of molecular biologists.
As the ability to sequence the nucleotides making up DNA has improved, it also has become possible to use genes to reconstruct the evolutionary history of organisms. Because of mutations, the sequence of nucleotides in a gene gradually changes over time. The more closely related two organisms are, the less different their DNA will be. Because there are tens of thousands of genes in humans and other organisms, DNA contains a tremendous amount of information about the evolutionary history of each organism.
Genes evolve at different rates because, although mutation is a random event, some proteins are much more tolerant of changes in their amino acid sequence than are other proteins. For this reason, the genes that encode these more tolerant, less constrained proteins evolve faster. The average rate at which a particular kind of gene or protein evolves gives rise to the concept of a "molecular clock." Molecular clocks run rapidly for
less constrained proteins and slowly for more constrained proteins, though they all time the same evolutionary events.
Cytochrome c proteins interact intimately with other macromolecules and are quite constrained in their amino acid sequences. The less rigidly constrained hemoglobins, interact mainly with oxygen and other small molecules. Fibrinopeptides, are protein fragments that are cut from larger proteins (fibrinogens) when blood clots.
The clock for fibrinopeptides runs rapidly; 1 percent of the amino acids change in a little longer than 1 million years. At the other extreme, the molecular clock runs slowly for cytochrome c; a 1 percent change in amino acid sequence requires 20 million years.
The hemoglobin clock is intermediate.
The concept of a molecular clock is useful for two purposes. It determines evolutionary relationships among organisms, and it indicates the time in the past when species started to diverge from one another. Once the clock for a particular gene or protein has been calibrated by reference to some event whose time is known, the actual chronological time when all other events occurred can be determined by examining the
protein or gene tree.
An interesting additional line of evidence supporting evolution involves sequences of DNA known as "pseudogenes." Pseudogenes are remnants of genes that no longer function but continue to be carried along in DNA as excess baggage. Pseudogenes also change through time, as they are passed on from ancestors to
descendants, and they offer an especially useful way of reconstructing evolutionary relationships.
With functioning genes, one possible explanation for the relative similarity between genes from different organisms is that their ways of life are similar--for example, the genes from a horse and a zebra could be more similar because of their similar habitats and behaviors than the genes from a horse and a tiger. But this possible explanation does not work for pseudogenes, since they perform no function. Rather, the degree of similarity between pseudogenes must simply reflect their evolutionary relatedness. The more remote the last common ancestor of two organisms, the more dissimilar their pseudogenes will be.
The evidence for evolution from molecular biology is overwhelming and is growing quickly. In some cases, this molecular evidence makes it possible to go beyond the paleontological evidence. For example, it has long been postulated that whales descended from land mammals that had returned to the sea. From anatomical and paleontological evidence, the whales' closest living land relatives seemed to be the even-toed hoofed mammals (modern cattle, sheep, camels, goats, etc.). Recent comparisons of some milk protein genes (beta-casein and kappa-casein) have confirmed this relationship and have suggested that the closest land-bound living relative of whales may be the hippopotamus. In this case, molecular biology has augmented the fossil record.
Describe the Basic Structure of DNA and its role in evolution
- Make a list of things that can self replicate without error. In many cases the object has not replicated itself. Instead, copies of an original blueprint have been made.
- Found in every cell of every organism, deoxyribonucleic acid is the only molecule known that is able to replicate itself and correct errors, thereby allowing cell division.
- DNA is used and copied thousands of times with very little change
- DNA provides the directions for the building of new cells and for the repair of worn cells.
- DNA is most often described as a double helix. DNA closely resembles a twisted ladder. Sugar and phosphate molecules form the backbone of the ladder, while the nitrogen bases form the rungs. Nitrogen bases from one spine of the ladder are connected by weak hydrogen bonds to the nitrogen bases on the other side of the ladder.
The double helix structure was first described in 1953 by James Watson and Francis Crick. Their discovery is one of the most significant of the twentieth century
- The DNA molecule is made up of individual units called nucleotides. Each nuleotide is composed of a deoxyribose sugar, a phosphate, and a nitrogen base.
- Complementary base pairing describes the behavior of the nitrogen bases. There are two families of nitrogenous bases: pyrimidines and purines. Pyrimidines have a single ring and include cytosine, thymine and uracil. Purines have a double ring and include adenine and guanine. A purine base always pairs with pyrimidine base. Can you suggest why?
[Using Mnemonics: "Thousands Count Pyramids" "Ads Guarantee Purity"]
- In DNA cytosine always pairs with guanine and adenine always pairs with thymine.
- If a stretch of one strand has the has the base sequence AGGTCCG, what would be the sequence of the same stretch of the other strand?
- A researcher finds a sample of DNA to have the four bases in the following percentages:
A = 30.9%
T = 25.4%
G = 19.9%
C = 25.8%
Why must there have been an error in his measurements?
Assuming the percentages of adenine and guanine are correct what should the values have been for thymine and cytosine?
- A genes meaning to the cell is encoded in its specific sequence of the four bases. The linear order of bases encoded in a gene specifies the amino acid sequence of a protein, which then specifies that protein's function in the cell.
- The two strands of the double helix are complementary, each the predictable counterpart of the other. It is this feature of DNA that makes possible the precise copying of genes that is responsible for inheritance. As a cell prepares to divide, the two strands of each gene seperate. Each existing strand serves as a template to order nucleotides into a new complementary strand.
- The genetic code is contained in 46 seperate chromosomes in your body.
Complete the case study on page 610. Read pages608 - 614 and complete review questions 1 - 8 0n page 614.
When you read the Historical Profile of Watson and Crick on page 612 and 613 notice that the two main process skills described are organizing data and communicating and answer the two questions that follow:
1. In a paragraph, how were open communication between scientists and and use of previous information essential to the discovery of the structure of DNA?
2. In a paragraph, how did politics prevent Linus Pauling from being named as a co-discoverer of the structure of DNA
What conditions are necessary for evolution to take place?
Differences among traits must occur among members of the same species. Therefore, no two individuals are exactly alike. Evolution does not refer to individual change or development. These variations must be passed on to the next generation. Variation among living organisms is not restricted solely to physical appearance. It can also be expressed in an organisms metabolism, fertility, mode of reproduction, behavior, or other measurable characteristics.
What is the role of DNA in evolution?
The modern theory of evolution recognizes that the main source of variation in a population lies in the differences in the genes carried by the chromosomes. Genes determine an organism's appearance, and mutations (permanent genetic changes) can cause new variations to arise. These variations can be passed on from generation to generation through DNA.
We have aready mentioned that DNA also provides biochemical evidence for the theory of evolution. The biochemical details shared by all cells are evidence that all life descended from a distant common ancestor - the first cell. The analysis and comparison of proteins and DNA from different organisms indicate that similar organisms also have similar chemical structures. When species of different orders are compared their DNA structures show greater differences than when species within the same genus are compared. The greater the number of mismatches in nitrogen bases, the greater the evolutionary distance between ancestors. One technique for measuring the similarity between two samples of DNA is DNA hybridization (pg. 73).
Complete the Case Study: Amino Acids and Evolution on page 49.
Evolutionary theory - prior to Darwin
Charles Darwin's name has become synonymous with the concept of evolution. However, he was not the first to consider that organisms (populations, species) change through time. For example, some early Greeks believed that the world had been created out of unformed matter. Empedocles offered that when animals were originally created they were random associations of body parts: a lion's head may have been on a frog's body, etc. Shortly after the earth was created, the animals began wandering about looking for their normal parts which when in close proximity would tend to be attracted to each other. This explained presence of imperfect, "monsters" Aniximander proposed the evolution of one form into another (e.g., fish to humans). Other early thinkers recognized the differences among organisms. For example, Aristotle ("father of biology") developed his "scale of nature" where living beings were arranged ladder-like in increasing complexity. Each form had an allotted location -- "rung"-- with every rung taken and these rungs never changed as this would disrupt the overall pattern. This view of the "fixity" of species was the dominant belief for the following 2000 years.
Even during Darwin's time, the view of the fixity and perfection of organisms was widely held. Specifically, the Judeo-Christian belief of the special creation -- and therefore immutability -- of all creatures was the prevailing view. This was reflected in natural theology which was based on the idea that the Creator's plan could be discovered by studying nature. However, there were a number of developments and discoveries prior to and at the time of Darwin's studies that were influential on Darwin.
1.James Hutton (scottish geologist) in 1795 proposed that the form a landscape has taken can be explained by examining mechanisms currently operating = gradualism; e.g., canyons wereformed by rivers cutting down through valleys.
2.Georges Cuvier (father of paleontology), in contrast, proposed catastrophism to explain the changes he observed in rock strata: each boundary between strata corresponded to huge catastrophes (of biblical proportion) such as floods, droughts. Staunch anti-evolutionist (especially against Lamarck, see below)
3.Charles Lyell (scottish geologist): Somewhat after Cuvier and Hutton, Charles Lyell (a contemporary of Darwin) extended the science of geology, publishing the leading geology text ofthe day, Principles of Geology, which would have an enormous influence on Darwin. In particular, Lyell's theory of uniformitarianism (which incorporated Hutton's gradualism) -- the basic geologic processes at work on earth have not changed throughout its history -- was a radical idea because it implied that: a.The earth must be very old b.Very slow and persistent forces can cause significant changes to occur
4.Jean Baptiste Lamarck (French naturalist and curator of invertebrates at Natural HistoryMuseum in Paris) proposed that species change (= evolve) (as had others) and proposed a mechanism for how the change could occur (as others had not effectively): inheritance of acquired characteristics (1809). Lamarck took Aristotle's scale of nature and interpreted it and the fossil record as indicating that nature was constantly striving for increased complexity (perfection).
"Darwin saw what every person saw, yet saw what no one saw."
Charles Darwin (1809-1882) was an avid naturalist even at an early age. His father (physician) sent him to medical school at the University of Edinburgh, but he was an uninspired student; he switched to Cambridge with intent of becoming a clergyman (early on Darwin was a dedicated creationist). After graduation (age of 22) he signed on with Captain FitzRoy for a round-the-world voyage as ship's naturalist on the HMS Beagle. On this 5-year voyage, Darwin made many collections and observations that would (later) have enormous influence on him. Most prominant among these:
a.Volcanism: processes discussed in Lyell's Principles of Geology which he read on the voyage
b.Unique species, both extant and extinct (e.g., armadillo & glyptodont)
c.But most influential were his collections and observations in the Galápagos Islands:
i.More unique organisms seen; was able to compare mainland and island forms and found differences and similarities
ii.Inter-island comparisons (e.g., giant tortoises differed among the islands)
iii.Finches: Darwin collected finch species that were unique to the islands
Quickly upon his return from the voyage of the Beagle, Darwin married and started writing up his notebooks and sifting through all his collections. Soon after return he concluded that species do in fact change (are mutable), but could propose no mechanism for how this change could occur.
In 1838, Darwin read "An Essay on Population" by economist Thomas Malthus (which suggested that populations tend to grow faster than their food supplies). This would create a 'struggle' among individuals for resources = a selective force: those individuals that possessed particular traits that made them better able to gather resources would tend to produce more offspring (= natural selection). Those traits would therefore increase in frequency within the population.
Darwin hesitated to publish his ideas, knowing how provocative they would be (including within his own family) and therefore ran the risk of being 'scooped' by other naturalists. This almost happened when Alfred Russell Wallace contacted Darwin about his independent development of essentially the same ideas, based on Wallace's work on islands in Indonesia.
Darwin and Wallace produced a joint paper that was read at a scientific meeting in 1858. Darwin then produced a book-length treatise detailing his ideas, published in 1859 as On the Origin of Species By Means of Natural Selection. A very rapid response followed and this is one of the most influential books ever written.
Lamarck vs. Darwin
Jean Baptiste de Lamarck put forth his first theory of evolution was in 1809. Lamarck described a mechanism known as "the inheritance of aquired characteristics". According to Lamarck' s theory, if there were salamanders living in tall grass which made their legs no longer useful and they instead began slithering on their bellies then the muscles of their legs would grow smaller from disuse. The salamanders would pass this acquired trait on to their offspring. In time, the salamander's legs would be used so rarely that they disappeared. Lamarck presented no experimental evidence or observation and his theory fell out of scientific favor.
The next significant idea came from Charles Darwin, when he combined the idea of competition with his other observations from his voyage on the Beagle to explain how evolution could occur. Today his is the most widely accepted theory of evolution. The essence of his theory was that individuals with advantageous heritable traits were more likely to survive and reproduce in an environment where resources were limited. This meant that this trait would flourish in each generation and become more and more predominant in the population. At the time (1837), Darwin had no idea what the biological mechanism (DNA) responsible for evolution was but his observations of how evolution occurred were correct and his theory of evolution is still the most widely accepted.
How would Lamarck explain the evolution of blind fish living in a cave?
How would Lamarck explain the evolution of blind fish living in a cave?
Explain the role of sexual reproduction in variation and evolution
Make a list of as many different breeds of dog as we can think of.
How many do you have?
How many breeds of dog are there?
How did all these dogs come to exist?
Why is artificial selection an appropriate term to describe the activities of breeders?
Reproduction is the act of one or more living things creating one or more similar living things. Evolution can only occur in a population whose existence is ongoing. The beauty of sexual reproduction is that both parents contribute half of the genetic material to the offspring. This mixing is random and therefor leads to variation. This variation can then be selectively bred for either by man as we see in dog breeds or by the environment as we see in the natural world.
When man selectively breeds for a variation we call this?
When nature selectively breeds for a trait we call this?
Describe the process of natural selection
We have previously mentioned the process of natural selection but we will now discuss it in more detail. Darwin's theory of evolution by natural selection can be divided into five distinct ideas:
Overproduction - Overproduction means that the number of offspring produced by a species is greater than the number that can survive, reproduce and live to maturity.
Struggle for existence (competition) - Because of overproduction, organisms of the same species, as well as those of different species, must compete for limited resources such as food, water, and a place to live.
Variation - Differences among traits occur among members of the same of the species. Therefore, no two individuals are exactly alike. Darwin believed that these variations are passed on to the next generation.
Survival of the fittest (natural selection) - Those individuals in a species with traits that give them an advantage are better able to compete, survive, and reproduce. All others die off without leaving any offspring. Since nature selects the organisms that survive, the process is called natural selection.
Origin of new species (speciation) - Over numerous generations, new species arise by the accumulation of inherited variations. When a type is produced that is significantly different from the original, it becomes a new species.
As an example of evolution by natural selection, Darwin noted the beaks of the finches he observed on the Galapagos. He counted 13 similar species that differed mainly in the structure and therefore the function of their beaks which were specialized for the food available on their island
As another example Darwin noted that the willow ptarmigan which is brown most of the year turns white in the winter. This color change he inferred, helped protect it from predators, which would have a hard time spotting the bird in the snow. Ptarmigans that didn't change color would be easily spotted and eaten. Ptarmigans that turned white in winter would be more likely to survive, reproduce, and pass this adaptation to future generations. Can you think of other examples?
How would Darwin explain the evolution of the legless salamander?
Complete the case study on pg. 96 of Nelson Biology
Suggest conditions under which the allelic frequencies in a population could change, including genetic drift, differential migration, mutation and natural selection.
Often the most meaningful way to way to understand how species change over time - that is, how they evolve - is to think of a population as simply a collection of genes. The collection of genes for all the traits in a population is called the gene pool. The gene pool of a population thus contains all the alleles for all the genes. An allele frequency is the percentage of a specific allele of a gene in the gene pool. A population in which allele frequencies do not change from generation to generation is said to be in genetic equilibrium.
Genetic Drift explains the disruption of the genetic equilibrium that occurs in small populations. In small populations chance can significantly affect allele frequencies. In a population of ten to twenty organisms it is possible that only one member might carry an allele for a particular gene. If that member failed to breed the allele would be eliminated from the gene pool forever, even if the reason for lack of reproductive success was caused by chance not by a lack of fitness. Elimination of the allele would change the allele frequncy and disrupt genetic equilibrium in the population. [an example of one such small population is the Galapagos tortoise. They numbered in the thousands before the 1900's until settlers introduced goats and pigs that competed directly with them for food. By 1970 the total population had shrunk to fewer than 20 members.]
Migration - the movement of individual organisms into or out a population - can alter allele frequencies in a population and thus disrupt genetic equilibrium. The movement of genes into or out of a population through migration is called gene flow. Gene flow does not always involve the movement of individuals from place to place. It may involve the movement of sperm, for example, when sperm in plant pollen is transported by wind.
Mutations are inheritable changes in the genetic material of an organism. Mutations disrupt genetic equilibrium by producing totally new alleles for a trait. They may take place in any cell. Germ cell mutations occur in sex cells, such as eggs and sperm. They do not affect the organism itself but are passed on to offspring. Somatic mutations take place in body cells. They are passed on to daughter cells through mitosis.
Chromosome mutations often occur during cell division. Deletion occurs when a piece of chromosome breaks off. All the information on that piece is lost. Inversion occurs when a piece breaks from a chromosome and reattaches itself to the chromosome in the reverse orientation. Translocation occurs when a broken piece attaches to a nonhomologous chromosome. Another kind of chromosomal mutation, called nondisjunction, occurs when a replicated chromosome pair fails to seperate during cell division. When nondisjunction occurs, one daughter cell recieves an extra copy of a chromosome, and the other daughter cell lacks that chromosome entirely. In humans, for example, if nondisjunction occurs during sperm formation, one sperm cell may have 22 chromosomes, and the other may have 24. If one of these gametes combines with a normal egg, the zygote will have either 45 (monosomy) or 47 (trisomy) chromosomes. An extra chromosome 21, for example, results in Down syndrome, a disorder characterized by mental retardation, a fold of skin above the eyes, and weak muscles. Klinefelter syndrome results from the trisomic genotype XXY. Klinefelter individuals may be mentally retarded and have low fertility. Turner syndrome is a monosomic condition with the genotype XO. An XO female is characterized by immature physical development, sterility, and a webbed neck.
Gene mutations arise from mistakes in DNA replication. When one nitrogen base is substituted for another, added, or deleted a point mutation has occured. The addition or deletion of a nitrogen base is a point mutation called a frameshift mutation. Cosmic rays, X rays, ultraviolet radiation, and chemicals that alter the DNA are called mutagenic agents (or mutagens). By changing the arrangement of the nucleotides in the double helix, the mutagen changes the genetic code. The shift in a single nucleotide will lead to the production of a new protein from the instructions. The new protein has a different chemical structure and, in most cases, is incapable of carrying out the function of the required protein. Without the required protein, cell function is impaired, if not completely destroyed. Although some mutations can, by chance, improve the functioning of the cell, the vast majority of mutations produce adverse effects. Some have a neutral effect. Mutations ultimately provide the variation upon which natural selection acts.
Sometimes the error may arise because of a shortage of a particular type of nucleotide during the replication process.
One of the most common types of error is created when one nitrogen base is substituted for another. [For example: hydroxylamine can modify cystosine so that it pairs with thymine. Normally, adenine pairs with ________ ? The hydroxylamine removes a nitrogen group attached to the adenine molecule, making it appear much like the guanine molecule. The guanine then bonds with a thymine-containing nucleuotide.
Occasionally, X rays will break the backbone of the DNA molecule. Special enzymes will repair the break but the spliced segment may not get placed in the proper position. The misplaced segment may alter the entire library of genetic information.
One well known genetic mutation is a human disorder called sickle-cell anemia. See Figure 25.17 on page 642. This genetic disorder affects the structure of the oxygen-carrying molecule found in red blood cells. The alteration of a single nitrigen base causes valine to replace glutamate as the sixth amino acid in one of the protein chains. Even this slight change has devastating consequences. The red blood cell assumes a sickle shape and is unable to carry an adequate amount of oxygen. To make matters worse the sickle-shaped cells clog the small capillaries, starving the body's tissues of oxygen.
[Read Pg. 646-647 THALIDOMIDE AND GENE MUTATIONS]
What type of agent would thalidomide be classified as?
[Read pg. 647-649 FRONTIERS OF TECHNOLOGY: THE AMES TEST]
How does the Ames test identify cancer causing agents?
Natural selection is an ongoing process in nature, and it is the single most significant factor disrupting genetic equilibrium. Natural selection results in higher reproductive rates for individuals with certain phenotypes, and, hence, certain genotypes. Thus by the action of natural selection allele frequencies change from one generation to the next. Each of four types of natural selection - stabilizing, directional, disruptive, and sexual - cause changes in the gene pool of a population.
Stabilizing selection is a type of natural selection in which individuals with the average form of a trait have an advantage in terms of survival and reproduction. The extreme forms of the trait, on the other hand, confer a disadvantage to the organism. Ex. Lizards [In some species of lizards conditions may exist where average size is an advantage in terms of survival and reproduction. Lizards that are larger than average will be more easily spotted, captured, and eaten by predators than average-sized lizards will. On the other hand, lizards that are smaller than average might not be able to run fast enough to escape predators and therefore will also be at a disadvantage. Thus conditions are such that the average size lizards would be more likely to survive and therefore to reproduce.] Stabilizing selection is most effective in a population that has become well adapted to its environment. Since most populations are reasonably well adapted to their environments most of the time, stabilizing selection is the most common type of natural selection.
Directional selection is a type of natural selection in which individuals with one of the extreme forms of a trait have an advantage in terms of survival and reproduction. Ex. Anteaters [Anteaters feed by breaking open termite nests, extending a stick tongue into the nest, and lapping up the termites. Suppose that the area was invaded by a new species of termite that built very deep nests. Anteaters with long tongues could more effectively prey on these termites than could anteaters with short tongues. In other words anteaters with long tongues would have a selective advantage.]
Disruptive selection is a type of natural selection in which individuals with either of the extreme forms of a trait have an advantage in terms of survival and reproduction. The average form of the trait on the other hand, confers a selective disadvantageto the organism. Ex. Limpets [The trait of shell color in marine animals called limpets varies from pure white to dark tan. On rocks covered with goose barnacles, which are white, white shelled limpets are at an advantage. Birds that prey on white-shelled limpets have a hard time identifying the white shells against the white background. On bare dark-colored rocks dark-shelled limpets are at an advantage. The limpet eating birds also have a hard time locating the dark shells against the dark background. However, it is easy for the birds to spot limpets with shells of an intermediate color, which are visible against both backgrounds.
Sexual selection is the preferential choice of a mate based on the presence of a specific trait. Ex. Tropical Beetles [In the tropical beetle, females preferentially mate with males having an elongated snout. Therefore male beetles with a longer snout are more likely to reproduce and pass their genes on to the next generation. Thus the longer snout is sexually selected for in the male of this species. Sexual selection may be stabilizing, directional, or disruptive.
Differentiate among and give examples of convergence, divergence, speciation
Convergence is the development of similar forms in geographically different areas in response to similar environments. In other words, unrelated species become more and more similar in appearance as they adapt to the same kind of environment. Ex. Two unrelated plants the cactus and the spurge have developed similar adaptations to desert environments. Though they are unrelated and one grows in the American desert the other in the African desert they both developed fleshy stems that store water and sharp spines that ward off predators.
Divergence is the development of different structures or characteristics in populations that were originally the same. Each population evolves to meet the demands of its own environment. In other words, two or more related species becoming more and more dissimilar. Ex. The red fox and the kit fox. Homologous structures provide evidence of divergence. Turn to pg. 70 and look at Figure 2.10. Notice that the flipper of a sea lion, the leg of a bat, and the human arm all have the same basic structure. They also have the same pattern of early growth. These homologous structures in some cases serve different functions, but they are sufficiently similar to suggest that they have the same evolutionary origin. [Many other examples of homologies can be found in living organisms. For example the eustachian tube in humans is homologous to the gill slits of fish, and the middle ear bones of humans are homologous to certain jawbones of fish.]
Speciation refers to the formation of new species. In the modern sense of the term, a species is a group of similar organisms that can interbreed and produce fertile offspring. Since a species can only arise from an existing species there must be some process or mechanism by which a single species can develop into one or more descendant species. For a new species to arise, either interbreeding or the production of fertile offspring must somehow cease among members of a formerly successful breeding population. Two ways in which this may arise are through geographic isolation and reproductive isolation.
Geographic isolation is when the seperation is caused by physical obstacles or barriers such as mountain ranges, and bodies of water, a river changing course, or even barriers created by humans such as a highway. When this happens, gene flow between the isolated group and the main population ceases. Eventually, the groups become so different that individuals of one population can no longer interbreed with those of another. Ex. When a group of American finches colonized the Hawaiin islands the group became geographically isolated from the parent population. These finches eventually gave rise to 23 species of Hawaiin honeycreepers. Ex. The desert of Death Valley, California, has a number of isolated ponds formed by springs. Each pond contains a species of fish that lives only in that pond. Scientists suggest that these species arose through geographic isolation. How did these fish become isolated in Death Valley? Geologic evidence from a study of wave patterns in sedimentary rocks indicates that most of Death Valley was covered by a huge lake during the last ice age. When the ice age ended, the region became dry. Only small spring fed ponds remained. Members of a fish species that previously formed a single population in the lake may have become isolated in different ponds. Another example is the speciation of snails diagramed in Figure 6.8 on page 188 of your text. The most famous example of geographic isolation being used to explain speciation are the species of finches that Darwin found on the Galapagos Islands.
Reproductive isolation occurs when barriers to successful breeding arise among population groups in the same area. Reproductive isolation is the inability of formerly interbreeding organisms to produce offspring. Factors that contribute to reproductive isolation include differences in mating habits and courtship patterns, seasonal differences in mating, and the ability of the sperm to fertilize the eggs. Ex. The wood frog and the leopard frog have become reproductively isolated, possibly as a result of disruptive selection In the ancestral frog frog species the frogs that bred earlier and frogs that bred later may have both been selected for, while frogs that bred between these times may have been selected against, perhaps because some predator was especially active at that time. The two groups may have become reproductively isolated because of differences in breeding times.
1. Speculate about animal species that may exist in the future.
2. Discuss what future anatomical and physiological changes you think might take place in existing species.
3. Speculate about possible environmental pressures that might lead to species changes you have described.
Compare and contrast the gradual change model with the punctuated equilibrium model of evolution
How fast do new species form? Because their generation times are short, new species of unicellular organisms may evolve in years, months or even days. For plants and animals Darwin theorized that new species formed gradually over millions of years. More recent examples show species having arose in only thousands of years. Ex. Today several species of moths, unique to hawaiin islands, feed on bananas. These moth species are closely related to other plant-eating moths in Hawaii. Archeological evidence suggests that settlers from Polynesia introduced banana trees to the Hawaiin islands about a thousand years ago. Thus scientists suggest that the banana-eating moths arose from other Plant-eating moths, undergoing adaptive radiation in less than the thousand years that banana trees have existed in Hawaii.
Evidence from the fossil record has led some scientists to propose that the speciation need not occur gradually but can occur in spurts. According to the theory of punctuated equilibrium, all populations of a species may exist for a relatively long time at or close to genetic equilibrium. Then this equilibrium may be interupted by a brief period of rapid genetic change in which speciation occurs.
Some scientists argue that if species evolved gradually, the fossil record should show many examples of transitional forms - species with characteristics intermediate between those of the ancestral species and the new species. However, for most organisms such transitional forms are absent from the fossil record. Instead the fossil records for most species show that they stay the same for millions of years. Then new related species suddenly appear.
Wether new species form gradually or rapidly is still a point of debate among scientists.
Identify the role of extinction in evolution
Extinction refers to the permanent disappearance of a species from the earth as a result of environmental events or human actions. Nearly 500 million different species have inhabited the planet earth. More than 90% have either become extinct or have evolved into new species to adapt to changes.
What causes extinction?
For a species to continue to exist, some members must have the traits that allow them to survive and pass their genes on to the next generation. Ex. The whooping crane has been on the endangered species list for several years now. Possible reasons include long migratory routes and poor reproductive success. [The whooping crane produces two eggs; however only one offspring matures. The first fledgling to crawl from the egg kills its brother or sister. This ensures enough food for the survivor, but limits reproductive capacity.] If the environment changes the species will become extinct unless some members of the species have adaptations that allow them to survive and reproduce successfully under the new environmental conditions. Environmental changes caused by humans have led to the extinction of hundreds of organisms in the past few centuries. Most of these changes involve destruction of habitats. Ex. The black-footed ferret [The black-footed ferret may be on the way to extinction because humans have encroached on the habitat of its prey, the prarie dog. The conversion of the praries of central North America into farmland and grazing ranges caused a decline in the large population of prarie dogs in the region. In turn the black-footed ferret, which preys solely on the prarie dog, has also greatly declined in numbers. This weasellike animal may become extinct may soon become extinct, because members of the species do not have variations that result in reproductive success in this changing environment.]
What is the role of extinction in evolution?
The history of our planet has been punctuated by large-scale disasters that have destroyed large numbers of animals.It is likely that the largest mass extinction took place 240 million years ago by a catastrophe that heralded the the beginning of the Triasic period. About 80% of the organisms living in the oceans and on land perished. This was followed by another mass extinction 205 million years ago, marking the beginning of the Jurassic period and the age of the dinosaurs. The mass extinction may well have removed competition, ensuring the success of the early reptiles, which were no larger than small dogs.
The best known case of mass extinction would have to be that of the dinosaurs which occured about 65 million years ago.
What killed the dinosaurs?
[Read pages 79 - 81 for the answer; be able to identify two theories that attempt to explain the extinction of the dinosaurs]
The evolution of altruism
Altruism is defined generally as concern for the welfare of others; in behavioral ecology, altruism is defined as acting so as to increase the fitness of another individual while decreasing one's own fitness. Based on what we have discussed throughout the semester, it should be apparent that we might have a difficult time explaining how such traits could be adaptations: if an individual helps to increase another's genetic representation in the next generation while decreasing its own genetic representation, this is working opposite to the action of natural selection. Selection should therefore eliminate such a "helping" trait from the population rather quickly. Yet such apparent helping is demonstrated by many species (e.g., cooperative breeding in birds, alarm calls in ground squirrels, suicidal defense by honeybees). OK, Mr. Darwin, how do you explain this? As in most difficult problems, there have been a number of solutions proposed.
Group selection - In the 1960s, the benefit of altruistic behavior was believed to lie in the effect it had on the entire group (e.g., population, species). The thinking was that altruistic behavior could evolve because altruistic groups had a higher average fitness and were more likely to persist. (For example, individuals that voluntarily curtailed their own reproduction would benefit everyone in the group.) However, as we saw when we discussed the evolution of sex, selection is generally believed to act at the level of the individual: the fitness differences among individuals are what selection is all about. A group of altruistic individuals could be easily "invaded" by a selfish phenotype that acted only in its own self-interest and not for the good of the group.
Kin selection - Let's look at the details of natural selection one more time. Suppose there are two alleles at a locus that determines whether an individual reduces its own reproduction in order to spend resources in helping others produce their own offspring: allele H1 determines non-helping while allele H2 determines helping. For simplicity, let's say that we are dealing with haploid individuals. Suppose that, on average, helping another individual reduces the reproductive output of the H2 phenotype by 0.5 offspring below that of the H1 phenotype. Also suppose that being helped increases your reproductive output by 1 offspring. We then have two different possibilities for each phenotype:
If an H1 individual is not helped by an H2 individual, then its reproductive output is X
If an H1 individual is helped by an H2 individual, then its reproductive output is X + 1
If an H2 is not helped by an H2 individual, then its reproductive output is X - 0.5
If an H2 is helped by an H2 individual, then its reproductive output is X - 0.5 + 1
If we start out with equal allele frequencies in a population (i.e., equal numbers of both phenotypes), what should happen over time? Because the H1 individuals reproduce at a higher rate (i.e., they don't waste resources by helping others) AND they may receive the benefit of being aided by H2 individuals, the fitness of the H1 phenotype should be higher and will increase in frequency in the population. So in our example above, starting out with equal numbers of both phenotypes, we would expect that the average fitness of each genotype to be:
Average reproductive output of H1 phenotype: [X + (X + 1)]/2 = X + 1/2
Average reproductive output of H2 phenotype: [(X - 0.5) + (X - 0.5 + 1)]/2 = X
The end result will be the elimination of altruistic behavior -- we've explained nothing.
But, what if the H2 individuals only helped other H2 individuals and did not help the H1 individuals? This would mean that each H2 individual helped one other individual and was helped by one other individual. Again, starting with equal numbers of each phenotype, the H2 phenotype should have a higher average fitness as a result of preferentially helping each other:
Average reproductive output of H1 phenotype: X
Average reproductive output of H2 phenotype: X - 0.5 + 1 = X + 1/2
Therefore, we've identified conditions under which altruistic behavior could evolve: when the help you provide is directed toward individuals that are genetically related to you AND when the benefit to helping outweighs the cost. (Actually, if the benefit and cost to helping were equal -- e.g., helping both increased the reproductive output of the helped individual by one and decreased the reproductive output of the helper by one -- then this helping behavior could still persist in the population, there would just not be a selective advantage to helping over non helping.)
We've stumbled upon a very important concept: the genetic representation (fitness) of an individual can be increased in the population not only by reproduction by the individual itself, but also by helping to enhance the reproductive output of genetically related individuals. Because individuals that are related to you share a certain proportion of your genetic material, enhancing their reproductive output actually enhances your own fitness. This is the basis of inclusive fitness: the total fitness of a genotype is determined by the genetic contribution made through producing its own offspring AND through the production of offspring of related individuals. The first component of inclusive fitness -- production of the individual's own offspring -- is termed the direct component of inclusive fitness while the second component -- the genetic contribution by way of relatives' offspring -- is termed the indirect component of inclusive fitness (i.e., inclusive fitness = direct component + indirect component).
The fitness payoff by way of the indirect component of fitness is the proposed basis for the evolution of altruism by way of kin selection: altruistic traits that increase the reproductive output of kin can be selected for because the inclusive fitness of the altruistic individual is increased.
Because there are varying degrees of relatedness between individuals, we can predict that altruistic behavior is more likely to evolve among particular sets of relatives. Generally, the more closely related two individuals are, the more likely it is altruistic behavior may occur between them. We can quantify how closely related two individuals are by determining the probability that any two individuals have the same copy of gene: we will designate this as r and call it the coefficient of relatedness. Hence, r is expected to be 0.5 between parent and offspring and among full siblings; 0.25 between grandchildren and grandparents, etc. If we know the value of r between individuals, we can specify the conditions under which altruistic behavior could evolve: we would expect an altruistic allele to increase in frequency when:
r > Cost (fitness loss to altruist)/Benefit (fitness gain to recipient)
Therefore, the "altruistic" behavior is consequently not really altruistic after all: there is a payoff in terms of increased genetic representation in the population (increased inclusive fitness).
Examples: eusociality in the Hymenoptera; alarm calls in ground squirrels; cooperative breeding in scrub jays
Reciprocal altruism - Is there a way that altruistic behavior could evolve among unrelated individuals?
Theoretically, yes, although it may be less likely to evolve than altruism under kin selection. The model of reciprocal altruism basically says that under certain circumstances, individuals may act altruistically toward unrelated individuals. However, there is still a payoff to the altruistic individual: he or she will be the recipient of altruistic behavior in the future. Hence, the model in simple form is "if you scratch my back, I'll scratch yours." For this type of behavior to evolve, there are a couple of necessary characteristics:
1.recipients must have the opportunity to reciprocate later
2.there must be some way to detect and punish cheaters
3.the benefit of receiving help must outweigh the costs of giving help
Examples: male-male coalitions in olive baboon; sharing of blood by vampire bats
Isn't evolution just an inference?
- No one saw the evolution of one-toed horses from three-toed horses, but that does not mean that we cannot be confident that horses evolved. Science is practiced in many ways besides direct observation and experimentation. Much scientific discovery is done through indirect experimentation and observation in which inferences are made, and hypotheses generated from those inferences are tested. For instance, particle physicists cannot directly observe subatomic particles because the particles are too small. They make inferences about the weight, speed, and other properties of the particles based on other observations. A logical hypothesis might be something like this: If the weight of this particle is Y, when I bombard it, X will happen. If X does not happen, then the hypothesis is disproved. Thus, we can learn about the natural world even if we cannot directly observe a phenomenon--and that is true about the past, too.
In historical sciences like astronomy, geology, evolutionary biology, and archaeology, logical inferences are made and then tested against data. Sometimes the test cannot be made until new data are available, but a great deal has been done to help us understand the past. For example, scorpionflies (Mecoptera) and true flies (Diptera) have enough similarities that entomologists consider them to be closely related. Scorpionflies have four wings of about the same size, and true flies have a large front pair of wings but the back pair is replaced by small club-shaped structures. If two-winged flies evolved from scorpionfly-like ancestors, as comparative anatomy suggests, then an intermediate true fly with four wings should have existed--and in 1976 fossils of such a fly were discovered. Furthermore, geneticists have found that the number of wings in flies can be changed through mutations in a single gene.
Something that happened in the past is thus not "off limits" for scientific study. Hypotheses can be made about such phenomena, and these hypotheses can be tested and can lead to solid conclusions. Furthermore, many key mechanisms of evolution occur over relatively short periods and can be observed directly--such as the evolution of bacteria resistant to antibiotics.
Evolution is a well-supported theory drawn from a variety of sources of data, including observations about the fossil record, genetic information, the distribution of plants and animals, and the similarities across species of anatomy and development. Scientists have inferred that descent with modification offers the best scientific explanation for these observations.
Is evolution a fact or a theory?
The theory of evolution explains how life on Earth has changed. In scientific terms, "theory" does not mean "guess" or "hunch" as it does in everyday usage. Scientific theories are explanations of natural phenomena built up logically from testable observations and hypotheses. Biological evolution is the best scientific explanation we have for the enormous range of observations about the living world. Scientists most often use the word "fact" to describe an observation. But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples. The occurrence of evolution in this sense is a fact. Scientists no longer question whether descent with modification occurred because the evidence supporting the idea is so strong.
Don't many famous scientists reject evolution?
No. The scientific consensus around evolution is overwhelming. Those opposed to evolution sometimes use quotations from prominent scientists out of context to claim that scientists do not support evolution. However, examination of the quotations reveals that the scientists are actually disputing some aspect of how evolution occurs, not whether evolution occurred. For example, the biologist Stephen Jay Gould once wrote that "the extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology." But Gould, an accomplished paleontologist and eloquent educator about evolution, was arguing about how evolution takes place. He was discussing whether the rate of change of species is slow and gradual or whether it takes place in bursts after long periods when little change occurs--an idea known as punctuated equilibrium. As Gould writes in response, "This quotation, although accurate as a partial citation, is dishonest in leaving out the following explanatory material showing my true purpose--to discuss rates of evolutionary change, not to deny the fact of evolution itself." Gould defines punctuated equilibrium as follows:
Punctuated equilibrium is neither a creationist idea nor even a non-Darwinian evolutionary theory about sudden change that produces a new species all at once in a single generation. Punctuated equilibrium accepts the conventional idea that new species form over hundreds or thousands of generations and through an extensive series of intermediate stages. But geological time is so long that even a few thousand years may appear as a mere "moment" relative to the several million years of existence for most species. Thus, rates of evolution vary enormously and new species may appear to arise "suddenly" in geological time, even though the time involved would seem long, and the change very slow, when compared to a human lifetime.