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Chapter 19 - Darwin

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I.         Evolution refers to the processes that have transformed life on Earth from its earliest form to the vast diversity that characterizes it today.

A.  Darwin published Origin of Species in 1859 with two points

1.  Species were not created in their present forms but had evolved from ancestral species.

2.       Natural selection is the proposed mechanism for evolution. 

a.       A population of organisms can change over time as a result of individuals with certain heritable traits leaving more offspring than other individuals.

B.  Evolutionary change is based on the interactions between populations of organisms and their environment.

C.  Evolutionary adaptations are inherited characteristics that enhance an organismís ability to survive and reproduce in a particular environment.


II.  Western culture resisted evolutionary views of life

            A.  The Scale of Nature and Natural Theology

                        1.  Many Greek philosophers believed in gradual evolution of life.

2.       Plato believed that evolution would be counterproductive because organisms that were perfect for their environment could not live in an imperfect world.

3.       Aristotle believed that all living forms could be arranged on a scale of nature.

4.       Judeo-Christian culture used the idea of natural theology that was dedicated to discovering the Creatorís plan by studying nature.  They saw adaptations as evidence that the Creator had designed each for a particular purpose.

5.       Carolus Linnaeus was the founder of taxonomy who sought to discover order in the diversity of life.  He developed a two-part system of naming organisms that is still used today.  His clustering of species implied no evolutionary kinship.

B.  Curvier, Fossils, and Catastrophism

1.  Fossils are relics or impressions of organisms from the past used to help lay the groundwork for Darwinís ideas.

2.       Most fossils are found in sedimentary rock formed from sand and mud that settle in marshes, lakes and seas.  Newer layers compress the older layers into strata.  Fossils in the layers show a succession of organisms.

3.       Paleontology was developed by Georges Cuvier to study fossils.  He documented the succession of the fossil record when he noted that each stratum contained layers that had a unique group of characteristics.  He recognized that extinction was evident.  He began the idea of catastrophism speculating that each boundary between strata corresponded in time to a catastrophe that had destroyed many species.

II.  Theories of geological gradualism helped clear the path for evolutionary biologists

            A.  James Hutton proposed the idea of gradualism where various landforms came                     about due to a gradual change.  Profound changes are a cumulative product                     of slow but continuous processes.

B.      Charles Lyell incorporated Huttonís theory in a theory known as uniformitarianism that says these processes have not changed throughout time. 

C.      Darwin learned from this that geological change results from a slow continuous action and that these slow changes have a substantial impact on change.

III.  Lamarck placed fossils in evolutionary context

            A.  Jean Baptist Lamarck compared current species to fossil forms and said you                     could see several lines of descent.  He felt that evolution was driven to                     greater complexity.  He incorporated two ideas.

            1.  Use and disuse where parts of the body that are used remain and                                 those that arenít are done away with.

2.       The inheritance of acquired characteristics that said modifications during a lifetime can be passed to offspring.

IV.  The Darwinian Revolution

            A.  Field research helped Darwin frame his view of life

                        1.  Darwin began his voyage on the HMS Beagle in December 1831.

2.       Mission was to chart the poorly known stretches of the South American coastline.

3.       He observed thousands of specimens.

4.       Noted various adaptations like plants were very different from those found in Europe but also varied as to whether they were found in a tropical climate versus a temperate climate.

5.       He also noticed that may of the species that inhabited the Galapagos Islands lived nowhere else.  He focused on the finches that were unique to individual islands.

B.  Darwin focuses on Adaptation

1.  Darwin perceived the origin of the species and adaptation to the environment were very closely related to each other.

2.       A new species could arise from an ancestral form by the gradual accumulation of adaptations to a different environment like a geographical barrier that isolates two populations.

3.       Over a long period of time, these two populations could become dissimilar enough to appear as two separate species.

4.       Darwinís ideas were reiterated by Alfred Wallace working in the East Indies.  The discovery of someone else who believed Darwinís theories catapulted him to write Origin of Species.

C.  Origin of species developed two main points

            1.  Descent with Modification

a.       Darwin believed that all organisms related through descent from some unknown organism and as descendants of that organism moved into various habitats the accumulated modifications that fit them into specific ways of life.

2.  Natural Selection

a.       Natural selection is differential success in reproduction.

b.       It occurs through an interaction between the environment and the variability inherent among the individual organisms making up the population.

c.       The product is the adaptation of populations to their environment.

d.       Organisms must have the ability to overreproduce.

e.       He also noticed that artificial selection was possible for domestication.

D.      Subtleties of Natural Selection

1.  The population is important for evolution.  Remember a population is a group of interbreeding individuals belonging to a particular species and sharing a common geographic area.

2.       A population is the smallest unit that can evolve.

3.       Natural selection involves interactions between individual organisms and their environment but individual do not evolve.

4.       Natural selection can only amplify or diminish heritable variations.

E.  Examples

            1.  Peter and Rosemary Grant studied the beaks of finches on Daphne        Major of the Galapagos Islands.

2.       Noted that the depth of the beak changed with the seasons of wet and dry.  The wetter the season, the smaller the beak and the drier, the larger the beak.

3.       Reinforced the point that natural selection is situational.

V.  Evidence of Evolution

            A.  Evidence of evolution pervades biology

                        1.  Biogeography

a.       The geographical distribution of species.

b.       Many species are indigenous yet closely related to species on the mainland.

2.  The Fossil Record

a.       The succession of the fossil record is compatible with other known types of evidence.

b.       The oldest known fossils are prokaryotes.

c.       Also see a chronological appearance of different classes of vertebrate animals.

d.       Fish to amphibians to reptiles to mammals to birds which is consistent with the fossil record.

e.       Evolutionary transitions should leave signs in the fossil record.

3.  Comparative Anatomy

a.       Descent with modification is evident in anatomical similarities between species groups in the same taxonomic category.

b.       There is some similarities between the forelimbs of humans, cats, whales, bats, and other mammals although the appendages are very different in function.

c.       Similarity in characteristics is called homology and the anatomical signs of evolution are called homologous structures.

d.       This shows the remodeling process from ancestral structures.

e.       Some homologous structures are vestigial organs or structures that have little or no use to the organism now but may have been important in ancestors.

4.  Comparative embryology

a.       Closely related organisms go through similar stages in their embryonic development.

b.       All vertebrates go through a stage where they have gill pouches.

c.       As development progresses, the various vertebrates diverge more and more taking on more distinctive characteristics.

d.       Often used to establish homology among structures.

e.       The ontogeny or development of the individual is a replay of the phylogeny or history of the species.

5.  Molecular biology

a.       Genes and gene products are now studied to reveal similarities.

b.       Molecular biology supports the idea that all forms of life are related to some extent.

c.       Mutations have helped with the changes in the expression of genes to change organisms.

                                    d.  A common genetic code is overwhelming evidence that all life                                             is related and has been passed down since the codeís                                             inception in an early life form.