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Tracing Phylogeny

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I.  The fossil record documents macroevolution

A.       How fossils form

1.        A fossil is any preserved remnant or impression left by an organism that lived in the past.

2.        Sedimentary rocks are the riches sources of fossils.

a.        Sand and silt weathered and eroded from the land are carried by rivers to seas and swamps where the particles settle to the bottom.  Deposits pile up and compress the older sediments below into rock.

3.        The organic substances of a dead organism buried in sediments usually decay rapidly.

4.        Hard parts that are rich in minerals may remain as fossils.

5.        Many of these relics are hardened even more and preserved by a process called petrifaction.

6.        The most common fossilized plant material is pollen because it has a hard organic case that resists degradation.

7.        Casts called trace fossils form in footprints, animal burrows, or other impressions left in sediments by the activities of animals.

B.       Limitations of the Fossil Record

1.        The discovery of a fossil is the culmination of a sequence of improbable coincidences.

2.        First, the organism had to die in the right place at the right time.

3.        Second, the rock layer containing the fossil had to escape geological process that destroy or severely distort rocks suck as erosion, pressure from superimposed strata or the melting of rocks that occurs at some location.

4.        Third, the fossil must later be exposed.

5.        Fourth is the remote chance that someone will find the fossil.

6.        A substantial fraction of species that have lived probably left no fossils, most fossils that formed have been destroyed and only a fraction of the existing fossils has been discovered.

II.  Paleontologists use a variety of methods to date fossils

A.       Relative Dating

1.        At any particular location, sedimentation is not continuous but occurs in intervals when the sea level changes or lakes and swamps dry up and refill.

2.        The rock forms in layers or strata.

3.        The fossils in each layer are a local sampling of the organisms that existed at the time that sediment was deposited.

4.        Younger sediments are superimposed upon older ones.

5.        The strata at one location can often be correlated with strata at another location by the presence of similar fossils known as index fossils.

6.        Geologists have established a geological time scale with a consistent sequence of historical periods.

7.        These periods are grouped into four eras:  Precambrian, Paleozoic, Mesozoic and Cenozoic

a.        Each era represents a distinct age in the history of Earth and its life.

b.        The boundaries are marked in the fossil record by explosive radiations of many new forms of life following mass extinctions.

c.        The boundaries between periods within each era also mark major transitions in the forms of life fossilized in rocks.

8.        The periods within each era are further subdivided into finer intervals called epochs.

9.        The record of the rocks is a serial that chronicles the relative ages of fossils.

10.     It does not tell us the absolute ages of the embedded fossils.

B.       Absolute Dating

1.        Radiometric dating is the method most often used to determine the ages of rocks and fossils on a scale of absolute time.

2.        Fossils contain isotopes of elements that accumulated in the organisms when they were alive.

3.        Because each radioactive isotope has a fixed rate of decay, it can be used to date a specimen.

4.        An isotopeís half-life is the number of years it takes for 50% of the original sample to decay and is unaffected by temperature, pressure and other environmental variables.

5.        Another method is using amino acids.

6.        Amino acids exist in two isomers with either left handed or right handed symmetry. 

7.        Organisms synthesize only left handed amino acids which are incorporated into proteins.

8.        After an organism dies, its population of left handed amino acids is slowly converted resulting in a mixture of left and right handed amino acids.  (L-amino acids = left handed and D-amino acids = right handed)

9.        Knowing the rate at which this chemical conversion takes place, called racemization, we can determine how long the organism has been dead.

10.     Racemization is temperature sensitive.

III.  Some Evolutionary novelties are modified version of older structures.

A.       One mechanism is the gradual refinement of existing structures for new function like the comparison of bird wings to the forelimbs of dinosaurs and modern reptiles.

B.       Most biological structures have an evolutionary plasticity that makes alternative functions possible.

C.       Preadaptation for a structure that evolved in one context and became co-opted for another function is an explaination.

IV.  Genes that control development play a major role in evolutionary novelty.

A.       The evolution of complex structures from the antecedents requires so much remodeling that changes are probably involved at a large number of gene loci.

B.       Genes that program development control the rate, timing, and spatial pattern of changes in an organismís form as it is transfigured from a zygote into an adult.

C.       Allometric growth is the difference in the relative rates of growth of various parts of the body and helps shape an organism.

D.       Change these relative rates of growth even slightly and you change the adult form substantially.

E.        Genetic changes can also alter the timing of developmental events like the sequence in which different body parts start and stop developing.