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Introduction to Evolution (DOC)

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					                                    Introduction to Evolution

                                      Comparative Anatomy


I. Comparisons of anatomical features of different organisms often provides evidence to support
the theory of evolution. Note that much of paleontology is based on comparative anatomy
between extinct and extant organisms

A. Comparative study of anatomical structures within a particular group of organisms often
reveals that they are based upon a common prototype or ground plan which varies with the
organisms particular environment

1. Vertebrate column demonstrates how, within a single phylum, an organ system seems to be
based upon a prototype that is varied simply from class to class. Throughout the vertebrates the
column has become modified within each group, however all share a common ancestor which
had the prototypic column

B. The interpretation of anatomy requires an understanding of homologous versus analogous
structures:

1. Homologous structures - similarity in structure due to common descent, irrespective of the
diverse uses to which they may be put. E.g. vertebrate forearms

a. Adaptive radiation - the progressive modification of a structure to serve many different
purposes. It is attained by mutual fitting of structure, function and environment. Readily apparent
in the forelimbs of mammals which have been modified by changes of proportions, fusion of
parts, or loss of parts., e.g. swimming, digging, running, flying

2. Analogous structures - similarity in structure based on adaptation for the same function, not
common descent. E.g. wings have developed independently in insects, reptiles, birds, and bats

a. Analogous structures are the result of morphological convergence - structures "fitted" for a
particular purpose tend to be similar, regardless of origin. E.g. body shape and flippers in
dolphins, penguins and fish (convergent evolution)

b. All vertebrate eyes are homologous - constructed of identical materials that are used in similar
ways

c. In the Cephalopods (squids, octopi) eyes develop from skin whereas in vertebrates they
develop from the brain, likewise in the former the lens is a crystalline secretion of skin cells
whereas in the latter it is cellular. Eyes are analogous structures in vertebrates and cephalopods

3. Organisms derived from one common ancestor are monophyletic, e.g. all vertebrates are
derived from one ancestor, all mammals from common ancestor
a. phylogeny = the evolutionary history of a group of organisms

4. A classification based on analogous structures, e.g. wings, would be polyphyletic, and
incorrect

C. Vestigial organs are reduced and rudimentary compared to the same complex structure in
other organisms. Vestigial characters, if functional, perform relatively simple, minor, or
inessential functions using structures that were clearly designed for other complex purposes:

1. Human appendix useless yet in other mammals, including primates, it is necessary to aid in
digestion of high cellulose diet

2. Human external ear muscles still present but useless

3. The coccyx (tailbone), is the remnant of a lost tail. All mammals have a tail at one point in
their development; in humans, it is present for a period of 4 weeks, and most prominent at 31-35
days old. Some babies occasionally have tails

4. Human wisdom teeth vestigial compared to other primates

5. Some snakes and dolphins have skeletal limbs, dolphins with fully developed hind limbs have
been found

      Cetacean (whale, dolphins, etc.) embryos initiate hind-limb bud development but the bud
       arrests and degenerates around the fifth gestational week

6. Cave dwelling crayfish have eyestalks yet no eyes, or may be blind, or the eyes are much
reduced and rudimentary

7. Sometimes vestigial organs may be adapted for new uses, e.g. penguin wings can't be used for
flight yet adapted for swimming

8. Yellow flowers of dandelion resemble those of insect-pollinated plants, but seeds develop
without pollinator (apomixis)

9. Kiwi, a flightless bird, has tiny stubby and useless wings

10. Other human vestigial features

				
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posted:12/31/2011
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