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
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. Humans have tailbones and some babies occasionally have tails
4. Human wisdom teeth vestigial compared to other primates
5. Some snakes have skeletal limbs
6. Cave dwelling crayfish have eyestalks yet no eyes
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
II. Additional readings