Homology
From EvoWiki
Homology refers to the fundamental similarities in genetic sequence, anatomical structure, behavior or other character of two taxa that result from common descent. Homology is distinguished from homoplasies, where similarities are not the result of common descent, such as analogy.
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Overview
Homology is generally recognized in two steps, "Remane's criteria", or the similarity criteria, and the congruence criteria.
The first demands that structural similarities outweigh functional necessity in a multitude of different ways, including general similarity, ontogeny, and body position relative to other elements. When these similarities do not outweigh functional necessity, the resemblance is more likely an analogy.
But these considerations alone are often times not sufficient. In certain circumstances, analogous traits can pass the tests, or they have become so dissimilar from millions of years of evolution that their resemblance is no longer clear. The second criteria, congruence, handles this by placing potential homologies in their phylogenetic context, testing them against other similarities. For example, while some have argued that the furculae of non-avian theropod dinosaurs and birds do not meet Remane's criteria (and accepting this for the sake of argument), the hundreds of other shared characters suggest that the two groups are closely related, and the structures the same. The hypothesis of homology of the furcula is congruent with the rest of the character data.
Examples
Examples of homology are widespread throughout biology. Some key examples are given below:
Pentadactyl limbs
A classic example of homology are the pentadactyl (having five digits) forelimbs of tetrapods (see diagram at left). The vast majority of tetrapods have the same forelimb bone arrangement: one arm bone (humerus), two forearm bones (radius and ulna), a number of wrist (carpal) bones and five metacarpals (located in the palm) joined to five sets of phalanges (bones of the fingers), but these bones perform many different functions, including digging, grasping, swimming and flying. There are, of course, variations to this basic plan, such as the fused forearm bones of order Anura (frogs and toads) and the loss of one digit in birds and some amphibians (eg salamander).
If the forelimb had evolved independently (or, for that matter, if it was designed separately), one would expect the forelimb to be arranged differently for different modes of use, but instead the same pattern is seen, indicating common descent.
Serial Homology
Homology can be present among different features of the same organism, a circumstance called serial homology. Many examples are trivial, like hairs or scales or leaves, but there are important nontrivial examples. A classic example is the limbs of arthropods; they have a similar overall architecture, but they get specialized in a variety of different directions. Likewise, flower parts are serially homologous to leaves. Serial homology is supported by the occurrence of "homeotic mutations", in which one body part develops like another. Thus, fruit flies can have antennae and mouthparts developing like legs and their third thoracic segments developing like their second ones:
http://www.ucl.ac.uk/~ucbzwdr/teaching/b250-99/flyheads.jpg
(wild type, antennapedia, antennapedia with proboscipedia)
http://www.ucl.ac.uk/~ucbzwdr/teaching/b250-99/bithorax.jpg
(bithorax and postbithorax; normal flies have their third-segment wings reduced to much-smaller halteres)
(Source: Hopeful Monsters)
The famous homeobox or Hox genes were discovered by the study of these and other such mutations; they specify front-to-rear identity, and mutations in them or changes in their expression patterns produce these sorts of misdevelopment.
Molecular Examples
The concept of homology extends to the molecular realm, where genes and proteins from different species often show much more sequence similarity than their functions would require -- with these similarity values following treelike patterns that often closely parallel traditional phylogenies. Serial homology also exists among molecules, and is easily interpreted as a result of gene duplications. A classic example is in the jawed-vertebrate hemoglobin molecule, which is a tetramer composed of two alpha and two beta chains arranged in a square pattern, with order a-b-a-b. Alpha-beta similarities are less than alpha-alpha or beta-beta similarities between different species, though their similarity is still recognizable; this suggests that some ancestral globin gene was duplicated in some early jawless or ancestral jawed fish.
Deep Homology
A remarkable result of molecular investigations has been the discovery of "deep homology" -- mechanisms of growth and development are widely conserved across the animal kingdom, despite numerous outward differences. Hox genes have been discovered in much of the animal kingdom, and when their expression patterns and functions have been explored, they have been found to specify front-to-rear identity in fruit-fly manner, though with numerous differences in detail. Likewise, the gene Pax6 starts eye development across arthropods, vertebrates, and polychaete worms, even though the resulting eyes look very different.
A particularly dramatic example of deep homology has been the revival of Geoffroy St. Hilaire's hypothesis of dorsoventral inversion, which states that the dorsal part of vertebrates is homologous to the ventral part of arthropods, and vice versa:
- Vertebrates: dorsal, CNS, gut, heart, ventral
- Arthropods: ventral, CNS, gut, heart, dorsal
This hypothesis was widely criticized for several reasons; the vertebrate CNS is tube-shaped, while the arthropod CNS is ladder-shaped, the mouth would have to change sides, etc. This hypothesis would be revived and shot down every 20 years, but recent molecular-level discoveries have strongly established it, presumably ending this cycle. The genes responsible for establishing CNS-side identity are homologous, as are the genes that establish the heart-side identity; this has been demonstrated by injecting the fruit-fly version into a frog embryo, and vice versa.
Related Creationist Claims
Some creationists, such as Jonathan Wells, claim that the case for homology is circular, arguing that the evidence for homology requires assuming evolution a priori. However, homology had been recognized by pre-Darwinian biologists, some of whom had argued that homology is a result of features being copies of some transcendent Platonic "archetypes".
Acknowledgments
JGK/LP/GFA
