Homology and Analogy

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The division is based on a distinction between similarity due to common ancestry, or homology, and resemblance which is due solely to similarity of function, called analogy. An example is the forelimbs of humans, horses, whales and birds which are judged homologous because ‘they are all constructed on the same pattern, and include similar bones in the same relative positions because these are all derived from the same ancestral bones. The wings of birds and insects, on the other hand, are analogous: they serve the same purpose, but do not constitute modified versions of a structure present in a common ancestor. The wings of birds and bats are homologous in skeletal structure because of descent from the forelimb of a common reptilian ancestor; but they are analogous in terms of their modification for flight—feathers in birds, skin membranes in bats.’

In other words, if a design similarity supports evolutionary assumptions, it is listed as an homology and is accepted as evidence for evolution. Conversely, if a design similarity does not support evolution, it is called analogy, and the conclusion is drawn that the similarity exists because a certain design is highly functional for a specific body part, and not because of a common ancestor. Many analogous structures are assumed to exist due to convergent evolution, which is defined as the separate evolution of similar structures because of similar environmental demands. Convergent evolution also is used to explain similar structures that have formed from different embryo structures or precursors.

Many examples of Homology are actually better explained by Analogy, and the resemblance that exists is often due to similarity of function and/or design constraints. The forelimbs of humans, whales and birds are similar because they serve similar functions and have similar design constraints. The conclusion that two homologous bones are similar because they are putatively ‘derived from the same ancestral bones’ (as Barr claims) is not based on direct evidence but instead on a priori conclusions demanded by macroevolution. Jones concluded that “ … the evolutionist argument from homology lacks scientific content. This particular lack has very serious implications; it strikes at the root of all attempts by evolutionists to give homology an objective basis and distinguish homology (similarities due to descent) from analogy (similarities not due to descent). The only way they can recognize analogous variation, especially when due to convergent evolution is by criteria (e.g. genetic or embryological) which we now know do not hold for organs of "unquestionable" homology. The evolutionist concept of homology is now shown to be entirely subjective.”

Stephen J. Gould suggested that ‘the central task of evolutionary biology is … the separation of homologous from analogous likeness’, and then emphasized that ‘homology is similarity due to descent from a common ancestor, period’. The problem with this definition is that without direct knowledge we cannot know ancestry. In answer to the question ‘Can we identify fossil ancestors of species alive today?’, University of Michigan Professor Mark Siddall contends that this is impossible and that the use of stratigraphic data when assembling phylogenies must be based on speculation.

Huxley understood as far back as 1870 that when dealing with fossils, which are the only evidence we have of past life, one cannot distinguish uncles and nephews from fathers and sons. Among the many reasons ancestors cannot be distinguished from sister taxa, as noted by Siddall and others, is that there can be no positive evidence of ancestry, only inferences. Lack of evidence can only allow it as a possibility or an ad hoc postulate.

Although many similarities exist in almost all animal structures, structural variations are the norm. Often the variations found in the animal world seem to exist solely to produce variety, and not for the purpose of conferring a survival advantage. Some examples in humans are Attached earlobes, Tongue rolling, Hitchhiker’ thumb, Bent little finger, Interlacing fingers, Widow’ peak.

No biological or logical requirement exists to vary the design of bones, muscles and nerves needlessly in every living form beyond what is necessary to adapt the animal to its environment. Although variety is universal in the natural world, variety that interferes with the life process or an animal’ survival usually is avoided in animal design. Design constraints severely limit the possible variations in an animal’ anatomy, and excess deviation from the ideal can interfere with the animal’ ability to survive.

The many similarities that exist among members of the animal kingdom is the result of the fact that a single designer created the basic kinds of living ‘systems’, then specially modified each type of life to enable it to survive in its unique environmental niche. Examples of major environments for which organisms must be designed include the air, ground and water. Structures that serve similar purposes under similar conditions and that are nourished by similar foods ought to possess similarity in both design and function. This is illustrated in a critique of Berra’ Corvette analogy cited previously:

“ … Berra’ primary purpose is to show that living organisms are the result of naturalistic evolution rather than intelligent design. Structural similarities among automobiles, however, even similarities between older and newer models (which Berra calls "descent with modification") are due to construction according to pre-existing patterns, i.e., to design. Ironically, therefore, Berra’ analogy shows that even striking similarities are not sufficient to exclude design-based explanations. In order to demonstrate naturalistic evolution, it is necessary to show that the mechanism by which organisms are constructed (unlike the mechanism by which automobiles are constructed) does not involve design.”