The concept of homology is another of Jonathan Well's 'icons of evolution' - ideas that he wrongly labels as 'key' to teaching evolution, and then describes as incorrect, misleading, or out-of-date. Let's see what he has to say about homology - & why he's wrong.
HOMOLOGY. Why do textbooks define homology as similarity due to common ancestry, then claim that it is evidence for common ancestry -- a circular argument masquerading as scientific evidence?
Here's what Alan Gishlick has to say about homology:
Homology is a specific explanation of similarity of form... Similarities can often be explained by common descent; features are considered homologous if they are shown to be inherited from a current ancestor. For example, although the arms of four-limbed vertebrates externally appear quite different, all have the same basic underlying skeletal and muscular pattern. Such shared patterns are best explained by the inference that they were inherited from a common ancestor that also had this pattern. Proposed homologies are evaluated using comparative anatomy, genetics, development, and behaviour.
In other words, homologies are diagnosed by assessing evidence from a range of sources. A biologist would perhaps recognise a possible homology between two structures because those structures have similar forms, or are found in the same place in the two organisms. But they would then have to test that hypothesis of homology by comparing the feature across many groups, lookng for patterns of form, function, development, biochemistry, and presence and absence (Gishlick, 2008). The fact that homology is tested, not simply assumed, means that Wells is again wrong in his claim that the statement that homologous featues are evidence of common descent is a circular one.
A.D. Gishlick Icons of evolution? Why much of what Jonathan Wells writes about evolution is wrong. Retrieved from the National Centre for Science Education website on August 25 2008)