Implications of Genetic Convergent Evolution for Common Descent
In the previous post, I discussed a recent paper in Trends in Genetics, "Causes and evolutionary significance of genetic convergence," which notes that that genetic convergence is not uncommon, even though only a "restricted number of substitutions" at the genetic level can create novel phenotypic traits. This data not only shows that functional genotypes are rare, but it also poses a much deeper problem for evolutionary thinking--one that challenges the very basis for constructing phylogenetic trees.
The main assumption behind evolutionary trees is that functional genetic similarity implies inheritance from a common ancestor. But "convergent" genetic evolution shows that there are many instances where functional similarity is not the result of inheritance from a common ancestor. So when we find functional genetic similarity, are we to assume that it represents a homologous DNA sequence, or a convergently similarity sequence? This poses great difficulties for those who wish to build evolutionary trees under the assumption of common descent.
I am hardly the first to recognize this. Evolutionary paleoecologist Simon Conway Morris--who is not an intelligent design (ID) proponent--quite explicitly observes that convergence poses a major difficulty for the construction of phylogenetic trees:
I believe the topic of convergence is important for two main reasons. One is widely acknowledged, if as often subject to procrustean procedures of accommodation. It concerns phylogeny, with the obvious circularity of two questions: do we trust our phylogeny and thereby define convergence (which everyone does), or do we trust our characters to be convergent (for whatever reason) and define our phylogeny? As phylogeny depends on characters, the two questions are inseparable. ... Even so, no phylogeny is free of its convergences, and it is often the case that a biologist believes a phylogeny because in his or her view certain convergences would be too incredible to be true. ...Another good example comes from a recent treatise published by Harvard University Press which states that convergent evolution causes "difficulties" for building trees:
During my time in the libraries I have been particularly struck by the adjectives that accompany descriptions of evolutionary convergence. Words like, 'remarkable', 'striking', 'extraordinary', or even 'astonishing' and 'uncanny' are common place...the frequency of adjectival surprise associated with descriptions of convergence suggests there is almost a feeling of unease in these similarities. Indeed, I strongly suspect that some of these biologists sense the ghost of teleology looking over their shoulders.
(Simon Conway Morris, Life's Solution: Inevitable Humans in a Lonely Universe, pp. 127-128 (Cambridge University Press, 2003).)
Cladistics can run into difficulties in its application because not all character states are necessarily homologous. Certain resemblances are convergent -- that is, the result of independent evolution. We cannot always detect these convergences immediately, and their presence may contradict other similarities, "true homologies" yet to be recognized. Thus, we are obliged to assume at first that, for each character, similar states are homologous, despite knowing that there may be convergence among them.Likewise, a paper in Annual Review of Ecology and Systematics explains the "difficulties" facing molecular phylogenies as a result of convergence:
(Guillaume Lecointre & Herv� Le Guyader, The Tree of Life: A Phylogenetic Classification, p. 16 (Harvard University Press, 2006).)
Given the difficulties associated with alignment and with the establishing the conditions of consistency and convergence, it is clear that molecular phylogenies should not be accepted uncritically as accurate representations of the degree of relatedness between organisms.Thus, the textbook Explore Evolution says the following:
(Rudolph Raff, Charles R. Marshall, and James M. Turbeville, "Using DNA sequences to unravel the Cambrian radiation of the animal phyla," Annual Review of Ecology and Systematics, Vol. 25:351-375 (1994).)
Convergence is a deeply intriguing mystery, given how complex some of the structures are. Some scientists are skeptical that an undirected process like natural selection and mutation would have stumbled upon the same complex structure many different times. Advocates of neo-Darwinism, on the other hand, think convergent structures simply show that natural selection can produce functional innovations more than once. For other scientists, the phenomenon of convergence raises doubts about how significant homology really is as evidence for Common Descent. Convergence, by definition, affirms that similar structures do not necessarily point to common ancestry. Even neo-Darwinists acknowledge this. But if similar features can point to having a common ancestor--and to not having a common ancestor--how much does "homology" really tell us about the history of life? (p. 48)When building evolutionary trees, evolutionists assume that functional genetic similarity is the result of inheritance from a common ancestor. Except for when it isn't. And when the data doesn't fit their assumptions, evolutionists explain it away as the result of "convergence."
Using this methodology, one can explain virtually any dataset. Is there a way to falsify common descent, even in the face of convergent genetic similarity? If convergent genetic evolution is common, how does one know if their tree is based upon homologous sequences or convergent ones? Critics like me see the logic underlying evolutionary trees to be methodologically inconsistent, unpersuasive, and ultimately arbitrary.