A number of advocates of intelligent design have written variations on the theme that convergence is a problem for evolution. I aim to show why this argument is daft.
First of all, what is convergence? Definitions differ, and I’m not going to get into an extended discussion of the differences. A definition that will serve well enough is Anurag Agrawal’s, “the independent evolution of similar phenotypes.” A phenotype, and this will be important, can refer to a single trait, multiple traits, or the entire set of traits expressed by an organism. Green-eyed is a phenotype. A calico pattern of fur color is a phenotype. All of the traits that make up a particular cat are also a phenotype. A phenotype can describe a trait (or set of traits) of an individual or of a species, so just as being 5’10” tall is a phenotype, so is being bipedal.
Convergence typically refers to the latter kind of phenotype, those that characterize a species. So if, for example, seasonal changes in coat color have independently evolved in a bird, a lagomorph, a mustelid, and a canid, that’s an example of convergence of a single trait.
If birds, bats, and pterosaurs independently evolved powered flight, that’s convergence of a whole suite of traits: wings, flight muscles, the behavior of flying, and so on. Convergence is often taken to imply that the species that have independently evolved the phenotype are distantly related, in contrast to parallelism, in which the species are closely enough related that the genetic or developmental basis of the shared phenotype is likely to be similar. For some authors, it’s the evolutionary distance that distinguishes convergence from parallelism; for others, it’s the underlying genetics. For the purpose of this discussion, I’ll do my best to use the same definition of convergence as that of the authors I’m criticizing, whether or not they explicitly define it.
Second, let’s establish that I’m not setting up a straw man; intelligent design advocates really do claim that convergence contradicts/falsifies/casts doubt on common descent or even on evolution more generally. Lest you think I’m exaggerating, I’ll quote a few examples (emphasis added). Here’s Granville Sewell:
This phenomenon of convergence is so ubiquitous that it has become a major problem for evolutionists.
The problem is it destroys the tree of life! —2018-08-06
…convergence violates the evolutionary pattern. Regardless of adaptation versus constraint explanations, and any other mechanisms evolutionists can or will imagine, the basic fact remains: a fundamental evidence and prediction of evolution is falsified. —2017-05-25
…it is an epic myth, willingly perpetuated by evolutionary biologists, that the similarities between organisms mostly fall in a hierarchic pattern of nested groups and thus suggest common ancestry and indicate phylogenetic relationship. In reality this claim is contradicted by a flood of incongruences and reticulate patterns that shed doubt on fundamental paradigms of evolutionary biology like the notions of homology and common descent. —2018-04-13
…similarity not stemming from genealogy – i.e., convergence – is just what evolutionary theory should not expect. —2017-06-26
Lee M. Spetner:
…the concept of convergence was invented to explain away many discrepancies in the phylogenetic trees, examples of convergence are inherently contradictions to the [Modern Synthesis]. —2016-09-26
and my favorite, from Casey Luskin:
Convergent Evolution Challenges Darwinism and Destroys the Logic Behind Common Ancestry —2015-02-09
These are just a few cherry-picked examples; convergence is a drum that various advocates of intelligent design have been beating for a long time, and at a fast tempo. It will thus take me some time to take these arguments apart.
I promised to show why the argument that convergence is a problem for evolution is daft, and I’ll get to that. For now, though, I’m going to restrict myself to debunking subtle and not-so-subtle misrepresentations of what evolutionary biologists think, and have historically thought, about convergence.
If your only source for learning about evolutionary biology were the misleadingly-named Evolution News & Science Today (formerly Evolution News & Views), you could be forgiven for thinking that convergence is a new discovery. For example, Günter Bechly says
…independent (convergent) origin of similar structures is a very common phenomenon in the history of life, which is quite unexpected if Darwinian evolution would be true. —2018-01-01
and even more recently,
When I studied in the 1980s at the University of Tübingen, where the founder of phylogenetic systematics, Professor Willi Hennig, was teaching a first generation of cladists, we still all thought that such homoplasies are the exceptions to the rule, usually restricted to simple or poorly known characters. Since then the situation has profoundly changed. Homoplasy is now recognized as a ubiquitous phenomenon… —2018-04-13
Homoplasy is synonymous with convergence, or close enough for our purposes. Lee M. Spetner:
Convergent evolution is the Darwinists’ lollapalooza. They made it up to keep their phylogenetic tree from falling apart, but they can’t say how convergence happens. –As quoted by Casey Luskin, 2014-10-19
“The Darwinists’ lollapalooza’? What does that even mean?
Granville Sewell says
This phenomenon of convergence is so ubiquitous that it has become a major problem for evolutionists. —2018-08-06
So convergence is ‘unexpected’, it’s ‘made up’ or ‘invented’ in response to failed phylogenetic trees, it’s ‘now’ (since the 1980s) recognized as ubiquitous, and it ‘has become’ a major problem. The impression I get from all of this is that convergence has only recently been recognized, or at least only recently recognized as common.
The truth, though, is that biologists have long known that convergence is widespread and common. Darwin was well aware of the phenomenon, and he wrote about it extensively in The Origin of Species:
For animals, belonging to two most distinct lines of descent, may have become adapted to similar conditions, and thus have assumed a close external resemblance…Numerous cases could be given of striking resemblances in quite distinct beings between single parts or organs, which have been adapted for the same functions. –p. 374, 6th edition
The Origin discusses convergence of jaw morphology between thylacines and dogs; of color patterns in “no less than ten” genera of butterflies; of electric organs in fishes; of luminous organs in insects; of “a mass of pollen-grains, borne on a foot-stalk with an adhesive gland” between two distantly related genera of plants; of eyes between vertebrates and cephalopods; of air-breathing apparatus between separate lineages of terrestrial crustaceans; of hair claspers among distinct lineages of mites; of wings among birds, bats, and insects; of body shape and fin-like anterior limbs among dugongs, whales, and fishes; and of general appearance among mice, shrews, and the marsupial Antechinus.
Suggestions that Darwin, or more recent evolutionary biologists, were unaware of convergence, thought it was rare, “made it up”, or lack an explanation for it are false.
Agrawal, A. A. 2017. Toward a predictive framework for convergent evolution: integrating natural history, genetic mechanisms, and consequences for the diversity of life. Am. Nat. 190:S1–S12. DOI: 10.1086/692111
Zimova, M., K. Hackländer, J. M. Good, J. Melo-Ferreira, P. C. Alves, and L. S. Mills. 2018. Function and underlying mechanisms of seasonal colour moulting in mammals and birds: what keeps them changing in a warming world? Biol. Rev. 93:1478–1498. DOI: 10.1111/brv.12405