Recently on this blog, I have been exploring and examining some of the genomic arguments for common descent. As I have been documenting in recent weeks, while the case for common ancestry — on the face of it — looks mightily strong, closer inspection reveals that the arguments don’t, in fact, stand up under more rigorous scrutiny. In the vast majority of instances, the corroborative data is very carefully cherry picked from the pertinent data set, and the non-congruent evidence is discarded or ignored. In some cases, non-congruent data is rationalised — sometimes plausibly. But then one ought not to think that an ad hoc rationalisation constitutes evidence for said position. As Casey Luskin notes,
…at the end of the day, we must call them what they are: ad hoc rationalizations designed to save a theory that has already been falsified. Because it is taken as an assumption, evolutionists effectively treat common ancestry in an unfalsifiable and unscientific fashion, where any data that contradicts the expectations of common descent is simply explained away via one of the above ad hoc rationalizations. But if we treat common descent as it ought to be treated — as a testable hypothesis — then it contradicts much data.
One popular argument for common descent is the case from the discipline of biogeography — that is, the study of the geographical and historical distribution of species in relation to one another. The argument is based largely around the observation that species are related in accordance with their geographical proximity with respect to one another. One well-known example of this is the concentration of marsupial mammals in Australia and South America. As the Internet encyclopedia, Wikipedia, explains,
The history of marsupials also provides an example of how the theories of evolution and continental drift can be combined to make predictions about what will be found in the fossil record. The earliest marsupial fossils are about 80 million years old and found in North America; by 40 million years ago fossils show that they could be found throughout South America, but there is no evidence of them in Australia, where they now predominate, until about 30 million years ago. The theory of evolution predicts that the Australian marsupials must be descended from the older ones found in the Americas. The theory of continental drift says that between 30 and 40 million years ago South America and Australia were still part of the Southern hemisphere super continent of Gondwana and that they were connected by land that is now part of Antarctica. Therefore combining the two theories scientists predicted that marsupials migrated from what is now South America across what is now Antarctica to what is now Australia between 40 and 30 million years ago. This hypothesis led paleontologists to Antarctica to look for marsupial fossils of the appropriate age. After years of searching they found, starting in 1982, fossils on Seymour Island off the coast of the Antarctic Peninsula of more than a dozen marsupial species that lived 35-40 million years ago.
I must confess that I have my doubts with regards the efficacy of this argument in establishing universal common descent, or even common descent of all marsupial mammals. After all, as noted in the textbook Explore Evolution, marsupials are not even restricted to the southern continents of Australia and South America. Some marsupials live in the northern hemisphere, and there is even some paleontological evidence for the oldest marsupials inhabiting China!
But be that as it may. As with the majority of arguments favouring common descent, the argument from biogeography is loaded with carefully cherry-picked data.
I have recently been enjoying reading a fascinating 2005 review paper from Trends in Ecology and Evolution. The paper’s author (Alan de Queiroz) notes several major conundrums for those of an evolutionary persuasion with regards biogeography. The author notes that,
‘Batrachians (frogs, toads, newts)’, Darwin noted in The Origin of Species, ‘have never been found on any of the many islands with which the great oceans are studded.’ He explained this absence by the fact that amphibians are quickly killed by seawater and are thus unlikely to cross oceans successfully. No biogeographer doubts that amphibians and certain other organisms (e.g. most terrestrial mammals, flightless birds) are especially poor oceanic dispersers. However, some recent studies show that it is unsafe to assume that such organisms never colonize new areas by crossing ocean barriers.
A striking example concerns two mantellid frog species found on Mayotte, an island of the Comoros archipelago some 300 km west of Madagascar. The two species had been described as conspecific with taxa on Madagascar (where nearly all other mantellids are found) and were assumed to have been introduced. However, morphology and DNA sequences indicate that the two Mayotte taxa are distinct new species and, therefore, are natural endemics. The Comoros are volcanic and have never been attached to other landmasses; thus, the results strongly imply origins by natural, overwater dispersal. Furthermore, the two species are not closely related within the Mantellidae, indicating two independent dispersal events.
Another case involves the carnivores and lemurs of Madagascar, medium-sized mammals that are considered poor oceanic dispersers. Yoder et al. found through molecular dating analyses that both groups diverged from African mainland relatives long after the separation of Madagascar from Africa. The estimated divergence dates also do not match the hypothesized existence of a Cenozoic land bridge between Africa and Madagascar. Thus, both groups seem to have reached Madagascar by oceanic dispersal, perhaps facilitated by the ability to go into torpor.
Other examples of unexpected oceanic dispersal include monkeys from Africa to South America, flightless insects from New Zealand to the Chatham Islands, multiple dispersals by chameleons in the Indian Ocean, several other amphibian cases, and, more controversially, flightless ratite birds to New Zealand. Although Darwin apparently was wrong in thinking that amphibians never cross saltwater, these cases reinforce a general message of the great evolutionist: given enough time, many things that seem unlikely can happen.
So, when the biogeographical data does not accord with the predictions and expectations made by common descent, one always has ‘oceanic dispersal’ as an ad hoc fudge factor — including the rather remarkable claim that Monkeys made it across the Atlantic from Africa to South America! As Casey Luskin notes here, molecular studies claim that the South American monkeys diverged from the African monkeys around 35 million years ago. But Africa became an isolated island continent around 80 million years ago!
Sound implausible? Well, we all know that “given enough time, many things that seem unlikely can happen”, right?