Fact-Checking Wikipedia on Common Descent: The Evidence from Biogeographical Distribution
We have now reached the fourth part of my series on Wikipedia and the evidence for common descent. In previous entries, I discussed Wikipedia's arguments for common descent based on comparative physiology / biochemistry, comparative anatomy, and paleontology. Now I am going to address the arguments from biogeographical distribution. Biogeography is essentially the study of the geographical and historical distribution of species in relation to one another. The argument holds that species are related in accordance with their geographical proximity to one another.
One well-known example is the concentration of marsupial mammals in Australia and South America. Marsupial fossils, dating to around 80 million years ago, have been found in North America. Fossils of marsupial mammals in South America have been found dating to 40 million years ago. The earliest evidence for the presence of marsupials in their current primary habitat, Australia, dates to about 30 million years ago. Between 30 and 40 million years ago, Australia and South America were connected by a continental land bridge, which is now Antarctica, to form the supercontinent known as Gondwana. Therefore, if evolution is true, we are told, we ought to find evidence for marsupial migration from South America to Australia via Antarctica. And, indeed, fossils of various species of marsupial, dating to 35-40 million years ago, have been identified on Antarctica.
I must confess that I have doubts about the efficacy of this argument in establishing universal common descent. Phenomena like island biogeography or "ring species" may well demonstrate limited common ancestry. In the case described above, it does not even compel the conclusion of the common descent of all marsupial mammals. For one thing, as noted by this article in National Geographic, there is even some paleontological evidence for very old lines of marsupials inhabiting China! Very ancient -- indeed, the oldest European -- fossil marsupials have been discovered in France. In fact, marsupial fossils have now been identified on every continent.
Wikipedia lists several other notable examples of island and continental biogeography. The main shortcoming of the argument lies in its failure to take into account all the evidence. In 2005, an interesting paper was published in the Journal of Ecology and Evolution by Alan de Queiroz (blogged about previously by ENV's Casey Luskin here), documenting several major biogeographical conundrums. The author observed,
"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 fit with the predictions and expectations of common descent, one always has "oceanic dispersal" at the ready to serve 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 plate tectonic history shows that the continental breakup that separated South America from Africa took place between 100 and 120 million years ago and that South America was an isolated island continent from at least about 80 million years ago until approximately 3.5 million years ago. 35 million years ago, at the time of the New World/Old World monkey divergence, Africa was not significantly closer to South America than it is today.
Casey Luskin also reports on various other examples of biogeographic non-congruence:
When proponents of neo-Darwinism "speculate" about the "luck" and "chance" needed to explain this "amazing" phenomenon and "challenging" biogeographical data, it's clear that they are lacking reasonable explanations. Yet rafting or other means of "oceanic dispersal" have been suggested to solve a number of other biogeographical conundrums that challenge neo-Darwinism, including:
- Lizards reaching South America
- Large caviomorph rodents reaching South America
- Bees arriving in Madagascar
- Lemurs arriving in Madagascar
- The arrival of other mammals in Madagascar, including the Tenrecidae (hedgehoglike insectivorous mammals), aardvarks, the hippopotamus, and the Viverridae (cat-sized carnivorous mammals)
- Dispersal of salamanders across the western end of the Mediterranean
- Dispersal of certain lizards across the western end of the Mediterranean
- The origin of certain lizards in Cuba
- The appearance of elephant fossils on "many islands," which are said to have arrived by swimming
- Dispersal of freshwater frogs across oceanic island chains
- Certain frogs reaching Madagascar
- The colonization of Anguilla by green iguanas
- Appearance of certain South American insects
- Dispersals of chameleons across the Indian Ocean
- Origin of certain insects in Caribbean islands
- The origin of mantellid frogs found on the island of Mayotte in the Comoros archipelago, despite the fact that "[a]mphibians are thought to be unable to disperse over ocean barriers because they do not tolerate the osmotic stress of salt water"
- The spread of flightless insects to the Chatham Islands
- The origin of indigenous gekkos in South America
- Origin of crocodile distributions
- The appearance of sloths in South America
- The origin of a group of Australian rodents
- The appearance of land mammals of the Mediterranean islands (also suggesting that "Hippos, elephants, and giant deer reached the islands by swimming")
- The origin of various land reptiles in Western Samoa
- The presence of Crotalus rattlesnakes in Baja California
See Casey's original article for a full listing of citations and references for those examples.
In addition to these cases, there is a plethora of other biogeographic anomalies and conundrums that cannot readily be explained by the nested hierarchical pattern predicted by common descent. Examples include certain fauna of central and southern Africa, which are more closely related to that of southern Asia than that of northern Africa; and crowberries which are only found in regions in the far north of the northern hemisphere and regions in the far south of the southern hemisphere; and flora found in Madagascar which bears similarity to those found in Indonesia. One particularly striking example is the discovery of what appears to be a placental fossil in Australia, dating to 120 million years ago. According to previous reckoning, placental mammals evolved in the northern hemisphere, and only appeared in Australia about five million years ago. This has prompted the evolutionary rationalization that these placental mammals, having first evolved in the southern hemisphere, migrated into the northern and subsequently became extinct in the southern. But such post hoc solutions can be invoked to explain any observation.
Here's the bottom line: While the argument from biogeography, at best, only demonstrates limited levels of common ancestry, and also the occurrence of speciation, the force of this argument is substantially undercut by mitigating adverse evidence such as the biogeographic anomalies to which I alluded. While one can account for some of those conundrums by invoking post hoc rationalizations such as convergence, oceanic dispersal, and unparsimonious extinctions, such anomalies significantly weaken the value of biogeographical distribution as positive evidence for descent with modification. In the next post, I shall consider Wikipedia's discussion of the "evidence from observed natural selection."