(Cartoon Courtesy of Pete Chadwell)
In its response to the chapter on biogeography in the supplementary textbook Explore Evolution: The Arguments For and Against Neo-Darwinism (EE), the National Center for Science Education (NCSE) asserts that EE “mangles the tiny fraction of biogeography covered.”* My response to the NCSE’s arguments, “The NCSE’s Biogeographic Conundrums: A Defense of Explore Evolution‘s Treatment of Biogeography,” notes that “[t]he NCSE’s approach is to cherrypick examples to support their arguments for universal common descent, but a large number of ‘biogeographic conundrums’ that challenge neo-Darwinism could be discussed.”
For example, in its response regarding marsupials, the NCSE admits, “If the [North American] opossum truly had roots in Australia, it would indeed be a biogeographic conundrum.” Since North American opossums are not descended from Australian “possums,” their high morphological similarity dictates to neo-Darwinian evolutionists that this must be another case of extreme convergent evolution that challenges the methodology by which neo-Darwinism infers homology and common descent.
But what if North American opossums were descended from Australian possums? Why does the NCSE observe that this hypothetical situation would pose a “biogeographic conundrum?” The NCSE says this because there would be no route by which Australian possums could have migrated to North America. The NCSE’s reasoning here is sound: they presume that if organisms in Locale B are descended from organisms in Locale A, then there must have been some migration route by which organisms could migrate from A to B. If there is no such route, then we’re presented with, in the NCSE’s own words, a “biogeographic conundrum.” Using such reasoning, the NCSE then argues that marsupials and other groups have biogeographic histories that are congruent with the tectonic history of islands and continents, thus allegedly supporting common descent:
The same pattern of diversification and migration seen in marsupials can also be seen in other groups of plants and animals. That consistency between biogeographic and evolutionary patterns provides important evidence about the continuity of the processes driving the evolution and diversification of all life. This continuity is what would be expected of a pattern of common descent, and is not what would be expected with the creationist orchard scheme.
With marsupials, the NCSE claims that the “continuity” of geography and evolution predicts that there will always be some land bridge or migratory pathway which terrestrial organisms can follow. This was claimed to allegedly show “consistency between biogeographic and evolutionary patterns” that demonstrates “what would be expected of a pattern of common descent.” Ignoring the NCSE’s continued inappropriate usage of the “creationist” label, their claim is simply not true, for there are many examples of terrestrial organisms existing and appearing in locations where no land-based migratory route is apparent. The NCSE’s approach is to cherrypick examples to support their arguments for universal common descent, but a large number of “biogeographic conundrums” that challenge neo-Darwinism could be discussed.
Traditional evolutionary explanations of biogeography fail when terrestrial (or freshwater) organisms appear on an island or continent but there is no standard migratory mechanism for them to have arrived there from some ancestral population. The NCSE boasts about the use of migration pathways or land bridges to explain the presence of marsupials or other plants and animals around the world. But what happens when organisms–even higher mammals–appear on isolated islands, and there appears no way for their purported ancestors to migrate there? At these points, evolutionary biogeographers appeal to a fallback position, a suite of mechanisms of “oceanic dispersal.” As a review by De Quieroz (2005) stated:
A classic problem in biogeography is to explain why particular terrestrial and freshwater taxa have geographical distributions that are broken up by oceans. Why are southern beeches (Nothofagus spp.) found in Australia, New Zealand, New Guinea and southern South America? Why are there iguanas on the Fiji Islands, whereas all their close relatives are in the New World?57
According to De Quiroz, such examples require “oceanic dispersal over tectonic vicariance as an explanation for disjunct distributions in a wide variety of taxa, from frogs to beetles to baobab trees.”58 But he recognizes a fundamental problem with overseas dispersal hypotheses: “cladistic biogeographers claimed that hypotheses of dispersal were not falsifiable because all patterns of relationships can be explained by some dispersal hypothesis.”59 He further states that, “A main objection to dispersal hypotheses is that they are unfalsifiable and thus unscientific,” continuing that, “this can be countered by noting that, if plausible vicariance hypotheses are falsified, then dispersal is supported by default.”60 In other words, evolutionists assume that traditional land-based migration pathways (the type of evidence the NCSE claims supports common descent) were taken, but when they aren’t an option, one can always fall back when necessary on to unfalsifiable ad hoc hypotheses of oceanic dispersal. After reviewing a number of “unexpected” biogeographic data that require oceanic dispersal, De Quiroz’s review concludes: “these cases reinforce a general message of the great evolutionist [Darwin]: given enough time, many things that seem unlikely can happen.”
Thus, neo-Darwinian evolutionists are forced to appeal to “unlikely” or “unexpected” transmigration of terrestrial organisms, in some cases requiring the crossing of oceans (“oceanic dispersal”) to account for some biogeographical data. Such data challenges the simplistic picture of biogeography put forth by the NCSE that biogeography lends support to universal common descent through congruence between migration pathways and tectonic history. If anything, the “disjunct distributions in a wide variety of taxa” would tend to lend prima facie support for an orchard model of life’s history suggested by EE; a single tree of life hypothesis can only be sustained through extremely unlikely ad hoc appeals to oceanic dispersal to save universal common descent from difficult biogeographical data. What follows are some notable examples of such data.
Sea Monkey Hypotheses
One of the most infamous examples of the very sort of “biogeographic conundrum” the NCSE fears is the origin of South American monkeys, called platyrrhines.61 Based upon molecular and morphological evidence, “New World” platyrrhine monkeys are thought to be descended from African “Old World” or catarrhine monkeys. The problem is that plate tectonic history shows that Africa and South America split off from one another between 100 and 120 Mya, and that South America was an isolated island continent at least from about 80 Mya until about 3.5 Mya.62 Molecular studies claim that the South American monkeys split from African monkeys perhaps around 35 Mya.63 Monkeys are thought to have first evolved in Africa, and so somehow proponents of neo-Darwinism must account for the subsequent appearance of monkeys in the Upper Oligocene in South America.64 As Walter Carl Hartwig puts it: “The platyrrhine origins issue incorporates several different questions. How did platyrrhines get to South America?”65
If the standard evolutionary story is true, and platyrrhines and catarrhines are both part of the same crown group radiation of monkeys, then how did platyrrhines come to be in South America if South America was then an isolated island continent and there was no land-based route for monkeys to migrate from Africa to South America? For those unfamiliar with the arguments that proponents of neo-Darwinian biogeography make when backed into a corner, the answer to these questions is almost too incredible to believe: they propose that monkeys floated on rafts across the Atlantic Ocean to colonize South America. And of course, we can’t have just one seafaring monkey, or the monkey will quickly die leaving no offspring. Thus, at least two monkeys (or perhaps a single pregnant monkey) must have made the rafting voyage.
If this proposal seems a little farfetched, consider the quite serious endorsement of the rafting hypothesis given in a recent authoritative book, Primate Biogeography: Progress and Prospects (2006). The authors of the chapter “The Biogeography of Primate Evolution,” John G. Fleagle and Christopher C. Gilbert, state the problem as follows:
The most biogeographically challenging aspect of platyrrhine evolution concerns the origin of the entire clade. South America was an island continent throughout most of the Tertiary, and most of the orders of mammals found in Paleocene through Miocene deposits are endemic families or orders almost exclusively restricted to that continent. Primates first appear in the Late Oligocene and become common only in the Early Miocene. Rodents also appear first in the Oligocene. Both groups are almost certainly immigrants from some other continent, and paleontologists have debated for much of this century how and where primates reached South America.66
Likewise, a Harper Collins textbook on human evolution states:
The origin of platyrrhine monkeys puzzled paleontologists for decades. … When and how did the monkeys get to South America? Prior to about 1970, paleontologists invoked the concept of parallel evolution. … It seemed so unlikely that monkeys from Africa could cross a water barrier like the Atlantic Ocean… Molecular evidence demonstrated that all monkeys shared a common ancestor prior to their separation. … The “rafting hypothesis” argues that monkeys evolved from prosimians once and only once in Africa, and that it is a primitive monkey (parapithecid), and not a prosimian, that made the water-logged trip to South America. … Other species colonizing South America must have arrived in similar ways over millions of years.67
As noted above, the high degree of molecular genetic similarity between platyrrhine and catarhine monkeys precludes the possibility that African and South American monkeys are similar simply because of convergent evolution. Yet as Fleagle and Gilbert state, similarities between monkeys across the oceans “raises a difficult biogeographical issue” because “South America is separated from Africa by a distance of at least 2600 km, making a phylogenetic and biogeographic link between the primate faunas of the two continents seem very unlikely.”68 They argue that in light of “[t]he absence of any anthropoids from North America, combined with the considerable morphological evidence of a South American-African connection with the rodent and primate faunas” that therefore “the rafting hypothesis is the most likely scenario for the biogeographic origin of platyrrines.“69
All kinds of arguments have gone back and forth about whether such a rafting journey is possible or plausible. Of course, millions of years ago Africa and South America were slightly closer than they are today, but they were still very far apart at the time monkeys supposedly colonized South America. Fleagle and Gilbert argue that at best, the position of the continents in the early Tertiary still requires a “journey from Africa to South America anywhere from 8 to 15 days.”70 This is called “plausible,” but a macroview must be taken here: Is there any real biogeographical evidence that can falsify common ancestry? If the presence of higher mammalian fauna on isolated island continents with no simple way to arrive there does not falsify neo-Darwinian explanations of biogeography, what will?
Indeed, the rafting hypothesis has serious problems, for monkeys and rodents have high metabolisms and require large amounts of food and water:
The case of platyrrhines is more difficult to explain as anthropoid primates have higher metabolic rates and do not have the ability for prolonged periods of topor. A two-week rafting event across the Atlantic must have involved a floating island with an adequate food and water supply.71
Such “floating islands” are said to exist, but they admit that “the prevalence of over-water dispersal during primate evolution seems truly amazing for a mammalian order.”72 They further admit that “[t]he reasons for the prevalence of rafting during the course of primate evolution remain to be explained.”73
Needless to say, not all feel comfortable believing that seafaring monkeys on rafts are “plausible.” As Hartwig puts it, “The overwhelming evidence for the late Cretaceous-Pliocene isolation of South America renders the mechanical aspect of platyrrhine dispersal virtually irresolvable,”74 for “any late Eocene origins model must invoke a transoceanic crossing mechanism that is implausible (rafting) or suspect (waif dispersal) at best.”75
And there are deeper problems: monkeys apparently made the journey, but other smaller African primates such as lorises and galagos never colonized South America. If it was so easy for monkeys to raft across the proto-Atlantic ocean, why didn’t these lower primates also make the voyage? The answer we’re given by Fleagle and Gilbert is that rafting is “clearly a chance event, an example of ‘sweepstakes’ dispersal” as “[o]ne can only speculate that by a stroke of good luck anthropoids where able to ‘win’ the sweepstakes while lorises and galagos did not.”76 As another authority wrote, “The evidence strongly suggests the existence of a Palaeogene transoceanic sweepstakes route between Africa and South America, and presumably also a similar route between Africa and Madagascar” to explain such primate distributions.77
Apparently the NCSE was not quite accurate when they claimed, “By comparing macroevolutionary patterns between different groups, we find that the same patterns repeat. This strongly suggests that the same forces drove the diversification of those different groups.” The truth is that whenever oceanic “sweepstakes” dispersal is required, we find an exception to expected neo-Darwinian rules of biogeography. And as will be seen in my next post, there are so many exceptions that one might reasonably question whether the inviolable neo-Darwinian rule of universal common ancestry is supported by biogeography.
* All quotes of the NCSE in this document were downloaded from the NCSE website’s response to EE on Biogeography on October 29, 2008.
[57.] Alan de Queiroz, “The resurrection of oceanic dispersal in historical biogeography,” Trends in Ecology and Evolution, Vol.20(2):68-73 (February 2005).
[61.] See John C. Briggs, Global Biogeography, pg. 124 (Elsevier Science, 1995); Alain Houle, “The Origin of Platyrrhines: An Evaluation of the Antarctic Scenario and the Floating Island Model,” American Journal of Physical Anthropology, Vol. 109:541–559 (1999).
[62.] Carlos G. Schrago and Claudia A. M. Russo, “Timing the origin of New World monkeys,” Molecular Biology and Evolution, Vol. 20(10):1620–1625 (2003); John J. Flynn and André R. Wyss, “Recent advances in South American mammalian paleontology,” Trends in Ecology and Evolution, Vol. 13(11):449-454 (November, 1998); C. Barry Cox & Peter D. Moore, Biogeography: An Ecological and Evolutionary Approach, pg. 185 (Blackwell Science, 1993).
[63.] Carlos G. Schrago and Claudia A. M. Russo, “Timing the origin of New World monkeys,” Molecular Biology and Evolution, Vol. 20(10):1620–1625 (2003).
[64.] Anthony Hallam, An Outline of Phanerozoic Biogeography, pg. 166 (Oxford University Press, 1994). See also Walter Carl Hartwig, “Patterns, Puzzles and Perspectives on Platyrrhine Origins,” in Integrative Paths to the Past: Paleoanthropological Advances in Honor of F. Clark Howell, pg. 80 (Edited by Robert S. Corruccini and Russell L. Ciochon, Prentice Hall, 1994).
[65.] Walter Carl Hartwig, “Patterns, Puzzles and Perspectives on Platyrrhine Origins,” in Integrative Paths to the Past: Paleoanthropological Advances in Honor of F. Clark Howell, pg. 69 (Edited by Robert S. Corruccini and Russell L. Ciochon, Prentice Hall, 1994).
[66.] John G. Fleagle and Christopher C. Gilbert, “The Biogeography of Primate Evolution: The Role of Plate Tectonics, Climate and Chance,” in Primate Biogeography: Progress and Prospects, pgs. 393-394 (Shawn M. Lehman and John G. Fleagle, eds., Springer, 2006) (emphasis added).
[67.] Adrienne L. Zihlman, The Human Evolution Coloring Book, 4-11 (Harper Collins, 2000).
[68.] John G. Fleagle and Christopher C. Gilbert, “The Biogeography of Primate Evolution: The Role of Plate Tectonics, Climate and Chance,” in Primate Biogeography: Progress and Prospects, pg. 394 (Shawn M. Lehman and John G. Fleagle, eds., Springer, 2006).
[69.] Id. at 394-395 (emphasis added).
[70.] Id. at 394.
[71.] Id. at 404.
[72.] Id. at 404 (emphasis added).
[73.] Id. at 403.
[74.] Walter Carl Hartwig, “Patterns, Puzzles and Perspectives on Platyrrhine Origins,” in Integrative Paths to the Past: Paleoanthropological Advances in Honor of F. Clark Howell, pg. 76 (Edited by Robert S. Corruccini and Russell L. Ciochon, Prentice Hall, 1994).
[75.] Id. at 84 (emphasis added). Note: “waif dispersal” in this case refers to “island-hopping.”
[76.] John G. Fleagle and Christopher C. Gilbert, “The Biogeography of Primate Evolution: The Role of Plate Tectonics, Climate and Chance,” in Primate Biogeography: Progress and Prospects, pg. 395 (Shawn M. Lehman and John G. Fleagle, eds., Springer, 2006) (emphases added).
[77.] Anthony Hallam, An Outline of Phanerozoic Biogeography, pg. 166 (Oxford University Press, 1994).