Fixity of species and common descent

Studying the biogeographical links between different parts of the world can deepen our understanding of the evolutionary relationships between different populations within a species, between different species, and among higher taxonomic groups. Because different groups diversify at different rates, the evolutionary history of a single species revealed by biogeography might help clarify the relationship between entire families of some different group. These testable predictions about biogeography are powerful tools for scientists.

By the account in Explore Evolution, this predictive power is nonexistent, and biogeography only allows us to reject the notion of species fixity, nothing more. Instead of examining the many ways that biogeographic studies can inform our understanding of evolution, Explore Evolution simply dismisses the field as irrelevant. In particular, the authors claim that biogeographic evidence cannot distinguish between common ancestry and creationist "orchard" models. Such "orchards" have no scientific basis. Despite the book's claim to be "inquiry-based," Explore Evolution never clarifies how readers might themselves investigate such an orchard.

Fixity of Species

As John Wilkins explains, "The idea that species were universally thought to be fixed prior to Darwin is simply wrong — many creationist thinkers of the classical period through to the 19th century thought that species could change." Linnaeus, the father of modern taxonomy, began his career committed to the fixity of species, but began accepting the evidence against such fixity approximately a century before Darwin's ideas were published. Nor was fixism widely accepted within the scientific community by the time Darwin wrote: "No sooner had natural history established a tradition of fixism of species [in the 1700s] than it was immediately under challenge, for example by Pierre Maupertuis in Vénus Physique in 1745," according to Species: A History of the Idea, by John Wilkins (p. 104). It was not Darwin's intention simply to refute the discredited notion of species fixity, but to describe the way that life on earth is related. Biogeography helps us see how that process works over longer time periods.

Despite that history, Explore Evolution claims:

Darwin was using this evidence [biogeography] to challenge a theory that was popular in his day but is almost unheard of now: the fixity of species. The fixity of species was the idea that each species is fixed in its physical form which it doesn't change (at least not enough to constitute a new species) and placed in its current habitat from which it doesn't move (at least not beyond significant geographic barriers such as mountain ranges or oceans). Nowadays, the idea of the fixity of species isn't even a blip on the radar.
Explore Evolution, p. 76

Regardless of this rejection of fixism, Explore Evolution cites unnamed "critics" who assert that biogeography does not demonstrate "macroevolution," idiosyncratically defined as "the origin of new large-scale features such as organs or body plans." This, like the later arguments about the limits on the evolution of finches (see discussion of chapter 8), is an argument for fixity of something. They are merely following the lead of creationists like George McReady Price in the 1930s, who replaced the notion of species fixity with fixity of Biblical "kinds."

In fact, biogeography is a powerful illustration of macroevolution (as it is conventionally defined: "Evolution on the grand scale. The term refers to events above the species level. The origin of a new higher group, such as vertebrates, would be an example of a macroevolutionary event." from Ridley's Evolution, 2nd ed., p. 669). Adaptive radiations of flies, finches or marsupials all demonstrate how rapidly a small population can speciate and diversify, producing the the sort of diversity normally associated with "higher taxonomic groups" in geologically brief periods of time. The few genera of Darwin's finches occupy as many ecological niches as several families of birds; African cichlids exhibit morphologies and ecologies greater than can be found in the many orders of fish found on coral reefs, and all from an ancestor a few million years ago.

Deeper evolutionary insights can come from comparisons of multiple groups with a shared evolutionary history. For instance, both rodent, bat, and insect populations in the Philippines show similar evolutionary connections between certain island populations. One recent study summarized:

The Philippine archipelago is an exceptional theatre in which to investigate the roles of past history and current ecology in structuring geographic variation. The 7000 islands originated as a set of de novo oceanic islands … of varying ages and geological histories… It is an area of high biotic diversity … [A]t least 111 of the 170 native species of terrestrial mammals (64%) are endemic, it is still more striking that 24 of 84 genera (29%) are endemic, implying much in situ diversification, and phylogenetic studies suggest that several large endemic clades are present among fruit bats and murid rodents. Each oceanic island that has remained continuously isolated from its neighbouring islands is a unique centre of mammalian endemism, with 25–80% of the non-volant mammals [mammals other than bats] endemic, even on islands of only a few hundred square kilometres. Similar patterns are evident among butterflies (Holloway, 2003) and trichopteran insects.

Lawrence Heaney, Joseph Walsh, Jr., and A. Townsend Peterson (2005) "The roles of geological history and colonization abilities in genetic differentiation between mammalian populations in the Philippine archipelago," Journal of Biogeography 32(2):229-247

The similarity of these results allows researchers to make predictions about other groups, and where in one case a researcher might look at the biogeography of a single species, in other cases the same pattern may be seen in the biogeography of an entire taxonomic family. Thus, if biogeography undermines the fixity of species, it also undermines the fixity of higher taxonomic groups (as often advocated by proponents of a creationist "orchard") by showing that all taxonomic groupings have responded to the same evolutionary pressures.

Biogeographic predictability

Explore Evolution asserts "the evidence [from biogeography] is completely consistent with other views of the history of life, in which small-scale changes in form and features do occur within separate but disconnected groups of organisms" (p. 79). In order for a claim to be good science, it is not enough that it be "consistent" with the evidence, it must actually make testable predictions. In the terms stated, this "orchard" model offers no testable predictions, as it is infinitely malleable. It can be adjusted to fit any evidence, but Explore Evolution never offers enough details on the "orchard" to allow any predictions. Given the book's claim to be "inquiry-based," this is at best a sad oversight.

By contrast, the consistency of multiple lines of biogeographic evidence is exactly what would be predicted if all life were evolving in response to the same events throughout earth's history. As discussed above, in standard textbooks, and in the extensive (and uncited by Explore Evolution) scientific literature on biogeography, biogeography generates powerful testable predictions, predictions generated by comparing the biogeography of one group to that of another with shared geography. Biogeographers can predict where new species will be found, and can predict the diversity of communities in areas never before investigated because of the power of biogeography and evolution.

Such tests are impossible for the neo-creationist "orchard" model hinted at by Explore Evolution. The book simply gives too little detail of such a model to allow readers to make any prediction. Since the neo-creationist advocates of this model state forthrightly that they believe that God created each tree in the orchard of life, and since God can do anything, this is a model which is consistent with everything, and predicts nothing. The authors of Explore Evolution, perhaps in order to hide the book's creationist heritage, chose not to explain where they think the many trees of life come from (who planted the "orchard"?), how many trees there are, or why the trees are "separate but disconnected" (who prunes them?). This lack of specificity makes the model potentially consistent with anything, but only because they have chosen to specify nothing.

The "Further Debate" section of this chapter (pp. 79-80) repeats one of the great failings of this text. It highlights two sets of views, presents a weak explanation of one side, anonymous critics on the other, and then simply abandons the students to decide for themselves what to think. The authors might claim that this is consistent with an inquiry-based approach, but as discussed in the critique of chapter 1, this is false. An inquiry-based approach would present a real source of scientific uncertainty (and not a lightly repackaged creationist attack on science) and would provide the students with the tools to investigate the subject further. An inquiry-based textbook would not simply declare "scientists sometimes disagree about how to interpret the various classes of evidence we have examined" (p. 79). That is not inquiry, that is surrender. Scientific inquiry takes disagreement as a starting point for further research, not as a chance to declare that "there may not be much further debate" and "the issue is likely to remain exactly where it is."

Not only does this chapter (and the book as a whole) mislead students about the actual state of scientific knowledge, it misinforms them about the way science works.

Doubting Common Ancestry

Explore Evolution claims that some people who doubt common ancestry accept fixity of species, so biogeography doesn't prove anything to them. One such 19th century scientist:

accepted that migration and adaptation would alter the features of species. Nevertheless, he doubted that species could undergo unlimited change, and did not accept that all species shared a common ancestor. Many modern critics of neo-Darwinism share this view.
Explore Evolution, p. 78

Cuvier shares little with the modern creationists he is being compared with here. Cuvier wrote in the early 19th century, decades before Darwin and Wallace, and the understanding of genetics and the fossil record which Darwin had, let alone which modern scientists enjoy. He can be excused for thinking species appeared from some unknown source and remained fixed in form thereafter. If this is the most recent genuine scientist Explore Evolution can cite who holds this view, it is hardly a ringing endorsement. Cuvier's claims had there day, but research in his day and since then have falsified his views.

This small aside at the chapter's end takes back the small concession to critics of creationism offered at the chapter's beginning. Before, the authors acknowledge that species fixity "isn't even a blip on the radar" today. But again, biogeography is not simply an exercise in disputing species fixity: it demonstrates that taxonomic ranks above the species level are not fixed, and shows the process by which species diversify, and by which the branching process of evolution has produced those higher taxonomic levels.

Nor is biogeography simply concerned with speciation and adaptive radiation (a process the authors denigrate in the following chapter). Biogeography shows a great deal more than that species can change. Even within the limited subset of biogeography that Explore Evolution chooses to address (adaptive radiation), there is clear evidence for evolution of new species, genera, families, etc., and illustrations of the power of evolution to produce morphological novelty with great speed, given the right conditions. This is exactly the opposite conclusion from the one Explore Evolution draws. That does not mean that a debate is underway, only that the selective use of evidence can produce misleading results. When this book declares it impossible for evolution to accomplish a task, and then ignores instances where evolution does explain how that impossible thing happened, it calls the book's credibility, not evolution's, into question.