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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.