The Role of Climatic Change in the Evolution of Mammals

June 2007 / Vol. 57 No. 6 • BioScience 523 D all the arguments over semantics, evolution is a fact. It is a fact in the sense of dictionary definitions— for example, “something having real, demonstrable existence...the quality of being real or actual” (Soukhanov et al. 1996). And it is a fact in the scientific sense—“an observation that has been repeatedly confirmed” (Kennedy et al. 1998). This is a bold statement, but it follows directly from incontrovertible observations.We know what genes are and how traits are inherited, and that the variation we observe between individuals within populations and between species is underlain by genetic differences. We know how those genetic differences arise and how they are maintained: through mutation, recombination, random drift, and selection.We can (and do) induce within-species evolution in domestic animal breeding programs—think of dogs, cattle, and racehorses.And we have seen natural selection cause evolution in such examples as industrial melanism in moths (Cook 2003, Rudge 2005) and the reduction of size in snow lotus plants (Law and Salick 2005). On the microbial level, we recognize the reality of evolution by spending money on it: As Palumbi (2005) pointed out, consumers and taxpayers spend billions of dollars to combat the ever-escalating evolutionary arms race between antibiotics and the new bacterial genomes for which they select, and to try to prevent such potentially disastrous pandemics as avian flu. Observations from the fossil record show us how those mechanisms we observe in such a short term play out over the long term—which is somewhat remarkable, given that the fossilization process requires such a complicated sequence of events that only a tiny fraction of all the life forms that have ever lived were preserved as fossils, and a yet tinier fraction of those have been discovered. Darwin (1859) recognized the limits of the fossil record in this regard, and his contemporary Thomas Huxley recognized its power in the evolutionary debate when he wrote,“Primary and direct evidence in favour of evolution can be furnished only by palæontology. The geological record, so soon as it approaches completeness, must, when properly questioned, yield either an affirmative or a negative answer: if evolution has taken place, there will its mark be left; if it has not taken place, there will lie its refutation” (Huxley 1880). Although the paleontological record is still far from complete—for the same reasons Darwin and Huxley recognized some 150 years ago—it now yields a resounding affirmative answer about evolution. Fossils demonstrate overwhelmingly that geologically older species are replaced by geologically younger descendent species (including the succession of species in our own human history). The younger species typically share certain traits with the older ones, but