When I teach or give seminars about climate change, I often start by reminding the audience that there are only four possible responses of any species to climate change: species can adapt, individuals of a species can acclimate, species can shift their geographic distributions, or failing in these species can become committed to extinction. I then usually go on to dismiss adaptation as a non-viable option due to the fact that climate is changing so fast and adaptation happens so slow. Now I have some support for my dismissal of adaptation.
Ignacio Quintero and John J. Wiens (of Yale and U. Arizona, respectively) have just published a new paper entitled, “Rates of projected climate change dramatically exceed past rates of climatic niche evolution among vertebrate species” the journal Ecology Letters. This article describes a study looking at past rates of evolution in the climatic niches of species. More specifically, the authors used time-calibrated phylogenies and estimates of species’ current climatic distributions/niches to calculate the time since divergence and most likely ancestral climatic niche for many different pairs of sister species (540 species in 17 clades of terrestrial vertebrates). For each species, the rate of climatic adaptation was then calculated as the difference between its current and ancestral climatic niche divided by time since divergence.
Based on this simple analysis, Quintero and Wiens find that rates of past climatic evolution were ‘slow’ across all the taxa examined. For example, in the case of mean annual temperature, the mean rate of evolution was generally less than 1°C change per million years. Given that global mean temperatures thave increased by approx. 0.6°C over the past 30 years and expected to increase by more than 4°C over the next century, 1°C of evolution per million years is about 10000 to 100000 times too slow.
As indicated above, the reported analyses are very simplified. This is fully acknowledged by the authors and indeed nearly the entirety of their discussion is spent addressing potential sources of error. In addition to several potential minor sources of error, the authors identify three potential major sources of error:
1) The assume a constant rate of evolutionary change through time. It is possible that species are capable of much faster bursts of evolution than indicated by the analyses which looked at the average rate across long periods of time including any periods of stasis.
2) Subspecies or populations may be capable of faster evolutionary change than entire species.
3) The species may not have been evolving at their maximum possible rates in the past. This is my biggest complaint with the study. Rate of evolution will depend on many factors including eh strength of selection. If climate change was slower in the past, selection pressure would have been less resulting in slower evolution. Now that climate change is fast, selection pressure is greater and species may be able to evolve faster. But even with the greater selection pressure, I think it is inconceivable that species can evolve the 100000 times faster that will be required to keep pace with future climate change.
Despite these and other concerns, the results of this study very strongly support the contention that climate change will simply be too fast for species to respond to through adaptation. If species can’t keep upwithclimate through evolution, then it becomes more and more likely that their only real option to escape extinction will be to move upwithclimate to higher elevations or latitudes.