The textbooks say that evolution happens most rapidly in response to environmental changes. You would think, then, that the cycling from glacial periods to warmer interglacial periods would be accompanied by significant changes in the species that survive the change.
Not so, according to a new paper in Quaternary Science Reviews, by Donald Prothero and colleagues. After examining all the common bird and mammal species preserved in the Rancho La Brea tar pits in California, the authors conclude:
[T]he data show that birds and mammals at Rancho La Brea show complete stasis and were unresponsive to the major climate change that occurred at 20 ka, consistent with other studies of Pleistocene animals and plants. Most explanations for such stasis (stabilizing selection, canalization) fail in this setting where climate is changing. One possible explanation is that most large birds and mammals are very broadly adapted and relatively insensitive to changes in their environments, although even the small mammals of the Pleistocene show stasis during climate change, too.
I work at the other end of the size spectrum, on protein molecules rather than whole animals. It’s interesting that attempts to catch evolution in the act of doing the amazing things that the textbooks attribute to it seem to fail at both ends of the spectrum.
Image: La Brea Tar Pits, universeist/Flickr.