Little Baa
It’s a little perplexing to see this press release from my very own institution, which unwittingly provides fodder for creationists:
Edinburgh researchers studying Soay sheep on the island of Hirta, St Kilda, have shown that the animals’ average body size has decreased by five per cent over the past 24 years. ... The reduction in body size opposes the trend that would be expected from evolutionary research. Large sheep have the most offspring, and body size is inherited from one generation to the next, so sheep should tend to become bigger over time. Researchers say that in the case of the Soay sheep, the dwarfing effect of the animals’ environment has been greater than the opposing effect of evolution.
Creationists love to trumpet “surprising” results that “oppose [what] would be expected from evolutionary research”. But this is surely a case of natural selection in action. Colder winters kill off the smaller sheep, selecting for larger ones. The corollary is that milder winters don’t kill off as many small sheep, which remain in the population and thus bring the overall average size down. This would be true even if large sheep have more offspring than small sheep, because the proportion of large sheep in the overall population must be reducing now that more small sheep are surviving. If the population gets so large that the food supply runs out, new selection pressures will come into play, although there’s no guarantee that those will favour large sheep, who presumably need more food than small sheep to survive.
However it goes, it’s all explicable by natural selection, and the shrinking size of the sheep is a sign that the population is evolving. Evolution isn’t a cause, it’s an effect.
The Imperial College press release portrays the subtleties better, but still says that “the decrease in average body size seen in Hirta’s sheep is primarily an ecological response to environmental changes over the last 25 years; evolutionary change has contributed relatively little”, overlooking that what we are seeing surely is evolutionary change. The Leeds press release points out that this study identifies new factors to consider in analyses of natural selection, rather than calling evolution by natural selection into question, but also suggests that some of the confusion comes from the researchers, who seem to be talking of evolution as something that happens separately from “the effects of environmental change”. Surely that was Darwin’s entire point? All the genetic mutations in the world won’t matter a bit if the environment becomes impossible to live in.
The ScienceDaily report is the most straightforward, reporting the study and its implications without calling the very existence of evolution into question. So we might have to chalk the rest up to university press departments, not least the one up the road.
10 July 2009