Evolution and climate change: Survival of the less fit
I noticed this story a couple of days ago and finally an email exchange with friends on the meaning of adaptation, fitness and success prompted me to write this.
The basic story is that the harsh winters of the St Kilda archipelago beyond the Outer Hebrides have been favoring sheep which grow faster and get bigger during the summers. In winters that big size is an advantage due to higher stored fat content. Smaller sheep - those that develop slower in summers - are at a disadvantage in these conditions and die off disproportionately. Recently though researchers have noticed that the population is shrinking in terms of the size of individual sheep. Smaller sheep are surviving in larger numbers. The reason could be climate change and warmer winters.
This has led to speculations that climate change has overridden natural selection.
That's just wrong. Climate change has not overridden natural selection. It has shifted the selection pressure on the sheep population. Under the previous climatic regime smaller sheep were at a disadvantage i.e. they were under negative selection. That negative selection has been eased off due to the recent warmer winters.
Yet news reports continue to put out sentences like:
Classical evolutionary theory suggests that over time the average size of wild sheep increases, because larger animals tend to be more likely to survive and reproduce than smaller ones, and offspring tend to resemble their parents.
This makes it seem as if there is some universal process that will develop trends towards larger size. "Classical" evolutionary theory says no such thing about populations getting bigger in the size of individuals as an inevitable outcome. It may just so happen that for a particular time period the conditions - like harsh winters- favor bigger animals and a trend develops in a population towards larger size. But that can just as easily be reversed if conditions change and start favoring smaller size. A trend towards smaller size is also predicted by the same "classical" evolutionary theory.
Headlines like survival of the less fit perpetuates the misconception that in evolution bigger or stronger is better. We can do without such thinking.