Dude, it's called "relaxed selection"

2 minute read

This is a doofy story running on MSNBC without an author byline: “Shrinking of Scottish sheep tied to warming”. Why do I say “doofy”? Take a look at the way it describes natural selection:

The study upends the belief that natural selection is a dominant feature of evolution, noting that climate can trump that card.
"According to classic evolutionary theory," [study author Tim] Coulson added, the sheep "should have been getting bigger, because larger sheep tend to be more likely to survive and reproduce than smaller ones, and offspring tend to resemble their parents."

Yes, and classic evolutionary theory also says that if you stop killing the small ones, the population average is going to get smaller. Duh. A reduction will happen in a single generation as small individuals remain to become adults who would otherwise have been removed. The reduction may continue for a few generations, either by chance, or by changing the environmental component of variance in size. It can go on for many generations if there is a heritable component to size that confers a disadvantage on the largest individuals. Plausibly, larger individuals take longer to develop, there may be advantages to smaller size in females that are no longer opposed as strongly by antagonistic selection for larger size in males, or any number of other possibilities.

Has climate “trumped natural selection”? No. Cold and consequent food scarcity in this case is one cause of selection (killing small lambs). Possibly, one or more causes of stabilizing selection remain in force (maybe longer development time, but there are other possibilities). Or maybe not. Climate change has caused a change in the pattern of selection, by relaxing selection against small individuals who would otherwise have died from food scarcity.

The way the article describes selection is an old-time fallacy – “survival of the fittest” is recast as “survival of the strongest”, where strongest means “biggest”. If the small are somehow fail to be eliminated, then natural selection is failing at its work. It’s the eugenic fallacy, brought to 21st century climate change. It makes an eye-catching headline – “Climate Change Overpowers Natural Selection”. But it’s false.

A more accurate headline would be “Wee Lambs, Once Doomed to Starve, Saved by Climate Change”

I happen to have been reading some of the earlier research on these sheep, so I know that the work is interesting because researchers actually know about the fitness outcomes for individuals across their study duration. The observed fitness outcomes indicate that larger individuals have more offspring within each generation, but the population nevertheless became smaller over time. That comes down to viability of small young individuals and the non-heritable (environmental) component of variance in size, in a fairly complicated way. I’ll revisit the paper later to describe the study more fully. I just wanted to point out that this news story gets it totally wrong. Climate change is one of the big causes of the pattern of natural selection, it doesn’t magically repeal it.