As many newspapers reported last Friday, a study by Professor Ian Coulson of Imperial College has suggested that climate change is to blame for a decrease in the size of wild sheep in the Outer Hebrides.
The average weight of Soay sheep on the island of Hirta has fallen by about 5 per cent in the last 24 years. After studying a wealth of data on the body sizes and life histories of the sheep, the researchers concluded that global warming is the culprit. According to Prof Coulson, smaller lambs that normally struggle to survive through winter now have a better chance of making it to spring as conditions have been getting milder. Consequently, a greater number of small sheep are reproducing, and propagating their genes for small bodies.
They used a formula called the “Price equation”, which was designed by evolutionary theorist George Price to predict how a physical trait, such as body size, will change from one generation to the next.
With all of this data, the team was able to “rearrange the equation” and use it to work out how much of a contribution each driver made to the sheep’s body size.
They found that the local environment had a stronger effect on the animals than the evolutionary pressure to grow larger.
A press release put out by Imperial College reads:
Their results suggest 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.
This statement seems to underlie a bit of confusion in the press about what sort of effect we’re looking at here. If we accept Prof Coulson’s conclusion, then clearly the decrease in body size is an ecological response to environmental changes. But is it not an evolutionary change as well? Evolution boils down to a change in allele frequencies in a population (alleles being the different variants of a particular gene). If a larger number of smaller lambs are reaching reproductive maturity and passing on their genes for small bodies, then what we’re seeing is a weakening of the selection pressure in favour of larger bodies, leading to genes for small bodies becoming more numerous in the population.
Yet many reports implied that natural selection is not at play here. The Times said this:
The scientists attributed the change to short-term changes in climate rather than to the long-term pressures of natural selection, which would favour a larger — not a smaller — body size.
The Independent went as far as to publish a subtitle heralding ‘Darwinism turned on its head’.
The Telegraph led with a strong contender for 2009′s worst opening sentence in a science article (I welcome further nominations in the comment thread).
Survival of the fittest and natural selection usually means that species grow bigger as they evolve
It really doesn’t bode well when the article starts with a howler like that.
These misinterpretations notwithstanding, the Soay sheep study has elucidated a fairly benign effect of climate change on ecology. But the rate at which the planet is heating up means that many animals may not be able to evolve quickly enough to cope with the changes in their environment. According to the IPCC, between a fifth and a third of all species could be at risk of extinction by the end of the century as a result of global warming.
We can expect to read more stories about climate change shaping evolution in the coming years, no doubt including some that elements of the media construe as anti-Darwinian. But the likelihood is that these will be outnumbered by sad tales of species disappearing altogether.