So much for adapting to the environment. Too bad Darwin didn't know about CO2.
"Sheep are getting smaller. Well, at least the wild Soay sheep living on a remote Scottish island are. But according to classic evolutionary theory, they 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," said study author Tim Coulson, professor of population biology at Imperial College London who teaches courses in ecology, evolution and conservation.
"Our findings have solved a paradox that has tormented biologists for years – why predictions did not match observation. Biologists have realized that ecological and evolutionary processes are intricately intertwined, and they now have a way of dissecting out the contribution of each. Unfortunately it is too early to tell whether a warming world will lead to pocket-sized sheep," Coulson said.
Coulson and his colleagues analyzed body-weight measurements and life-history data (which record the timing of key milestones throughout an individual's life), for the female members of a population of Soay sheep. The sheep live on the island of Hirta in the St. Kilda archipelago and have been studied closely since 1985.
The researchers plugged their data into a numerical model that they say predicts how a trait such as body size will change over time due to natural selection and other factors that influence survival and reproduction in the wild. They selected body size because it is a heritable trait, and because the sheep have, on average, been decreasing in size for the last 25 years.
Their results lead them to state that the decrease is primarily an ecological response to environmental variation over the last 25 years. Evolutionary change has contributed relatively little.
More specifically, lambs are not growing as quickly as they once did. As winters have become shorter and milder, due to global climate change, lambs now do not need to put on as much as weight in the first months of life to survive to their first birthday. So, even the slower-growing ones now have a chance of surviving, according to Coulson.
Also contributing to this trend is what Coulson and his colleagues call the "young mum effect." The researchers found that younger mothers are physically unable to produce offspring that are as big as they were at birth. The reasons behind this are still unclear, but the young mum effect counters the effect of natural selection, which favors larger lambs, the authors report.
Article: Arpat Ozgul , Shripad Tuljapurkar, Tim G. Benton, Josephine M. Pemberton, Tim H. Clutton-Brock, Tim Coulson, 'The Dynamics of Phenotypic Change and the Shrinking Sheep of St. Kilda', Published Online July 2, 2009 Science DOI: 10.1126/science.1173668
This research was funded by the Natural Environment Research Council and the National Institute on Aging, NIH.