Theories related to climate change appear in green.
Concerning size
Survival - Thanks to climate change, small lambs survive winters better. Warmer temperatures means smaller lambs can live through the winter, thus decreasing the sheep’s average body size.
Growth - For some still unclear reason, lambs are growing differently. We could explain this phenomenon through several theories: a change in primary production, different vegetation availability or even parasites.
Genetics (size) - A. Wilson wrote that shrinking sheep could be explained by several factors, including “genetic evolution (but we have no evidence to support this)”. There is no evidence to support that genetics plays a role in decrease of body size.
Young mum effect - Young mums give birth to smaller lambs. This explains why Soay have not been getting bigger, as we expected them to, but it is not enough to explain why they are shrinking.
Selection on reproductive investment - As winters get warmer, during gestation, selection will favor the production of larger litters even at the expense of offspring size. This is why ewes will have smaller but more numerous lambs after a relatively warm winter (knowing her offspring has higher chances to survive) or will only have one or two babies if the winter is colder (knowing chances of offspring survival are slimmer).
Concerning color
Genetics - The dark color gene would be associated with an unknown locus having negative effects on fitness. Dark coat sheep who have 2 dark color genes are less fit than the sheep with other genetic combinations (dark sheep who have 1 dark color/1 light color gene, or light sheep who have 2 light color genes). Dark sheep with two dark color genes are less likely to survive, and as a result, the number of dark sheep in proportion to light coat color ones is decreasing.
Sunlight absorption - Dark sheep used to have a survival advantage because their coat allowed them to absorb more heat than light sheep. However, milder winters make this advantage unnecessary, thus making lighter sheep survive better and changing the average size of the whole population.