Since annual birth weight had not increased over the 20 years, the researchers hypothesized that a variable environment could limit its micro-evolution by limiting heritability, selection, or both.
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Wilson et al. To study the micro-evolutionary dynamics of a wild population, they adapted a theoretical model long used by animal breeders.
Quantitative genetic models measure the genetic contribution which typically involves multiple genes to variation in a given trait, such as milk production in dairy cows. With these models, plant and animal breeders maintain or develop desired traits by predicting an evolutionary path based on the strength of selection and the amount of genetic variance underlying the trait.
Some of these models assume a constant environment, which is fine for breeders, who can artificially control their subjects' environment, but proves problematic for evolutionary studies of wild populations. To overcome these limitations, the researchers incorporated a statistical tool called random regression analysis.
With this approach, they could not only model the effects of environmental variation on selection and heritability but also maximize the statistical power of their dataset.
This is a particularly effective technique for natural populations, for which the data are inevitably limited. It also allowed the researchers to model the genetic variance underlying lamb birth weight as a function of environmental quality poor if lamb survival was low, good if survival was high , creating a more realistic evolutionary picture.
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Lamb birth weight is primarily influenced by "maternal genetic effects," which derive from gene products of the mother's genome, rather than those of the lambs themselves. The most powerful modeling result showed that the extent to which birth weight is heritable--that is, the variance caused by maternal genetic effects--increased along with environmental quality. But when Wilson et al.
[PDF] Soay sheep: dynamics and selection in an island population. - Semantic Scholar
Fitness increased with birth weight, and with environmental quality, but the positive relationship between fitness and birth weight--indicating the strength of selection--became weaker in better environments. Environmental quality shapes the trajectories for both genetic variance and the strength of selection. Pemberton 2. The sheep of St Kilda T. Clutton-Brock, J. Pemberton, T.
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Coulson, I. Stevenson and A.
MacColl 3. Population dynamics in Soay sheep T.
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Clutton-Brock, T. Coulson, A. MacColl, A.
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Natural selection in island sheep
Crawley, S. Albon, D. Bazely, J. Milner, J.