Literature

Soay sheep live in glorious isolation on the tiny island of St Kilda, off the NW coast of Scotland. They provide interesting case studies for geneticists, ecologists and evolutionary biologists. Their "coat color is either dark brown or light tawny; the two phenotypes have been documented for at least 90 years." Changes in the frequency of these colours has stimulated the new research.

Dark and light coat colour morphs in Soay sheep. (For larger image, click here)

According to the authors, "variation in coat color is controlled by a single autosomal locus at which the dark allele is dominant to the light allele". They explain that the "dark coat color is associated with large size, which is heritable and positively correlated with fitness". Consequently, a reasonable prediction to make would be that the numbers of sheep with darker coats will increase in the population. But this is not happening! Observations show very clearly that "the frequency of dark sheep has decreased". This unexpected finding provides the rationale for further research. The authors do not consider the Soay Sheep case to be unusual. On the contrary, it is one of many examples of predictions not matching the data.

"Evolutionary theory states that directional selection on a heritable trait should result in evolutionary change. Analyses of long-term data sets from wild vertebrate populations reveal directional selection on heritable traits, yet many studies report no microevolutionary change (stasis) or, in some cases, responses in the opposite direction to that predicted."

The problem appears to be that genes affect more than one trait (something that we knew already in principle). The researchers found that the allele that is dominant for dark coat colour is associated not only with large body size (and increased fitness) but also with decreased lifetime. This finding was made possible because of the extensive documentation of the life histories of these wild sheep. The nature of this association is considered to be linkage disequilibrium rather than pleiotropy. Once the linkage became clear, the researchers could say:

"Our results actually imply that the light mutation should be increasing in frequency, although genetic drift will also play a role in determining the allelic frequencies."

A general conclusion can therefore be drawn:

"This study shows that selection acting on simple Mendelian traits in natural populations can have a complex genetic basis. This has implications for the study of microevolutionary change in natural populations, because fitness variation at the level of the genotype may not be evident in an analysis of selection on phenotype. Consequently, phenotypic studies may wrongly conclude that selection is not acting on genomic regions containing the loci underlying focal traits and may be unable to explain the microevolutionary dynamics of trait variation."

These comments are important, because nearly all the Darwinian just-so stories make use of the 'one gene - one trait' concept, which is now outdated. Although their scenarios are presented as plausible and reasonable, failure to address linkage disequilibrium and pleiotropy issues opens the door to faulty presuppositions and flawed logic. Educational programmes need to do more to create awareness of the unscientific nature of these Darwinian speculations.
It is to be welcomed that the authors qualify the word evolution using the prefix "micro". Phillip Johnson led the way in showing how many different meanings there are to the word "evolution", but there has been a marked reluctance for editors, authors and journalists to respond positively. If the term "microevolutionary trend" could be used to refer to the vast majority of textbook examples of "evolution", this would represent a major improvement in communication.

A Localized Negative Genetic Correlation Constrains Microevolution of Coat Color in Wild Sheep
J. Gratten, A. J. Wilson, A. F. McRae, D. Beraldi, P. M. Visscher, J. M. Pemberton, and J. Slate
Science, 319, 18 January 2008: 318-320.

Abstract: The evolutionary changes that occur over a small number of generations in natural populations often run counter to what is expected on the basis of the heritability of traits and the selective forces acting upon them. In Soay sheep, dark coat color is associated with large size, which is heritable and positively correlated with fitness, yet the frequency of dark sheep has decreased. This unexpected microevolutionary trend is explained by genetic linkage between the causal mutation underlying the color polymorphism and quantitative trait loci with antagonistic effects on size and fitness. As a consequence, homozygous dark sheep are large, but have reduced fitness relative to phenotypically indistinguishable dark heterozygotes and light sheep. This result demonstrates the importance of understanding the genetic basis of fitness variation when making predictions about the microevolutionary consequences of selection.

See also:

Images of St Kilda Archipeligo, Hirta and Boreray