Adaptive population divergence : markers , QTL and traits

Molecular genetic markers have played a major role in evolutionary biology.As molecular methods have become cheaper, faster and involve less invasive sampling, they have become increasingly popular in conservation [2,3], where there is often a clear need for rapid decision making. For example, because genetic differences among populations are often considered worthy of conserving [2–5], many studies apply a criterion, assigning conservation priority to populations (or clades of populations) that are reciprocally monophyletic, because these probably represent independently evolving clades or evolutionarily significant units (ESUs) [5]. There are several definitions of ESU in the literature, which vary in the degree of emphasis placed on molecular versus ecological criteria [2–6]. Such definitions have crucial importance in the light of conservation legislation, where boundaries of ESUs must be drawn before legal protection status can be assigned [2–4]. However, locally adaptive genetic diversity within units is likely to be of greater importance when choosing populations that are most suitable as translocation or restoration sources. In this case, adaptive genetic differences among populations can lead to outbreeding depression if divergent populations are mixed [7]. Several reviews have identified testing the assumption of an association between marker diversity and adaptive diversity as a pressing research concern [1,8–10], and warn against the gradual replacement of ecological data with molecular criteria when diagnosing units of conservation [2,3,10]. Two recent studies use a formal meta-analysis to test the relationship between molecular and quantitative variation, both within [10] and among populations [11]. Here, we focus on measures of divergence or population structure [11], because these are most relevant to the Molecular markers appear to be poor indicators of heritable variation in adaptive traits. Direct comparison of population structure in markers with that in traits is made possible by the measure Qst, which partitions quantitative genetic variation in a manner analogous to Fst for single gene markers. A survey of the literature reveals that mean Qst is typically larger than and poorly correlated with mean Fst across 29 species. Within species, Qst varies widely among traits; traits experiencing the strongest local selection pressures are expected to be the most divergent from molecular Fst. Thus, Qst will be particularly relevant to conservation efforts where preserving extant adaptation to local environments is an important goal. Recent theoretical and simulation studies suggest however that Fst is a better predictor of the pattern of allelic differentiation at quantitative trait loci (QTLs) than is Qst in random mating populations, in which case allelic variation at QTLs might be better assessed by molecular markers than will extant variation in the traits themselves.