Academic journal article Genetics

F^sub ST^ and Q^sub ST^ under Neutrality

Academic journal article Genetics

F^sub ST^ and Q^sub ST^ under Neutrality

Article excerpt

ABSTRACT

A commonly used test for natural selection has been to compare population differentiation for neutral molecular loci estimated by F^sub ST^ and for the additive genetic component of quantitative traits estimated by Q^sub ST^. Past analytical and empirical studies have led to the conclusion that when averaged over replicate evolutionary histories, Q^sub ST^ = F^sub ST^ under neutrality. We used analytical and simulation techniques to study the impact of stochastic fluctuation among replicate outcomes of an evolutionary process, or the evolutionary variance, of Q^sub ST^ and F^sub ST^ for a neutral quantitative trait determined by n unlinked diallelic loci with additive gene action. We studied analytical models of two scenarios. In one, a pair of demes has recently been formed through subdivision of a panmictic population; in the other, a pair of demes has been evolving in allopatry for a long time. A rigorous analysis of these two models showed that in general, it is not necessarily true that mean Q^sub ST^ = F^sub ST^ (across evolutionary replicates) for a neutral, additive quantitative trait. In addition, we used finite-island model simulations to show there is a strong positive correlation between Q^sub ST^ and the difference Q^sub ST^ - F^sub ST^ because the evolutionary variance of Q^sub ST^ is much larger than that of F^sub ST^. If traits with relatively large Q^sub ST^ values are preferentially sampled for study, the difference between Q^sub ST^ and F^sub ST^ will also be large and positive because of this correlation. Many recent studies have used tests of the null hypothesis Q^sub ST^ = F^sub ST^ to identify diversifying or uniform selection among subpopulations for quantitative traits. Our findings suggest that the distributions of Q^sub ST^ and F^sub ST^ under the null hypothesis of neutrality will depend on species-specific biology such as the number of subpopulations and the history of subpopulation divergence. In addition, the manner in which researchers select quantitative traits for study may introduce bias into the tests. As a result, researchers must be cautious before concluding that selection is occurring when Q^sub ST^ ≠ F^sub ST^.

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IDENTIFYING signatures of natural selection based on patterns of genetic variation within and among populations is a long-standing goal in population genetics. Distinguishing between natural selection and neutral processes such as genetic drift and gene flow is central to testing the hypothesis that selection is primarily responsible for patterns of phenotypic variation in natural populations. In the past 15 years, comparisons of two statistics, F^sub ST^andQ^sub ST^, have been used to test for the action of natural selection on quantitative traits in subdivided populations. The fixation index F^sub ST^ is a measure of population differentiation defined as the ratio of amongdeme to total variance at the allelic level (Wright 1951), and Q^sub ST^ is the analogous ratio of among-deme to total additive genetic variation for quantitative phenotypes (precise definitions of both these quantities are given below).

Theoretical analyses and simulations have indicated that under neutrality, F^sub ST^ and Q^sub ST^ should be equal. In particular, Lande (1992) showed that in a finite-island model and in a metapopulation model with extinction and recolonization, F^sub ST^ described the differentiation of a neutral quantitative trait among subpopulations, i.e., that F^sub ST^ = Q^sub ST^. Spitze (1993) used Lande's result as a null hypothesis to argue that diversifying selection was acting in populations of Daphnia obtusa; in this work Spitze coined the name Q^sub ST^. Whitlock (1999) used a coalescent argument to generalizeLande's (1992) results to additional population structures, including steppingstone models, under drift-mutation equilibrium. Le Corre and Kremer (2003) related Q^sub ST^ to F^sub ST^ for traits under selection, arguing that Q^sub ST^ should be greater than F^sub ST^ under diversifying selection (i. …

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