Adam H Porter | PubMed Health

PubMed2003

A test for deviation from island-model population structure.

The neutral island model forms the basis for several estimation models that relate patterns of genetic structure to microevolutionary processes. Estimates of gene flow are often based on this model and may be biased when the model's assumptions are violated. An appropriate test for violations is to compare FST scores for individual loci to a null distribution based on the average FST taken over multiple loci. A parametric bootstrap method is described here based on Wright's beta-distribution to generate null distributions of FST for each locus. These null distributions account for error introduced by sampling populations, individuals and loci, and also biological sources of error, including variable alleles/locus and inbreeding. Confidence limits can be obtained directly from these distributions. Significant deviations from the island model may be the result of selection, deviations from the island model's migration pattern, nonequilibrium conditions, or other deviations from island-model assumptions. Only strong biases are likely to be detected because of the inherently large sampling variation of FST. Nevertheless, a coefficient, Nb, describing bias in the spread of the beta-distribution in units comparable to the gene flow parameter, Nm, can be obtained for each locus. In samples from populations of the butterfly Coenonympha tullia, the loci Idh-1, Mdh-1, Pgi and Pgm showed significantly lower FST than expected.

Animals↗

PubMed2002

Speciation despite gene flow when developmental pathways evolve.

Evolutionary biologists assume that species formation requires a drastic reduction in gene exchange between populations, but the rate sufficient to prevent speciation is unknown. To study speciation, we use a new class of population genetic models that incorporate simple developmental genetic rules, likely present in all organisms, to construct the phenotype. When we allow replicate populations to evolve in parallel to a new, shared optimal phenotype, often their hybrids acquire poorly regulated phenotypes: Dobzhansky-Muller incompatibilities arise and postzygotic reproductive isolation evolves. Here we show that, although gene exchange does inhibit this process, it is the proportion of migrants exchanged (m) rather than the number of migrants (Nm) that is critical, and rates as high as 16 individuals exchanged per generation still permit the evolution of postzygotic isolation. Stronger directional selection counters the inhibitory effect of gene flow, increasing the speciation probability. We see similar results when populations in a standard two-locus, two-allele Dobzhansky-Muller model are subject to simultaneous directional selection and gene flow. However, in developmental pathway models with more than two loci, gene flow is more able to impede speciation. Genetic incompatibilities arise as frequent by-products of adaptive evolution of traits determined by regulatory pathways, something that does not occur when phenotypes are modeled using the standard, additive genetic framework. Development therefore not only constrains the microevolutionary process, it also facilitates the interactions among genes and gene products that make speciation more likely-even in the face of strong gene flow.

Animals↗