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Sexually selected traits often depend on an individual’s physical condition, or otherwise indirectly reflect the ecological performance of individuals. When individuals disperse between populations that are locally adapted to different environments, their ecological performance may decline. This in turn may result in more poorly expressed sexual traits, and therefore in a lower reproductive success. Hence, sexual selection may reduce the effective gene flow between populations, and thereby maintain or even enhance population divergence. This hypothesis was published in a highly visible journal (van Doorn et al., 2009, Science). Here I review the subsequently published empirical tests of this hypothesis. I downloaded all metadata (incl. abstracts) of papers citing van Doorn et al. (2009) and read those papers that undertook relevant tests. To my surprise, only very few papers provided explicit tests of the hypothesis, this never involved plants, and only one study found support for it. While sexual selection may therefore not often reduce gene flow between locally adapted populations, some improvements to experimental design and choice of study system are noted. I therefore also provide a detailed list of suggestions for high quality tests of this hypothesis. This hopefully acts as a catalyst for more and better studies to test whether sexual and natural selection can work in synergy to reduce effective dispersal, and thereby protect and promote adaptive population divergence.

Background colour matching reduces predation risk of grasshoppers

Model details

Effect of time of day and thermal conditions on microhabitat use by grasshoppers

Male-female interactions and assortative mating

Explanations of how organisms might adapt to urban environments have mostly focused on divergent natural selection and adaptive plasticity. However, differential habitat choice has been suggested as an alternative. Here, we test for habitat choice in enhancing crypsis in ground-perching grasshoppers colonizing an urbanized environment, composed of a mosaic of four distinctly coloured substrates (asphalt roads and adjacent pavements). Additionally, we determine its relative importance compared to present-day natural selection and phenotypic plasticity. We found that grasshoppers are very mobile, but nevertheless approximately match the colour of their local substrate. By manipulating grasshopper colour, we confirm that grasshoppers increase the usage of those urban substrates that resemble their own colours. This selective movement actively improves crypsis. Colour divergence between grasshoppers on different substrates is not or hardly due to present-day natural selection, because observed mortality rates are too low to counteract random substrate use. Additional experiments also show negligible contributions from plasticity in colour. Our results confirm that matching habitat choice can be an important driver of adaptation to urban environments. In general, studies should more fully incorporate that individuals are not only selective targets (i.e. selected on by the environment), but also selective agents (i.e. selecting their own environments).

Conservatism of ecological niches can cause geographical ranges or the formation of new species to be constrained, and might be expected in situations where strong trade-offs result in ecological specialization. Here we address the flexibility of resource use in European crossbills by comparing the ecological and genetic similarities between four Mediterranean and three northern European crossbill populations, all specialized in feeding on a different resource. We used sequence data of one mitochondrial and two nuclear genes from between 211 and 256 individuals. The northern crossbills were genetically too similar to infer which population was more related to the southern ones. Crossbills from the island of Mallorca showed genetic signatures of a stable and isolated population, supporting their past treatment as a locally (co)evolving taxon, and seem to have evolved from an ecologically distinct ancestor. Previous studies in other populations also suggest that genetic similarity does not predict morphological and resource similarity. We estimate that the divergence of all western European crossbills has occurred within the last 11 000 years. Overall, it appears that crossbills can diversify rapidly and with little niche conservatism, but that such potentially reproductively isolated specialists are evolutionarily short-lived

Divergent selection stemming from environmental variation may induce local adaptation and ecological speciation whereas gene flow might have a homogenizing effect. Gene flow among populations using different environ- ments can be reduced by geographical distance (isolation-by-distance) or by divergent selection stemming from resource use (isolation-by-ecology). We tested for and encountered phenotypic and genetic divergence among Spanish crossbills utilizing different species of co-occurring pine trees as their food resource. Morphological, vocal and mtDNA divergence were not correlated with geographical distance, but they were correlated with differences in resource use. Resource diversity has now been found to repeatedly predict crossbill diversity. However, when resource use is not 100% differentiated, additional characters (morphological, vocal, genetic) must be used to uncover and validate hidden population structure. In general, this confirms that ecology drives adaptive divergence and limits neutral gene flow as the first steps towards ecological speciation, unprevented by a high potential for gene flow.

Background: Southern caracaras are medium-sized raptors with a large range stretching to the southern tip of South America. An aberrant, leucistic plumage is found commonly along the coast of Chubut Province (Patagonia, Argentina). Leucistic birds do not produce dark melanin in their feathers. However, they are not albinos because their eyes are not red. No genetic studies of caracara plumages are known. Hypothesis: The high frequency of leucistic birds in Chubut Province arises because of natural selection. Methods: Map the distribution of leucistic individuals relative to normal individuals. Combine a variety of anecdotal natural history observations, collected over 20 years, into a logical inference. Observations: Leucistic caracaras were found only along a 250-km stretch of rocky oceanic islands and continental outcrops with large seabird colonies in Chubut Province. In the rest of their range, Southern caracaras have dark plumage. Where they do occur, leucistic birds are frequent and co-occur with dark-plumaged birds. Intermediate individuals, presumably heterozygotes, exist. Leucism is not related to age or sex. Leucistic individuals are restricted to a particular habitat. Gene flow has not homogenized the coastal and inland populations. Results: Leucism is not simply due to inbreeding producing more homozygous individuals. Leucism is not due to genetic drift. Leucism is not an environmental effect on individual physiology or development. Leucism is not a transient (plastic) phenomenon. Where they occur frequently, leucistic Southern caracaras are apparently favoured by natural selection, either directly or by pleiotropy

The comparison between quantitative genetic divergence (QST) and neutral genetic divergence (FST) among populations has become the standard test for historical signatures of selection on quantitative traits. However, when the mutation rate of neutral markers is relatively high in comparison with gene flow, estimates of FST will decrease, resulting in upwardly biased comparisons of QST vs. FST. Reviewing empirical studies, the difference between QST and FST is positively related to marker heterozygosity. After refuting alternative explanations for this pattern, we conclude that marker mutation rate indeed has had a biasing effect on published QST–FST comparisons. Hence, it is no longer clear that populations have commonly diverged in response to divergent selection. We present and discuss potential solutions to this bias. Comparing QST with recent indices of neutral divergence that statistically correct for marker heterozygosity (Hedrick’s G′st and Jost’s D) is not advised, because these indices are not theoretically equivalent to QST. One valid solution is to estimate FST from neutral markers with mutation rates comparable to those of the loci underlying quantitative traits (e.g. SNPs). QST can also be compared to ΦST (PhiST) of amova, as long as the genetic distance among allelic variants used to estimate ΦST reflects evolutionary history: in that case, neutral divergence is independent of mutation rate. In contrast to their common usage in comparisons of QST and FST, microsatellites typically have high mutation rates and do not evolve according to a simple evolutionary model, so are best avoided in QST–FST comparisons.