Note that because gene flow occurs among adjacent populations, the clines established by gene flow are expected to be steep ( Endler, 1973). Gene flow among populations makes the cline smooth around the balancing point, indicating that the cooperation of divergent selection derived from G × E and ongoing gene exchange over the equilibrium point may establish a smooth cline in morph frequency. The first mechanism is based on gene flow. Theoretically, a smooth cline in morph frequency has been suggested to be produced by the homogenizing evolutionary forces ( Endler, 1977). However, G × E itself leads to creation of a stepwise pattern in morph frequency across an equilibrium point, because in each population, a single morph with the highest fitness in each population should dominate. In two of these three mechanisms, divergent selection derived from gene-by-environment (G × E) interactions underlies the establishment of a cline in morph frequency the fitness advantage of each morph differentially changes across the environmental gradient and reverses across an equilibrium (balancing) point, where each phenotype has equal fitness ( Endler, 1973). The mechanisms that establish clines in morph frequency can be roughly classified into three categories ( Endler, 1977). Thus, quantifying the relative roles of selection and historical/ongoing stochastic factors is important to understand the process of population divergence in adaptive phenotypic and genotypic traits ( Schmidt et al., 2008).Ĭlines in genotype-specific morph/allele frequency are suitable model systems to study the relative importance of selection and historical/ongoing genetic drift in phenotypic population divergence, because the establishment of clines is potentially affected by both selection and historical/ongoing genetic drift ( Endler, 1973 Whibley et al., 2006). However, spatial variation in adaptive traits, including qualitative and quantitative traits, may be produced by both selection and genetic drift ( McKay and Latta, 2002 Strand et al., 2012). Spatial patterns in selectively neutral genetic variation result from random factors alone. Understanding mechanisms responsible for establishing spatial patterns in traits or allele frequencies is a central issue in evolutionary biology and ecology ( Endler, 1977 Huey et al., 2000 Saccheri et al., 2008). Our results suggest that a combination of two antagonistic selective factors, rather than stochastic factors, establishes the geographic cline in morph frequency in this system. Although comparison between population divergence in color and neutral loci using all populations detected only divergent selection, we detected two antagonistic selective factors acting on the color locus, that is, balancing and divergent selection, when considering geographical distance between populations. Comparisons between population pairwise F ST for neutral loci and for the color locus did not detect any stochastic factors affecting color locus. We here evaluated the role of selection and stochastic factors in population divergence of morph frequencies by comparing the divergences in color locus and neutral loci. In the damselfly Ischnura senegalensis, females exhibit color dimorphism and morph frequencies change geographically. However, it is difficult to separate these factors and detect the effects of selection when two or more contrasting selective factors are simultaneously acting on a single locus. Understanding the relative importance of selection and stochastic factors in population divergence of adaptive traits is a classical topic in evolutionary biology. Comparisons between population pairwise FST for neutral loci and for the color locus did not detect any stochastic factors affecting color locus.
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