Antagonistic selection factors induce a continuous population divergence in a polymorphism

Abstract

Understanding the relative importance of selection and stochastic factors in population divergence of adaptive traits is a classical topic in evolutionary biology. 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. In the damselfly Ischnura senegalensis, females exhibit color dimorphism and morph frequencies change geographically. 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. Comparisons between population pairwise F ST for neutral loci and for the color locus did not detect any stochastic factors affecting color locus. 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. 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.

Non-adaptive radiation in damselflies

“…Molecular work across 22 European I. elegans populations demonstrates low genetic differentiation (mean F ST = 0.06), presumably because of efficient dispersal (Wellenreuther et al 2011) (Table 1). Similar levels of genetic differentiation were detected across 30 populations of I. senegalensis in Japan (F ST = 0.10), another wide ranging species of the Palearctic 'elegans' clade (Takahashi et al 2013). Even species with more restricted distributions show similar levels of genetic differentiation (data derived from four populations of each species covering their distribution: I. graellsii F ST = 0.03; I. genei F ST = 0.13; and I. saharensis F ST = 0.09, R.…”

Evolution of increased phenotypic diversity enhances population performance by reducing sexual harassment in damselflies

“…Here we describe the ecological assumptions underlying this model and present the mathematical model in more detail. We derive predictions for equilibrium morph frequency, population density and the stability of populations by considering the variation in model parameters caused by a morph-specific geographic cline in female fitness 22,23 . Female colour morph diversity within a population (D) was calculated using the Shannon species diversity index 36 …”

“…In natural populations, the female morph frequency exhibits clinal variation, which is generated by the difference in potential fecundity between female morphs in each population 22,23 . Previous studies have shown that the potential fecundity of the different female morphs of I. senegalensis changes with latitude 22 .…”

“…Morph frequencies in both I. senegalensis and I. elegans geographically vary, with higher andromorph frequency at higher latitudes 22,23,42 . At least for I. senegalensis, this cline in morph frequency is established by the combination of negative frequency-dependent selection, which is derived from frequency-dependent sexual harassment, and genotype-byenvironment interaction set by different temperature conditions along the north-south cline 22,23 . Because andromorphs have slightly higher fitness than gynomorphs at lower latitudes but lower fitness at higher latitudes; therefore, the equilibrium frequency changes with latitude.…”

“…In natural populations of I. senegalensis, morph frequency exhibits clinal variation 22,23 . Our previous study suggested that this variation in morph frequency is generated by the gene-byenvironment interaction; the potential fitness of andromorph females was lower than that of gynomorph females at low latitudes, but was higher at high latitudes 22 .…”