Temporally Variable Multivariate Sexual Selection on Sexually Dimorphic Traits in a Wild Insect Population

Abstract

A widely held view is that the strength and form of natural selection varies in time and space in response to varying ecological forces; however, adequate quantitative evaluations of this are relatively scarce. In this study, we measured the strength and form of sexual selection acting on a suite of male morphological traits in a wild ambush bug (Phymata americana) population at 10 sampling dates over 2 years. We tested the prediction that the strength and direction of sexual selection would be associated with one or more important ecological variables. We found that patterns of multivariate selection varied considerably over time, and even within a season. Yet, for this population, a sexually dimorphic color pattern trait was consistently a target of directional selection. The strength of sexual selection on this trait was related to both sex ratio and density, which is consistent with the idea that ecological factors can play an important role in generating patterns of sexual selection. We also demonstrate that the median strength of linear selection obtained from replicated cross-sectional methods was qualitatively similar to the estimates obtained from longitudinal methods, providing multiple lines of evidence that the evolution of sexual color dimorphism in this species is attributable to sexual selection.

Differences in the temporal dynamics of phenotypic selection among fitness components in the wild

“…Although temporal variation in natural selection has long been appreciated [8], studies of sexual selection have also increasingly shown it to vary temporally and, much like natural selection, to be driven by variation in environmental conditions [9]. For example, temporal variation in sexual selection has been attributed to variation in ecological factors such as density [10 -12], climate [13] and sex ratio [10,12], and many of these same factors may also impact temporal variation in selection through survival or fecundity [6,7]. In fact, the dynamics of sexual selection are especially prone to temporal environmental fluctuations because of the often environmentally context-dependent nature of female preferences [14 -16], and environmentally dependent expression of many sexually selected traits [17,18].…”

Ecological correlates of daily mating frequency in a wild population of ambush bugs

“…This might explain episodes of negative selection on (i.e. disfavouring of) dark male coloration reported in the wild (Punzalan et al ., 2010), and a trend towards negative selection under warmer ambient conditions in an experimental setting (Punzalan et al ., 2008a). The mechanistic causes are unclear but excessively high temperature is known to hinder various aspects of insect physiology and fitness (e.g.…”

“…The importance of demographic variables for mating success is generally established in insects (e.g. Thornhill & Alcock, 1983;Rowe et al ., 1994) and, with respect to P. americana, previous work showed that the strength of sexual selection on male colour covaried positively with density and negatively with sex ratio (Punzalan et al ., 2010). Our findings suggest that fluctuating selection on male secondary sex coloration is tied to the effects of sex ratio on female mating frequency.…”

“…On 27 days between 29 July and 26 September in 2003, we observed individuals during 2-6 h walks along 15 parallel transects (within a 2700 m 2 plot), noting sex and mating status. Observations were pooled across transects and based on a total of 1276 observations from 476 uniquely marked individuals (described in Punzalan et al ., 2010). Many individuals were observed repeatedly on the same as well as on different days.…”

“…Previous work has shown that the strength and direction of sexual selection covary with population density and sex ratio (Punzalan et al ., 2010). These studies suggest the importance of these ecological variables on mating frequency, but a formal examination has been previously lacking.…”