Temporal variation in selection on male and female traits in wild tree crickets

Abstract

Understanding temporal variation in selection in natural populations is necessary to accurately estimate rates of divergence and macroevolutionary processes. Temporal variation in the strength and direction of selection on sex‐specific traits can also explain stasis in male and female phenotype and sexual dimorphism. I investigated changes in strength and form of viability selection (via predation by wasps) in a natural population of male and female tree crickets over 4 years. I found that although the source of viability stayed the same, viability selection affected males and females differently, and the strength, direction and form of selection varied considerably from year to year. In general, males experienced significant linear selection and significant selection differentials more frequently than females, and different male traits experienced significant linear selection each year. This yearly variation resulted in overall weak but significant convex selection on a composite male trait that mostly represented leg size and wing width. Significant selection on female phenotype was uncommon, but when it was detected, it was invariably nonlinear. Significant concave selection on traits representing female body size was observed in some years, as the largest and smallest females were preyed on less (the largest may have been too heavy for flying wasps to carry). Viability selection was significantly different between males and females in 2 of 4 years. Although viability selection via predation has the potential to drive phenotypic change and sexual dimorphism, temporal variation in selection may maintain stasis.

A potential role for restricted intertactical heritability in preventing intralocus conflict

“…The yearly variation in the strength of linear selection, and the pattern of stronger selection on males than in females, is consistent with studies of other salmonids (Seamons et al, 2007 ). Studies of other wild animal populations have also found that selection varies by year (pied flycatchers Ficedula hypoleuca (Visser et al, 2015 ), tree crickets Oecanthus nigricornis (Ercit, 2016 ), oystercatchers Haematopus ostralegus ostralegus (Pol et al, 2010 ), soay sheep Ovis aries (Milner et al, 1999 )).…”

“…The yearly variation in the strength of linear selection, and the pattern of stronger selection on males than in females, is consistent with studies of other salmonids (Seamons et al, 2007). Studies of other wild animal populations have also found that selection varies TA B L E 1 Linear (β) and quadratic (γ) selection gradients on length and their statistical significance for each year and tactic (females, hooknose, and jack males) by year (pied flycatchers Ficedula hypoleuca (Visser et al, 2015), tree crickets Oecanthus nigricornis (Ercit, 2016), oystercatchers…”

Predation promotes diversification in the mean and variance of antipredator traits

“…For instance, variation in body size is predicted to increase, if larger individuals, that are safe from predation, will proportionally grow more than smaller and more vulnerable individuals because size‐dependent behaviours change food availability and subsequently growth rate (Eklöv & Werner, 2000). In contrast, we can also expect shifts in mean body size, if predators cause different selective strength on foraging behaviour (Ercit, 2016; Reznick, 1982). In this case, body size variation is predicted to be lower under a stronger reduction in predator‐mediated behavioural changes, because decreased food intake leads to reduced growth and subsequently decreased individual variations (Peacor et al, 2007; Ziemba et al, 2000).…”