Habitat Complexity Drives Experimental Evolution of a Conditionally Expressed Secondary Sexual Trait

Abstract

The conditional expression of alternative phenotypes underlies the production of almost all life history decisions and many dichotomous traits, including male alternative reproductive morphs and behavioral tactics. Changes in tactic fitness should lead to evolutionary shifts in developmental switch points that underlie tactic expression. We used experimental evolution to directly test this hypothesis by rearing ten generations of the male-dimorphic mite Rhizoglyphus echinopus in either simple or three-dimensionally complex habitats that differed in their effects on morph fitness. In R. echinopus, fighter males develop weapons used for killing rivals, whereas scrambler males do not. Populations evolving in complex 3D habitats, where fighters had reduced fitness, produced fewer fighters because the switch point for fighter development evolved to a larger critical body size. Both the reduced mobility of fighter males and the altered spatial distribution of potential mates and rivals in the complex habitat were implicated in the evolutionary divergence of switch point between the habitats. Our results demonstrate how abiotic factors like habitat complexity can have a profound effect on evolution through sexual selection.

Paternal Effects on the Expression of a Male Polyphenism

“…In R. echinopus, male dimorphism is environmentally cued by body size and male density (Radwan 2001;Tomkins et al 2011), and in a previous study we suggested that there is large genetic variation for the switch point that links male morph expression to the body size cue (Buzatto et al 2012). However, the heritability of switch point variation is constrained by the fact that male dimorphism is sex limited in its expression: genes that affect the switch point controlling male dimorphism are not expressed in females.…”

“…In contrast, scramblers' legs are all equally thin and without a sharp tip, and scramblers search for unguarded females to mate with (Radwan 1993;Radwan 2009). Male dimorphism in the bulb mite R. echinopus is environmentally cued by body size and male density (Radwan 2001;Tomkins et al 2011). …”

“…We studied this question in the acarid mite Rhizoglyphus echinopus, a species in which males are conditionally fighters or scramblers. Male dimorphism in R. echinopus is environmentally cued by body size and male density (Radwan 2001;Tomkins et al 2011), but there is a great deal of overlap in the sizes of males that become either fighters or scramblers, even under the same male density. This overlap suggests that there is either a large genetic variation for the switch point that links male morph expression to the body size cue, or alternatively some genotypes in the population are entirely canalized to expressing one of the morphs regardless of body size and male density (Lively et al 2000;Buzatto et al 2012).…”

Investigating the genetic architecture of conditional strategies using the environmental threshold model

“…Evolutionary changes in body size do sometimes track changes in thresholds of alternative male phenotypes [37,38], and this could just be due to frequency-dependent selection acting on male morphs [39]. However, across numerous island populations of these earwigs, it has previously been shown that the frequency of male morphs in the population varies widely due to changes in the position of the threshold relative to the body size distribution ( [40], see also [41]).…”

“…The closest to such a test was a study on the environmentally cued pupal colour dimorphism in butterflies [42], where the authors found no evidence for a correlation between the preference for green or brown pupation sites and the tendency to produce green or brown pupae. Similarly, by manipulating habitat complexity, an artificial selection experiment with bulb mites found that male body size and threshold responded to selection in opposing directions; increased habitat complexity caused lines to evolve towards smaller bodies, whereas the thresholds for major morph expression evolved towards larger bodies, both changes causing a decrease in the frequency of majors [37].…”