Alternative phenotypes and sexual selection: can dichotomous handicaps honestly signal quality?

Considerable theoretical and empirical effort has been focused on the potential of continuously variable sexual traits to honestly indicate male quality, but relatively little effort has been devoted to a similar evolutionary role for dimorphic traits. Male dimorphisms, associated with conditionally expressed alternative reproductive tactics, represent extreme phenotypic plasticity. Evidence suggests that considerable heritable variation exists in the 'liability' underlying many threshold traits; if this liability is correlated with the genetic quality of males, dimorphic traits have the potential to be reliable indicators. We investigated the genetic architecture of phenotypically plastic morph expression in the context of condition-dependent signalling theory. Male morph in the mite Sancassania berlesei is condition dependent: 'fighters' armed with thickened and sharp third pairs of legs emerge from heavier nymphs than unarmoured 'scramblers'. We selected on male morph in three replicate 'fighter' and 'scrambler' lines and recorded a significant response to selection over seven generations; this was due to a shift in the threshold reaction norm but the lines showed no correlated response in condition. This is inconsistent with models predicting a substantial genetic correlation between condition and sexual trait expression. We discuss why dimorphic sexual traits may show more condition-independent genetic variance than continuous sexual traits.