An eco-evolutionary feedback loop between population dynamics and fighter expression affects the evolution of alternative reproductive tactics

Surprisingly, little is known about how eco-evolutionary feedback loops affect trait dynamics within a single population. Polymorphisms of discrete alternative phenotypes present ideal test beds to investigate this, as the alternative phenotypes typically exhibit contrasting demographic rates mediated through frequency or density dependence, and are thus differentially affected by selection. Alternative reproductive tactics (ARTs), like male fighters and sneakers, are an extreme form of discrete phenotype expression and occur across many taxa. Fighters possess weapons for male-male competition over access to mates, whereas sneakers are defenceless but adopt tactics like female-mimicking. Because fighters in some species mortally injure conspecifics, this raises the question whether fighter expression can feed back to affect population size and structure, thereby altering the selection gradient and evolutionary dynamics of ART expression in an eco-evolutionary feedback loop. Here, we investigated how the eco-evolutionary feedback loop between fighter expression and population size and structure affects the evolution and maintenance of ARTs. We introduced intraspecific killing by fighters in a two-sex, two-ART population model parameterized for the male dimorphic bulb mite (Rhizoglyphus robini) that includes life-history differences between the ARTs and a mating-probability matrix analogous to the classic hawk-dove game. Using adaptive dynamics, we found that the intraspecific killing by fighters can extend the range of life-history parameter values under which ARTs evolve, because fighters that kill other fighters decrease fighter fitness. This effect can be nullified when benefits from killing are incorporated, like increased reproduction through increased energy uptake. The eco-evolutionary feedback effects found here for a dimorphic trait likely also occur in other fitness-related traits, such as behavioural syndromes, parental care and niche construction traits. Current theoretical advances to model eco-evolutionary processes, and empirical steps towards unravelling the underlying drivers, pave the way for understanding how selection affects trait evolution in an eco-evolutionary feedback loop.