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

Abstract

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.

The diversity of eco‐evolutionary dynamics: Comparing the feedbacks between ecology and evolution across scales

“…The system dynamics, that is the complexity and strength of the link between the eco‐to‐evo and evo‐to‐eco pathways, will vary with the different combinations of pathways. In this issue, Croll, Egas, and Smallegange () highlight a closed eco‐evolutionary feedback loop at the individual and population level in the male‐dimorphic bulb mite ( Rhizoglyphus robini ) by parameterizing a model with life‐history data. They show that the ecological effect induced by intraspecific killing by individual fighter males extends the range of life‐history parameters under which the alternative reproductive tactics evolve (eco‐to‐evo pathway).…”

“…These example scenarios show how sexual conflicts emerge when traits that benefit individual males’ fitness increase in frequency in the population at a cost to female fitness and hence population maintenance. The contribution by Croll et al () on the eco‐evolutionary dynamics of alternative male tactics in the bulb mite provides a worked‐out example of this case.…”

“…tenance. The contribution byCroll et al (2018) on the eco-evolutionary dynamics of alternative male tactics in the bulb mite provides a worked-out example of this case.To reveal the role of sexual selection in eco-evolutionary dynamics, it will be important to firstly identify the genetic basis oftraits evolving under sexual selection. It will be crucial to understand confounding factors linked to the nature of sexual selection such as the evolution of sex-specific traits as well as the evolution of sexual conflict.…”

Coevolution of female fidelity and male help in populations with alternative reproductive tactics

“…These models typically focus on the competition between different male tactics while ignoring the interactions with females. Even in those that explicitly considered the female part of the population [26,27], the fecundity of females was unaffected by the phenotypes of males that they interacted with.…”

Toward an understanding of the chemical ecology of alternative reproductive tactics in the bulb mite (Rhizoglyphus robini)

“…Importantly, ART expression in R. robini does not depend on population density, unlike in its sister species R. echinopus [41], nor on ART frequency [42]. Upon maturity, male R. robini develop into one of two distinct morphs (see Figure 1): (1) juvenile males that are relatively large mostly mature as "fighters", which possess a thickened third leg pair with a sharp end that can be used to kill conspecifics [45,46], and (2) juvenile males that are relatively small mostly mature as "scramblers", which lack the weaponized leg pair (although a rare third morph, the megascrambler, has recently been suggested; [47]). Although scrambler expression is regulated by a (partially) genetically determined threshold for body size [17; 48; 49], gene-by-environment interactions also play a key role [50], with diet quality and quantity thought to be the primary drivers of body size and therefore ART expression [18].…”