A new manuscript accepted for publication from the Dr. Courtney Fitzpatrick Laboratory challenges a core tenet of sexual selection theory by demonstrating that costly, elaborate sexual traits can evolve in the sex traditionally considered to be limiting—such as females in many polygynous species—simply because they need to increase their mate encounter rates.
Using a quantitative genetic model, the research offers a novel mechanism for sexual selection that is distinct from the long-standing focus on competition for mates.
Overturning a Traditional View of Sexual Selection
Traditional sexual selection theory centers on relative mate limitation (competition), where individuals must compete for mating opportunities because same-sex rivals outnumber available mates. By this logic, the sex with an abundance of mates (the "limiting" sex, often females) is not expected to develop costly traits to attract partners. However, evidence from natural populations shows that individuals of the limiting sex often fail to mate at all, even when rivals are scarce. The new study resolves this discrepancy by shifting the focus to absolute mate limitation, which arises when low population density leads to low encounter rates with potential partners. "Costly signals are favored in these contexts not because individuals bearing them are better at outcompeting rivals, but because they are better at locating a mate in the environment," explains the lead researcher, Dr. James Colton Watts.
The Eco-Evolutionary Discovery
The model investigated a costly sexual trait (such as a conspicuous visual, acoustic, or chemical signal) expressed in the limiting sex that increases its detectability to mates.
The key findings demonstrate:
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Costly traits evolve and persist in the absence of competition: Sexual selection favors the elaboration of this costly trait as long as the marginal increase in the number of offspring produced outweighs the trait's marginal increase in mortality (e.g., from higher predation risk due to increased conspicuousness).
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Stable equilibrium: The evolution of the trait is governed by complex eco-evolutionary dynamics where the trait's evolving value and the population's ecological variables (like abundance) constantly influence each other. This process generally causes the sexual trait to converge on a single, stable eco-evolutionary equilibrium value that is resistant to replacement by other trait values.
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Hard selection impacts population growth: Unlike selection driven purely by competition, selection caused by absolute mate limitation constitutes hard selection, meaning the trait's evolution directly affects the mean fitness of the population and, consequently, its overall size and dynamics.
"These findings suggest a broader set of ecological contexts in which sexual selection can in principle occur," the authors write, noting that the model offers a mechanism to explain why conspicuous traits evolve in the limiting sex even where competition is weak or absent.
Implications for Biodiversity and Conservation
The research broadens the theoretical understanding of sexual trait evolution and highlights the strong, reciprocal link between evolution and population ecology. By showing that low population density and encounter rates can be a driving force for the evolution of costly sexual signals, the model suggests a critical area for future research, particularly in threatened species where small, sparse populations may be experiencing a high degree of absolute mate limitation.