How sexual conflict can cause population collapse






Evolutionary dynamics of trait values ​​and their population effect when the state is environmentally determined. (ONE and B) show trait expression and population sizes respectively from a simulation where the internal female fertility is high (b = 50, eq. 5), so that the population persists in an ecoevolutionary equilibrium. (Cand D ) show trait expression and population sizes respectively from a simulation where the internal female fertility is low (b= 10), so that the population dies out due to male injury (evolutionary suicide). In property development panels (ONE and C), dots show trait values ​​for high- and low-state males and females (dark blue for high-state male traits, xH, t; light blue for male traits in low state, xL, t; dark red for female traits in high condition, yH, t; light red for low state female characteristics, yL, t). Dashed lines show expected trait expression at ecoevolutionary equilibrium (xH* and xL* from Eq. 8and yH* and yL* from Eq. 10), with the same color scheme as for dots. In demographic panels (Band D ), dots show the number of high and low condition males and females (dark blue for high condition males, NmH, t; light blue for men with low fitness, Nml, t; dark red for women with high fitness, NfH, t; light red for women with low fitness, NfL, t). Dashed lines show expected population sizes at ecoevolutionary equilibrium (calculated by plugging phenotypic equilibria from Eqs. 810 into Eqs. A-1 and A-10). Values ​​for all dots are calculated every tenth generation of individual-based simulations (SI Appendix, Appendix A.4 for details of simulation procedure). Other parameters used in all panels: r = 0.5, β= 0.002, vH= 0.8, vL= 0.2, k= 2, kz= 1, ka= 1, χm= 1, χf= 1, γ= 1, d= 0.8, Pm= Pf= 0.5. Credit: Proceedings of the National Academy of Sciences(2023). DOI: 10.1073/pnas.2211668120

Males of a species that develop traits for sexual conflict can cause problems for females, and eventually the entire population.

A new model by researchers from Imperial College London and the University of Lausanne, published in Proceedings of the National Academy of Sciencesshows how so-called ‘good genes’ can sometimes cause a population to collapse.

Males of all species can compete for females, either by fighting other males for access or impressing females to win their approval. In both cases, males who express the most competitive traits—such as the best ornaments, such as peacock feathers, or the best weapons, such as large body size—have access to more females.

To have the best qualities, the males must be in good condition, for example to be in better shape or carry less disease. Over time, as better-fit males mate with more females, the prevalence of “good genes” increases throughout the animal population, causing the population as a whole to improve in fitness.

However, it can also backfire. Traits other than enhancing a male’s competitiveness can also harm females. For example, some male insects have evolved penises that tear the insides of females, and in many species, including mammals, males have evolved to harass females to induce mating. These behaviors reduce female fertility or can even kill them.

The team’s model tested theories of sexual competition in which men harm women, and compared the results with data from various population experiments. Previous experiments have shown conflicting accounts of whether sexual selection is positive or negative for the population as a whole. The new model provides an explanation for why some experiments show that male condition improves, without female fitness or viability improving.

Lead author Dr Ewan Flintham, from Imperial College London and the University of Lausanne, said: “Where men develop selfish traits that help them win individually, they can actually end up causing the population to crash – it’s a form of evolutionary suicide . Even when females evolve to counteract male damage and prevent population collapse, populations are still significantly reduced, reducing viability.”

Sexual interactions such as these are an important component of understanding population demography and conservation. For example, where there are more males, sexual competition increases, meaning that harm to females is more likely. This also applies in human-managed populations, for example domestic carp, where males and females must be isolated during the spawning season.

Dr Flintham completed the research as part of the Center for Doctoral Training in Quantitative and Modeling Skills in Ecology and Evolution at Imperial.

His project supervisor and study co-author Professor Vincent Savolainen, director of the Georgina Mace Center for the Living Planet at Imperial, said: “Male harm evolved in nature as something that was supposed to be good, but which is harmful to women and whole Questions like how and why this happening can only be answered with quantitative methods – data and mathematical models – which can be just as important as field studies.”

More information:
Ewan O. Flintham et al, Male injury outweighs the demographic advantages of good genes,Proceedings of the National Academy of Sciences(2023). DOI: 10.1073/pnas.2211668120

Journal information:
Proceedings of the National Academy of Sciences

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