How sexual strife can lead to population collapse if 'good genes' turn bad

Darwinism indicates that in nature, the fittest survive. But could that be a bad thing?

A new model by Imperial College London and the University of Lausanne researchers is showcasing how ‘good genes’ can sometimes cause a population to collapse, and the result is not pretty.

This is according to a press release by the first institution published on Friday.

It all has to do with sexual competition. The theory suggests that the fittest and healthy males of each population in nature will logically mate more, producing more of its powerful genes.

But what if that weren't; a good thing?

The researchers’ new model tested theories of sexual competition and compared the results with data for various population experiments in order to provide an explanation for why some experiments show male condition improving without female fitness or population viability improving alongside.

Evolving selfish traits

 “Where males evolve selfish traits that help them individually win, they can actually end up causing the population to crash – it’s a form of evolutionary suicide. Even when females evolve to counter male harm and prevent population collapse, the population still decreases significantly, reducing its viability,” said first author Dr. Ewan Flintham, from Imperial College London and the University of Lausanne.

This indicates that traits that improve a male’s competitive prowess can sometimes damage females. A good example of this is how some insect males have evolved penises that tear the females’ insides, and in many species, including mammals, males have evolved to harass females to induce mating. 

“Male harm evolved in nature as something that was supposed to be good, but is detrimental to females and the whole population. Questions like how and why this happens can only be answered with quantitative methods – data and mathematical models – which can be just as important as field studies,” concluded in the statement project supervisor and study co-author Professor Vincent Savolainen, Director of the Georgina Mace Centre for the Living Planet at Imperial.

The study is published in the Proceedings of the National Academy of Sciences.

Study abstract:

Sexual conflict can arise when males evolve traits that improve their mating success but in doing so harm females. By reducing female fitness, male harm can diminish offspring production in a population and even drive extinction. Current theory on harm is based on the assumption that an individual’s phenotype is solely determined by its genotype. But the expression of most sexually selected traits is also influenced by variation in biological condition (condition-dependent expression), such that individuals in better condition can express more extreme phenotypes. Here, we developed demographically explicit models of sexual conflict evolution where individuals vary in their condition. Because condition-dependent expression readily evolves for traits underlying sexual conflict, we show that conflict is more intense in populations where individuals are in better condition. Such intensified conflict reduces mean fitness and can thus generate a negative association between condition and population size. The impact of condition on demography is especially likely to be detrimental when the genetic basis of condition coevolves with sexual conflict. This occurs because sexual selection favors alleles that improve condition (the so-called good genes effect), producing feedback between condition and sexual conflict that drives the evolution of intense male harm. Our results indicate that in presence of male harm, the good genes effect in fact easily becomes detrimental to populations.