New Species by Ladies’ Choice

Translation: Patricia Ruka-Ahr (

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Schindler, S., Breidbach, O., & Jost, J. (2013). Preferring the fittest mates: An analytically tractable model Journal of Theoretical Biology, 317, 30-38 DOI: 10.1016/j.jtbi.2012.09.018

Female animals who prefer healthy and fit partners are having more and fitter descendants. A gene that brings forth such mating preferences is going to spread quickly. This gene can even forward the split-up of one species into two.

Display of Greater crested terns. The more healthy a male is, the bigger are its mating chances. (c) Glen Fergus / Wikimedia Commons / Public Domain

Display of Greater crested terns. The more healthy a male is, the bigger are its mating chances.
(c) Glen Fergus / Wikimedia Commons / Public Domain

The choice of a partner has big influence on the course of evolution. On the one hand it is determining the success of reproduction of an individual. On the other hand the adaptation of species to their environment depends on which individuals are reproducing. A third consequence is, that if mating only happens within groups and not across groups, these groups will differ more and more genetically and can develop into separate species.

Females of many species are looking for a fit partner. Qualities that influence fitness are for instance weight, ability to fly and hunt, or the state of body ornaments such as plumage or horns. In most cases a competition with candidates showing their fitness precedes mating. Some male spiders are, for instance, performing dances that only healthy individuals can bear. Male dancing flies use gifts in the form of prey to buy cooperation of female flies. A male that is able to care for the supply, transportation and even packaging has better chances of mating.

Already in Darwin’s lifetime it was known that fitter individuals are more attractive as partners than average or even sickly ones. Until now this mating preference has only been described verbally. There existed no way to calculate the consequences of the choice of a partner exactly. Me and my colleagues have developed a theoretical model that now offers these kind of calculations.

One assumption of the model is that mating preferences are genetically determined. There is a gene (the “preference gene”) that causes mating behaviour. Female carriers of this preference gene prefer fit candidates. Those who do not carry the gene mate without considering the fitness of a candidate.

The reproductive success of females can be calculated using the model, depending on their preferring a fit partner or not. Hence it is possible to find out if preferring a fit partner is worthwhile, and under what conditions the preference is evolutionary successful. The evolutionary success is derived from the fact whether, and if yes, how fast the preference gene spreads within the population.

Now one can show that at individual level a preference gene pays off thrice. First, selective females can produce more descendants than non-selective ones. Furthermore, the descendants of choosy females inherit the father’s gene for high fitness. In addition to that, sons are more attractive mating partners if they have inherited the high fitness gene. Thus, choosy females are more likely to have more and fitter third generation descendants than the non-choosy ones. The preference gene is therefore evolutionary successful. Only few generations after its first appearance, for instance by mutation, all individuals will carry the preference gene.

There is a second effect of the choice of a partner regarding the adaptation of the species. The model shows that the preference gene accelerates the adaptation of the population to its habitat. For fitter partners are chosen more often, the gene of high fitness will spread within the population.

The third effect of the choice of a partner concerns speciation. Populations that settle in different habitats will differ more because a population with choosy females can adapt to their environment more quickly. This could mark the beginning of a new species. The more two populations of the same species differ genetically and phenotypically, the more likely is their evolution into two new species. The divergence is aided by different individuals being fitter in different environments.

Applying this model, researchers are able to hypothesise on the speed of the origin of species that can be tested by experiments or studies. Using merely verbal arguments this has not been possible before. Thus, the model makes an important contribution because it allows the evaluation of environmental changes or other factors interacting with the choice of a partner.