Brotherly Love and the Evolution of Sexual Conflict

Common fruit flies (Drosophila melanogaster) limit their «harassment» to females when they compete with their brothers, helping them age more slowly and leave more offspring. The romantic idea of sexual reproduction as a cooperative phenomenon disappeared in the last decades thanks to many examples in nature in which competence for a female ends up damaging her. Some of the best-known examples include spiked penises that destroy the female’s reproductive tract to keep her from mating with other males in the future, or the presence of toxins in the ejaculate of some fly species such as the vinegar fly. These toxins manipulate the nervous system of the female to favour their own offspring (at the expense of others’) but cause the premature death of the female. More commonly, males of many other species woo females with such an intensity to defeat their rivals that they end up causing irreparable damage.

From the point of view of males, females are just another finite resource in the environment for which they need to compete, and evolution will favour any adaptation that helps males win this battle (leave more offspring than their rivals), independently of the damage to females. The evolution of harming the females is a paradigmatic example of what is known in evolutionary theory as the «tragedy of the commons». As a result from selfish individual fighting (of males, in this case), the productivity of the group (a population or species) is diminished, sometimes to the point of extinction. However, in nature, conflict and cooperation in social interaction are only the extremes of a continuum in which we can place every species, and one of the challenges for evolutionary biology is to explain how selection pressure (ecological factors) pushes one species one direction or another along this continuum. In other words, figuring out the processes that explain the evolution of conflict and cooperation in reproduction.

«From the point of view of males, females are just another finite resource in the environment for which they need to compete»

«In nature, conflict and cooperation in social interaction are only the extremes of a continuum in which we can place every species»

A paper by researchers at the University of Oxford published in Nature this week proves that one of the factors favouring the transition from conflict to cooperation is kin selection. The first author of the paper, Pau Carazo, usual Mètode’s contributor got his degree at the University of Valencia, and his PhD researching at the Cavanilles Institute for Biodiversity and Evolutionary Biology. He took a Marie Curie Fellowship in Oxford, where he currently works.

Kin selection refers to all those behaviours that are favoured by evolution because they increase not only the rate of breeding success of an individual (his genes are directly transmitted to the next generation), but also indirectly through the offspring of kin with whom he shares genetic material (his genes are indirectly transmitted). According to the results of the project, when males of the vinegar fly species compete against brothers, they reduce their aggressiveness and intensity, allowing females to age more slowly and produce more offspring. The reason being that any collateral damage caused to females will finally harm their own brothers and, indirectly, the transmission of their genes to the next generation. The research proves that males have evolved to behave in a flexible way depending on the composition of the group in which they are inserted, behaving less aggressively and thus favouring group productivity when they compete against kin. In the past, kin selection explained the evolution of seemingly altruistic social behaviour in animals. This last project proves it can be also very important to understand the evolution of conflict and cooperation in breeding.

© Mètode 2014.

«The research proves that males have evolved to behave in a flexible way depending on the composition of the group in which they are inserted»