Sexual selection leads to more rapid evolution of new species

When males are forced to compete for females, new species form more rapidly. This has been shown in a new study where the researchers compared beetles allowed to mate freely with groups of the same species where competition for mating was restricted. The experiments, which have been going on for 40 years and have followed 200 generations of beetles in direct lineages, explain how important sexual selection is for the emergence of new species.

Species are formed when existing species diverge and become new species. Today, researchers are able to track this divergence process directly from species’ DNA. But identifying exactly which biological factors and processes lead to species formation has proven to be a challenge.

One of the processes proposed is sexual selection. It is a form of natural selection that favours traits that provide a competitive advantage in reproduction with other individuals of the same sex, rather than for survival itself. For example, it is common for males to develop gaudy colours to attract attention or weapons to fight off their rivals. Sexual selection tends to be stronger in groups of animals where one female pairs with multiple different males – which means stronger competition among males for who gets to fertilise their eggs. Sexual selection is weaker where mating only occurs once, such as in mute swans and many mosquito species.

First stage of species formation can be observed in the lab

But species formation takes a long time, making it difficult to directly observe this evolution in animals and plants in real time. A research team from the universities of Uppsala and Belgrade have now succeeded in doing just this in a long-term study of a beetle species – the seed beetle Acanthoscelides obtectus. By comparing groups of beetles that were allowed to mate freely, with other groups where competition for mating was restricted for 200 generations, they were able to observe that the populations evolved in different directions.

“Previous research has suggested that competition between males plays an important role in the formation of new species. The unique thing about our study is that we ‘ve been able to confirm this through experiments in the lab that have been going on since 1986. Although the evolution of new species takes longer than 200 generations, we can clearly observe the signs of the earliest stages of species formation. The populations that evolved under conditions of strong sexual selection ended up being far more different from each other – both genetically and in reproductive characteristics. We have also observed that they evolved in different directions more than twice as rapidly when sexual selection was strong,” says Göran Arnqvist, Professor of Animal Ecology at Uppsala University and the study’s lead author.

Based on almost 40 years of studies of seed beetles

In 1986, the researchers divided a large population of these beetles into two groups of smaller populations, where half were able to evolve under conditions of strong sexual selection, and half under conditions of weak sexual selection for about 200 generations in direct lineages. In one group, the females were able to mate with several different males, which created strong sexual selection. In the second group, the females were allowed to mate only once, which reduced the competition between males. One of the findings was that the males that evolved under conditions of strong sexual selection were larger and mated more often and for longer. They also saw changes in the proteins that affect which sperm fertilise the eggs.

“In animals with internal fertilisation, the male not only transmits sperm during mating, but also a complex cocktail of proteins and peptides. This cocktail is crucial for which sperm gets to fertilise the egg. We observed that the cocktail changed more in the seed beetles that were living under conditions of strong sexual selection. This also applied to the receptors in the female’s bodies,” says Göran Arnqvist.

It is this type of small change that in the long run leads to the formation of new species.

“This protein cocktail, but also the beautiful feathers or plumage that males develop in certain animal species, are all characteristics that have to do with reproductive success and can lead to evolution taking off. 200 generations are not enough, but when we’re talking about 20,000 or 200,000 generations – in that case the populations may become so different that they can no longer breed with each other. Among insects and fish, for example, it’s been observed that there are more species in groups of these animals where the males are brightly coloured or adorned in various ways than in groups where the sexes look more similar,” Arnqvist says.