A team of EU-funded researchers has demonstrated how sexual selection and environmental pressures could interact to cause a species to split into two, even where there are no geographical barriers separating the new species. EU support for the study, which was published online by the journal Science, came from an Outgoing International Fellowship from the Marie Curie researcher mobility programme.
That natural selection is behind the birth of new species is now widely acknowledged. However, 150 years after the publication of Charles Darwin's book 'On the Origin of Species', the exact mechanisms that drive the development of new species (a process known as 'speciation') remain the subject of debate.
In a simple speciation scenario, two populations of a species are separated by an insurmountable geographical barrier, such as a sea or mountain range. Over time, the populations drift apart genetically until they are so different that if individuals from the two different populations were to mate with each other, any resulting offspring would be sterile.
Yet new species also arise in situations where populations are not completely cut off from one another in this way. In this study, the researchers created a mathematical model in which two populations of a species live in different environments. The model assumes that 'specialist' individuals that are highly adapted to their own local environment will survive better and have more offspring than 'intermediate' individuals with parents from both populations that are not specialised to thrive in either environment.
As long as individuals are able to move from one environment to another, this scenario will not result in the development of new species. However, that changes when sexual selection is added into the mix.
The researchers invented an ornament in the males of their imaginary species, such as colourful plumage, whose condition is a good indicator of whether an individual is well adapted to its local environment or not. 'Individuals adapted to the local environment are likely to be in a better condition, allowing them to develop brighter plumage than individuals that are less well adapted,' the researchers explain.
In this situation, the females evolve a marked preference for males with brighter plumage, because these males are best adapted to the local environment and it follows that their offspring will most likely be well adapted to the local environment too.
Once sexual selection has started to operate in this way, interbreeding between the two populations tails off sharply, as 'hybrid' males are not well adapted to either habitat. As a result, they do not develop brightly coloured plumage, and fail to attract females.
'Thus mate choice on the basis of an indicator of local adaptation enhances reproductive isolation between specialists and should therefore increase the likelihood of speciation,' the paper reads.
The model demonstrates that even relatively weak natural selection can split a species in two if boosted by strong sexual selection.
The researchers now plan to test their theory in the field on species such as crossbill birds and sticklebacks, where biologists are studying whether individuals' attractiveness reflects their level of adaptation to the local environment.