Choosing a mate is key to the evolution and survival of a species, contributing to whether the most attractive and beneficial traits are passed on to future generations. It is generally thought that the female of a species should choose a genetically dissimilar male to mate with. This introduces a greater amount of genetic diversity to resultant offspring, potentially conferring enhanced fitness, and notably avoiding the negative impacts of inbreeding. However, in certain environments the opposite has been shown to be true. In a recent study in BMC Evolutionary Biology, Gabrielle Sorci from the University of Burgundy, France, and colleagues reveal how an isolated population of house sparrows prefer genetically similar mates. Here Sorci discusses what led to these surprising findings and whether this may occur more widely.
How much do we know about the use of genetic similarity as a criterion in mate choice?
Genetic similarity is very often mentioned as being one of the most important factors shaping mate choice in a wide range of organisms. The conventional wisdom was that animals should avoid mating with close relatives because of the risk of inbreeding depression. However, there is accumulating evidence showing a continuum of mate preference, from genetically distant to genetically similar mates.
What inspired you to conduct this project and why use an insular island population of birds?
Insular populations offer the opportunity to tackle questions that can be hard to address in other environmental conditions. Insular populations are often small, and poorly connected with other populations. This increases the risk of losing genetic variation, and we wished to study the mechanisms that might contribute to maintain genetic variation in such populations.
What did you expect to find when you began the study and how different were your actual results?
Given the small population size and the isolated nature of the population, we expected that there should be strong selection pressures to avoid mating with close relatives. Our results did not support this prediction and if any tended to show that females might prefer to mate with genetically close males. It is important to remember that we could only assess the ‘realised’ mate choice and that this might differ from an ideal ‘preference’ because many constraints can operate on this ideal preference.
You found an especially high level of extra-pair paternity, where offspring raised by one pair would be the result of a female mating with a male outside of this pairing. How does that tie into your overall conclusions?
This was also quite a surprise because insular populations are usually expected to have low levels of extra-pair paternity. The choice of extra-pair mates tended to confirm the idea that females might prefer to mate with genetically close individuals.
How widely would you expect these patterns to be replicated, both in larger populations of house sparrows and in other isolated and non-isolated species?
This is difficult to say and is actually one aspect that we would like to explore in the future. To be more precise we would like to know not only how spatial variation affects the pattern of mate choice that we observed for this population, but also the importance of temporal variation (variation among years). This of course requires a large effort and long term monitoring of several populations, which is difficult to achieve with the prevailing funding scheme.
It has long been held that animals will in general avoid inbreeding. Do you think this idea needs rethinking?
This idea has already been reassessed. More than 30 years ago Patrick Bateson published a paper in Nature showing that quails prefer to mate with first cousins. The idea that mate choice should balance the cost of both inbreeding and outbreeding is now relatively well established.
Questions from Christopher Foote, Executive Editor for the BMC Series.