Bird species that breed with several sexual partners have fewer harmful mutations, according to new research.
The study, led by the Milner Centre for Evolution at the University of Bath and published in Evolution, shows for the first time how polygamy increases the efficiency of natural selection in wild populations.
Most birds pair up with a single mate each season. Some species, such as swans or geese, mate for life. In contrast, some bird species are polygamous, having several partners per breeding season, however it is unclear why they have evolved a different mating system.
The team of researchers sequenced the genome of polygamous Red-necked Phalaropes for the first time and found they carried fewer harmful mutations (Ed Stubbs).
An international team of scientists led by Bath analysed the genomes of 150 bird species, spanning all the major bird families and from locations across the world, including six species that were sequenced for the first time.
By counting differences between the genes inherited from the individual's mother and father (termed the heterozygosity), they were able to estimate the level of genetic diversity across each species.
They also looked at the frequency of gene mutations in each species and whether they changed the sequence of proteins for which they coded or were 'silent'. The former, called non-synonymous polymorphisms, are often detrimental to the individual, whereas silent mutations are generally harmless.
Contrary to their expectations, they found that polygamous species on the whole were no more diverse than monogamous ones, although the small number of species with polygamous females did have higher than expected genetic diversity.
They also found that, relative to the number of silent mutations, the polygamous species had significantly fewer potentially damaging mutations that changed the protein sequence.
Kees Wanders, PhD student in the University of Bath's Department of Life Sciences, is first author of the paper and explained: "Species evolve through natural selection, where harmful mutations are removed from the population in the long term because individuals with the mutation don't survive long enough to breed, or individuals with beneficial adaptations survive for longer.
"However, species also evolve through sexual selection, where evolution is shaped by individuals competing for access to mates, so that only the most desirable characteristics are successfully passed down to offspring.
"This research suggests that sexual selection aligns with natural selection in birds, so that harmful mutations are removed more efficiently in polygamous populations, where sexual selection is particularly strong. We still don't know exactly why some bird species are polygamous when most pair up for a breeding season or even for life.
"There are many different theories about why polygamy evolved in these species, but we've found the first evidence that it increases the efficiency of natural selection by rooting out harmful mutations and avoiding the effects of inbreeding.
"It's been previously observed by others in the lab in fruit flies, but this is the first time it's been observed in wild populations of birds."
The data for this study was collected as part of the B10K Project, with scientists from China, Wales, Denmark, Hungary, Germany, The Netherlands and Mexico contributing to the reseearch.
Reference
Wanders, K, Chen, G, & 7 others. 2023. Polygamy and Purifying Selection in Birds. Evolution 77: 276-288. DOI: https://doi.org/10.1093/evolut/qpac010
