Birds make use of a range of different mating systems to maximize their reproductive output. Ninety percent of bird species are monogamous, meaning that they have a single partner during a breeding season. Partnerships may last only the time it takes to raise the chicks, or they may last a lifetime, as in the case of swans and geese. Having two parents sharing the work in this way reduces the load on each parent and is considered the most effective way of ensuring chick survival.
However, a small percentage of birds are polygamous. This means that a bird will have two or more partners of the opposite sex at the same time, and parents will not share equally in chick-rearing duties. In the case where a male can monopolize a group of females, this is called polygyny. He will protect and fertilize all the females, but will play no part in brood rearing. Where a female mates with several males (polyandry), she leaves each male to brood and rear the chicks alone, while she seeks out more breeding opportunities.
Scientists do not know why these different mating systems have evolved. They assume that each system maximizes the reproductive output of birds in different environmental circumstances, but this has not been proven. In a new study, an international team led by scientists from Bath university has analyzed the genomes of 150 bird species, spanning all the major bird families and from locations across the world, and has come up with a novel explanation for the evolution of polygamy in birds.
The researchers measured the levels of heterozygosity in the genomes of the birds. This refers to the extent of differences between the genes inherited from a bird’s mother and from its father. Greater heterozygosity is an indication of increased genetic diversity, which is important for bird populations to be able to adapt to changing environmental circumstances. But when the researchers estimated the levels of genetic diversity across each species, there was no evidence that polygamous species were any more diverse than monogamous species.
They also looked at the frequency of gene mutations in each species and whether these mutations affected the production of the proteins they coded for, or whether they were “silent”. The former, called non-synonymous polymorphisms, are often detrimental to the individual, whereas silent mutations are not expressed and are generally harmless.
The results, published in the journal Evolution, showed that, relative to the number of silent mutations, the polygamous bird species had significantly fewer potentially damaging mutations that changed the coding sequences for proteins.
Kees Wanders, PhD student at the University of Bath and first author of the paper, explained the significance of this result: “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.”
Previous studies on fruit flies (Drosophila melanogaster) in the laboratory have also identified genetic differences that occur in male and female flies that are polygamous, in comparison with those that are monogamous. These genetic changes are not necessarily beneficial to the females, but appear to serve the interests of the males.
“There are many different theories about why polygamy evolved in these [animal] 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,” said Wanders. “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.”
“I had a hypothesis that in polygamous species, where individuals can’t easily find a mate and have to travel further to find a breeding partner, that this would mean there would be more genetic diversity in these species. However, we were surprised to find there was no evidence for this – instead we found that these species had fewer harmful mutations,” said study co-author Dr. Araxi Urrutia.
“Despite this apparent evolutionary advantage, most birds tend to stick together to raise their chicks because it gives their offspring a better chance of survival.”
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