From a meta-analysis, published in Nature Ecology & Evolution, scientists have compiled 139 experimental studies that indicate "little support" for the inbreeding aversion hypothesis in 88 animal species.
For ethologist Raïssa de Boer, co-author of the study, "evolutionary theory tells us that animals must tolerate, or even prefer, to mate with a family member."
Inbreeding would have certain advantages. In particular, it could improve the "selective value" of an individual, that is, its ability to reproduce and obtain fertile offspring - preferably in large numbers.
Incest also increases the risk of developing a genetic disease: within a family, two individuals are more likely to have the same genetic deficiencies. They are compensated for by better genes, but if these individuals mate, their children may inherit two defective genes. There is no way to compensate for this.
Inbreeding in passerines is common.
A population that regularly resorts to incest sees the number of sick people increase. The chances of conceiving large and fertile offspring are reduced. But in some cases, the ill effect of incest may not occur if there is a 'purge' - all the individuals who received defective genes die before reproducing. Gradually, the individuals who survive inherit only the good genes.
"Animals don't seem to care whether their potential mate is a sibling, cousin or unrelated individual when choosing who to mate with," said Regina Vega Trejo, also an ethologist and co-author.
The researchers conclude that while there may be a price to pay for inbreeding, in animals this may not be heavy enough to create a selective pressure that outweighs other aspects of mate choice. The pressure to avoid kin is probably weakest in animals where populations are large and well-mixed.
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