How does nature prevent incest and therefore inbreeding in animals? Without social conditioning, what happens to discourage (if not prevent) this?
鈥� In nature, inbreeding is not always undesirable or prevented. Intensive inbreeding can in certain cases purge harmful recessive genes from a population, while ensuring that essential genes are passed on. In a sparse population, it can ensure a mate. In its most extreme form, some plants self-pollinate so intensively that outcrosses are exceptional. Conversely, many plants separate pollen production from stigma ripening to reduce self-pollination, either by wind or by pollinators. Others, such as almonds, are self-sterile, having a sort of immunity to their own pollen.
Some animals are inhibited from incest by clues to shared genes, such as communal odours, although commonly this is incidental to the ejection of adolescents from parental territory, particularly young males when they begin to show cues that identify them as competitors.
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While some rodents accept incest, a pregnant female may resorb embryos conceived this way if she smells an unknown male. Analogously, in animals that live in social units, such as lions and some primates, incoming males commonly kill the immature young of their predecessors, or cause pregnant females to abort. This is actually a competitive measure, but it does reduce incestuous reproduction.
Jon Richfield, Somerset West, South Africa
鈥� One factor thought to deter inbreeding is the idea of attractiveness. In a study looking at what makes people seem more attractive to the opposite sex, people were shown two images of the same face. When pictures of the faces were doctored to look slightly more symmetrical, people found them more attractive than the original photographs. When animals inbreed, the chance their offspring is in some way defective, or asymmetrical, is higher. This means that the asymmetrical offspring appear less attractive than their interbred counterparts, and are less likely to find a mate and repeat the habits of their parents or siblings.
In another study, participants were shown pictures of people of the opposite sex who resembled them. In general, the greater the similarity, the weaker the attraction people felt. The likeness that carries through families seems to inhibit incest. If a sibling looks like you, you are less attracted to him or her. So, in theory, even if society was impartial to incestuous relationships, we are genetically hardwired not to find members of our families attractive.
鈥淟ikeness in families inhibits incest. If a sibling looks like you, you are less attracted to him or her鈥�
Rob Hayes, Skibbereen, Ireland