What’s Outbreeding?

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Outbreeding is a phenomenon in which individuals within a species will tend to breed with others who are neither close relatives nor distant genetic relationships, but somewhere in between the two. The process involves what’s known as recognition of kinship, which all species appear to have, including humans. Parental recognition is an innate ability to recognize that members of a species are genetically closely related, and thus avoid mating with them to prevent genetic deformities from occurring in the offspring. This was considered to be just a trait in humans up until 30 years ago, and is now considered to be inherent in everything from frog tadpoles to birds and monkeys.

Recognition of kin to maintain outbreeding is considered so important that it may be a dominant feature of cognitive processing in lower life forms. In a scientific study of the North African desert louse, Hemilepistus reaumuri, thousands of field observations established that there was not a single case of mistaken identity in the family groups in which they live. Woodlouse individuals identify one another by smell and have a brain of 10,000 neurons, of which 6,000 are dedicated to processing chemical odors. They live in burrows of up to 80 closely spaced individuals near other burrows. The fact that they devote more than half of their mental capacity to identifying close relatives is evidence of the importance of function in reproduction.

Optimal outbreeding theory is the broader conceptual framework for the effect of outbreeding and states that mating occurs in species neither too close nor too far from a genetic center to prevent mating of defective alleles or genes, which can lead to unexpected mutations. Even mating with individuals that are too far from the genetic norm is seen as dangerous because it can lead to destabilizing characteristics in the population of the species. While the theory remains somewhat controversial with the discovery of inbreeding in some species, the evidence continues to mount.

Examples of kin recognition encouraging outbreeding have been detected in a variety of life forms on Earth. Bank swallows remember both nesting places and the sound of offspring’s voices to avoid inbreeding. Ground squirrels use scent to distinguish between kin and non-kin, and are so accurate that males can tell full sisters from stepsisters.

The process of kin recognition has even been detected in plants. The English plane tree grows faster in the presence of related than unrelated plane trees, and scientists theorize that the plants release chemicals through their root systems to distinguish them from related and unrelated ones. Other plants such as Mountain Delphiniums distinguish between close relatives and non-relatives by the pollen they release. They use this ability to avoid breeding with closely related and extremely different versions of other delphiniums in the area.

The concept of outbreeding may have revolutionary effects on evolutionary biology, suggesting that the principle of natural selection is flawed. Natural selection promotes the idea that whichever species produces the most offspring is most likely to survive and come to dominate the environment. Researcher William D. Hamilton promoted the concept of kinship recognition in 1964 at the University of Oxford as an alternative approach to conventional natural selection. By claiming that superior genes gave a species better adaptability, he was laying the groundwork for the kinship recognition and outbreeding that are now widely known in nature. The social or mental complexity of the organism also appears to be irrelevant, and outbreeding is a dominant feature of successful organisms, regardless of their place in the natural order.

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