What Is Outbreeding?

Ray Hawk

Outbreeding is a phenomenon where individuals within a species will tend to breed with others who are neither close relatives nor distant genetic relations, but a middle ground of the two. The process involves what is known as kin recognition, which all species appear to have, including humans. Kin recognition is an innate ability to recognize that members of a species are genetically closely related, and therefore avoid breeding with them to avoid genetic deformities from arising in the offspring. This was considered only a trait in humans as recently as 30 years ago, and now is considered inherent in everything from frog tadpoles to birds and apes.

Prairie dogs, a type of ground squirrel, can recognize family members by scent.
Prairie dogs, a type of ground squirrel, can recognize family members by scent.

Kin recognition to maintain outbreeding is considered so important that it can be a dominant feature of cognitive processing in lower life forms. In a scientific study of desert woodlouse in North Africa, Hemilepistus reaumuri, thousands of field observations established that not one case of mistaken identity occurred in familial groups in which they live. Woodlouse individuals identify each other by scent, and have a brain of 10,000 neurons, where 6,000 of these are devoted to processing chemical odors. They live in burrows of up to 80 individuals closely spaced near other burrows. The fact that they dedicate over half of their mental ability to identifying close relatives is evidence of the importance of the function in reproduction.

Optimal Outbreeding theory is the larger conceptual framework for the outbreeding effect and states that mating occurs in species neither too close or too far from a genetic center to prevent the pairing up of defective alleles or genes, which can lead to unexpected mutations. Mating with individuals too far from the genetic norm is also seen as dangerous because it can bring destabilizing traits into the species population. While the theory remains somewhat controversial with the discovery of inbreeding in some species, evidence for it continues to mount.

Examples of kin recognition that encourage outbreeding have been detected in a diversity of life forms on Earth. Bank swallows remember both nesting locations and the sound of offspring voices to avoid inbreeding. Ground squirrels use scent to distinguish between kin and non-relatives, and are so precise that males can recognize full sisters from half sisters.

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

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

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