Most of us know that incest is wrong, either through cultural conditioning or what appear to be innate evolutionary cues, yet some humans can’t seem to resist the urge to have sex with their relatives. In fact, since the DNA of every living human is 99.9 percent the same, it may be much more common than we think. Still, that doesn’t mean incest can be acceptable in any case. As we will see, breeding with a close relative can result in some dire consequences.
It’s fairly common for dog breeders to preserve desirable traits within their litters through inbreeding, but frequent inbreeding over many generations causes serious problems. Most animals suffer the same adverse effects from inbreeding as humans, such as reduced fertility, slower growth rates, increased incidence of disease, and higher mortality rates. Also like humans, many creatures have evolved to form their own anti-incest mechanisms, either by migrating away from their homes or using various cues to identify their kin. Female lemurs, for instance, can tell if a male’s genes are too similar to her own by smelling the pheromones that emit from his genitals.
Mice also use smell to identify ideal potential mates, much like the human females in the smelly t-shirt test. Experiments have shown that keeping a brother and sister mouse isolated in a cage will often result in their mating, but only if there are no other viable partners. Introducing an unrelated male compels the female to abandon her brother. If the female has already been impregnated by her brother, she can even abort the pregnancy before mating with the unrelated male.
Because hyenas don’t use scent or other cues to identify relatives, females only mate with males that are new to their group. This preference for the “new guy” forces young male hyenas to leave their families upon adulthood in search of a pack of unrelated females, resulting in low levels of inbreeding among the species.
Of course, there are exceptions. The estimated 500-percent growth of the bedbug population in recent years was partially caused by a chemical resistance to insecticides, but also the bedbug’s ability to thrive on inbreeding. When bedbugs that have this resistance to insecticide continue to inbreed, doubling up on those resistant genes strengthens the resistance of the next generation, making the parasites even harder to kill and demonstrating one of the few instances in which inbreeding can actually benefit a species.