Inbreeding and Outbreeding - Left, Right, and Mother Nature

Sex, Power, and Partisanship: How Evolutionary Science Makes Sense of Our Political Divide - Hector A. Garcia 2019

Inbreeding and Outbreeding
Left, Right, and Mother Nature

Xenophobia and xenophilia among humans may be related to inbreeding and outbreeding. Much of what I share on this specific topic comes from evolutionary anthropologist Avi Tuschman's book Our Political Nature.23 Here Tuschman operationalizes inbreeding as “when kin mate with one another more than would be expected by chance” and outbreeding as “mating between individuals that have a greater genetic distance between them than one would expect at random, given the size of the population.” For humans, outbreeding could mean mating with members of a different tribe or race, or, even at one time in our distant past, mating with Neanderthals; recent genetic research has revealed that most non-Africans have up to 4 percent Neanderthal genes,24 which means when our modern human ancestors left Africa and found Neanderthals living in Europe, they mated with these fairly distant hominids.

The Benefits of Outbreeding

Outbreeding can help avert what biologists call inbreeding depression. Too much inbreeding increases risk of genetic disorders while also causing other problems, such as birth defects and higher infant mortality rates related to congenital diseases.25 Cystic fibrosis—a disorder that mostly damages the lungs but also the kidneys, liver, pancreas, and intestines—is one condition that can arise from inbreeding and one that for most of our species’ history killed young children before they reached reproductive maturity. Like other genetic diseases, this condition occurs when two copies of a problematic, recessive gene are transmitted to offspring.

Given the disastrous consequences of cystic fibrosis, why hasn't natural selection weeded out this gene eons ago? As Tuschman points out, this gene has positive as well as negative potential outcomes, as is the case for other often fatal genetic conditions like sickle cell anemia, which, in addition to causing health problems, provides resistance to malaria. Likewise, having one recessive copy of the cystic fibrosis allele is thought to help us resist a host of deadly diseases, such as cholera, typhoid, tuberculosis, and diarrhea.26 Thus a degree of outbreeding can reduce the risk of offspring with two copies of the allele.

Inbreeding can also create overspecialization for a certain environment, making populations more vulnerable during periods of environmental change. Weather can produce drastic environmental change, such as ice ages, periods of global warming, drought, or flooding, all of which can impact the availability of critical resources like water or edible plant life. Imagine the fate of an animal that is highly specialized to eat a certain fruit should weather eradicate that species of fruit tree. Similar risk can come from things like blights or pests, which often target specific plant species. The right amount of outbreeding, then, brings the genetic diversity needed to roll with the punches of environmental change.

Much of the environmental change in the lives of early humans came from migrations. Having first emerged in Africa, humans have migrated across the whole planet and continue to do so today. But migration for our early human ancestors was a far more dangerous affair than simply boarding an airplane, train, or bus. Migrations often brought violent conflict with outside groups, moving through dangerous environments like deserts or jungles, and encounters with predators, severe weather, famine, or disease, all of which culled populations and created bottlenecks where the genetic diversity of populations became constrained. Notably, bottlenecks often reduce genetic resistance to infectious disease.27 However, mating with local populations, who had had many generations to adapt to their environment, including time to develop resistance to local pathogens, would have produced better adapted offspring and restored needed genetic diversity to a population.

In making this point, Tuschman cites genetic research that has uncovered some fascinating facts about a family of genes related to the human immune system, called human leucocyte antigens (HLAs). HLAs play a key role in helping the body to determine self from nonself, allowing it to identify harmful invaders, an essential survival task in the world of microorganisms. Recent genetic research has found that a high percentage of variations in one kind of HLA in modern humans came from outbreeding with Neanderthals and Denisovians, another extinct early human who ranged from Siberia to Southeast Asia, as we migrated from Africa into Europe and Asia.28 Here we can see how inbreeding depression can be a selective pressure shaping xenophilia, the draw toward other people that allows us to outbreed.

Much of what makes people xenophilic, open to experience, and liberal, then, is evolutionarily pragmatic. Openness to new experiences, particularly to things like travel, puts us in contact with new people. Openness to new people allows us to interact with those we encounter in our travels and to exchange knowledge, goods, and technology. There is even research showing that liberals possess adaptations that allow them to be more open to eating a greater variety of foods,29 which would be valuable in novel environments or cultures. Moreover, sexual openness, a hallmark of being liberal, would allow us to exchange useful genes. Essentially, being liberal has its survival advantages.

The Benefits of Inbreeding

On the other hand, being xenophobic and closed to experience (i.e., conservative) also would have had its survival advantages. For one, a tendency to prefer the company of the in-group would have made inbreeding more likely, which can increase altruism in a population by increasing the amount of shared genes (something we will discuss in greater detail in the next chapter). Outbreeding depression has shaped these tendencies as well. All of the migration-related dangers that cull populations, cause genetic bottlenecks, and make outbreeding necessary can be avoided by simply not migrating. Thus those who are closed to new experiences, travel, or outsiders gain a certain fitness advantage by simply staying put.

Tuschman also reminds us that too much outbreeding can also cause obstetric problems. Preeclampsia is a condition that can lead to a host of health problems in the pregnant mother, such as kidney and liver failure, and even death. One large study examining 23,358 pregnancies in Turkey found that women who were married to first or second cousins (which is common in the Muslim world) were far less likely to suffer from preeclampsia, whereas outbreeding women were 60 percent more likely to experience it.30 While the causes of the condition are debated, some believe it is related to immunological incompatibility; for women, reproduction involves taking in foreign DNA into their bodies, and sometimes women's bodies will reject the fetus.

Other problems of outbreeding relate to immunology. Through natural selection, populations develop genetic immunities to local pathogens. But when humans begin breeding with outsiders whose immune systems were not adapted for their locality, the resulting offspring have less genetic resistance to those local diseases. Further, mating with “foreigners” by definition involves exchanging bodily fluids, which can carry foreign diseases that could kill outbreeders. And here is where it gets interesting. If xenophobia among conservatives reflects an adaptation that helped our ancestors avoid contagious diseases from outsiders, we would also expect to see conservatives exhibiting more fear of contagion.