When closely related individuals mate, geneticists call this inbreeding. Though largely uncommon in developed nations, inbreeding continues in some societies where marriages among family members are common. A new Massey University study of the Rindi, an isolated Indonesian tribe, finds their relaxed compliance with traditional rules of inbred, arranged marriages produced a genetic diversity similar to random mating.
Indonesia is an archipelago between the Indian Ocean and the Pacific Ocean in Southeastern Asia. Comprised of thousands of islands, the estimated population is over 252 million people who are descended from a variety of ethnic groups, including the Javanese, Sundanese, Malay, Batak, Madurese, and Chinese.
The Rindi tribe, which numbers just a few thousand people who exist in isolation on the island of Sumba, have established a traditional culture. Their marriage rules dictate a man marry his first cousin on his mother’s side. Wealth and power, the elders believe, is consolidated by tightening the family’s scope of social influence in this way. Clearly, the Rindi perceive a social advantage to this form of inbreeding. However, geneticists would argue the results of these traditional practices are more likely to bring about disadvantages in the form of decreased vigor, size, and fertility in their children. To prove this point, scientists would point to the unusually high rates of hemophilia and color blindness found in the British and Russian monarchies.
For the current study, Dr. Murray Cox, a professor at the Institute of Fundamental Sciences, and his colleagues hypothesized the Rindi would experience a reduction in genetic diversity by following their marriage rules. To examine this theory, they posed two questions: What are the expected genetic consequences to the Rindi tribe created by following the marriage rules? How closely do the Rindi actually follow their stated rules?
To discover the truth, the researchers used an open-source computer tool called SMARTPOP and DNA sequencing. SMARTPOP allowed the team to simulate the effects of the marriage rules, while the genetic data showed them what had really transpired within the tribe and the actual effects of their mating practices. Surprisingly, after gathering and analyzing data, the researchers discovered the Rindi don’t always follow their own rules. In fact, the tribe’s relaxed compliance, which amounted to two out of three adhering to the rules, produced a genetic diversity similar to random mating.
“People like to say they follow the rules, but actually we’re all really good at looking the other way if people don’t,” Cox stated in a press release. “Our work suggests that sometimes that’s a good thing.” In the end, the Rindi treat their rules with enough flexibility to form social connections without harming their own biological diversity, their own continued futurity.
Importantly, it shows how a mixture of endogamy and exogamy allows for healthy amounts of genetic diversity, even in the presence of prevalent consanguineous marriage. As long as a significant portion of the population is exogamous, then an endogamous minority (or in this case, majority) will have little to no negative impact on the health of the whole gene pool. This makes perfect sense, since mating with close cousins is something that all wild animals do plenty. (They can’t exactly hop on a plane.)