|
|
Incest in Nature |
|
|
|
||
|
Six years ago, I wrote about the science and ethics of incest ("The
Love That Dare Not Speak Its Surname"). At the time, a study
showed that having a
child with your first cousin raised the risk of a significant birth defect
from about 3-to-4 percent to about 4-to-7 percent. The authors concluded
that this difference
wasn't enough to
justify genetic testing of cousin couples, much less bans on cousin
marriage.
Now the incest taboo has taken
another hit.
Ecologists Kelly Zamudio and Chris Chandler have published a study in
Molecular Ecology on
sexual selection among spotted
salamanders. From this and other research, Science News
reporter Ewen Callaway has teased out a fascinating theme: Incest,
apparently for sound Darwinian reasons, is
surprisingly common
in nature.
Through interviews with biologists and ecologists, Callaway looks at several
cases. Among spotted salamanders, DNA analysis shows inbreeding "at the
level of first cousins, on average. Despite having hundreds of possible
mates to choose from, females tended to fertilize their eggs with sperm from
related males." Another study found that "Japanese quail prefer first
cousins over brothers and sisters and over less-related birds." Among
ambrosia beetles: "Brothers and sisters tend to mate." A comparison over two
generations of mating found that "inbred beetles fared no worse than outbred
insects, and the eggs produced by brother-sister pairs were likelier to
hatch than the eggs of unrelated pairs."
At least one fish species similarly prefers brother-sister mating.
Scientists "found that fathers from brother-sister couples spent more time,
on average, defending their caves and that both parents tended to pay more
attention to their kids than unrelated couples." This makes obvious sense.
The ecologist who supervised the study reports, "Couples which are full
siblings are more cooperative in brood care. ... [T]he males and females
stay with the offspring for several weeks and guard them—they defend
them—and there's less aggression between full siblings."
These aren't the only rationales for inbreeding. Paraphrasing a Cambridge
biologist, Callaway notes, "Many organisms might have slight genetic tweaks
or adaptations tuned to their local habitats, and too much genetic mixing
with outsiders can dilute these adaptations." Among ambrosia beetles, the
practice "may cement the slight genetic differences between the insects,"
thereby helping to "create new species."
Nor is inbreeding universally taboo among humans. A study in Pakistan found
that "three out of five marriages were between first cousins." Another in
India that found "one-fifth of marriages occurred between uncles and nieces
and a third between first cousins." And before you dismiss this as Eastern
barbarism, read up on
Charles Darwin and
Rudy Giuliani.
The incest taboo does have a firm biological basis. As Callaway explains,
"Inbreeding ups the chances that a child will inherit two versions of a
disease-causing gene." Data show higher mortality among infants born from
first-cousin pairs. But beyond that range, there's evidence that breeding
within the family has advantages. Two months ago, a study in Science
reported "a
significant positive association between kinship and fertility,"
with a likely "biological basis." The study found "the greatest
reproductive success" among "couples related at the level of third and
fourth cousins." On average, these cousins produced more kids than less
related—and more related—pairs did.
The upshot seems to be that there are advantages and disadvantages to
breeding with a relative, and as far as nature is concerned, the ideal
course is to strike a balance. You're free to argue that incest is wrong, of
course. But be careful what you call unnatural.
= |