In their reaction to the Iceland study, which showed clearly reduced fertility associated with outbreeding above about the third or fourth cousin level, many commenters seemed to prefer a default explanation of cultural causation, discounting biological cause.
Razib Khan in Why cousin lookin’ fertile…(?) nicely contracts possible cultural explanations into a single abstract model:
[P]erhaps this dynamic is a function of the difference between genetic variance and cultural variance, and the reproductive sweep spot is the region where the former is maximized across the reproductive pair and the latter minimized. […] Gene flow across demes quickly equilibrates allele frequencies so that drift can’t fix differences. But culture may be different; whereas it is very difficult to maintain greater between group variance than within group variance with genetics for coterminous populations, it is not as difficult when it comes to culture.
Comparing with Figure 1B from the Iceland paper, this looks plausible, if only because it looks just like what any kind of additive interaction between simple inbreeding and outbreeding depression curves would look like, but still:
Yet. If cultural drift over a relatively short time in a relatively homogenous population has enough pull to cause this kind of clear reproductive disadvantage for outbreeders, can we really, as a default assumption, discount biological causation?
In the experiment equal numbers of males and females of both species were placed in containers suitable for their survival and reproduction. The progeny of each generation were examined in order to determine if there were any interspecific hybrids. These hybrids were then eliminated. An equal number of males and females of the resulting progeny were then chosen to act as progenitors of the next generation.
The fly species chosen were sufficiently interfertile to breed into almost 50% of hybrid progeny at the beginning of the experiment, but that changed very quickly:
|Generation||Percentage of hybrids|
Of course, the experimental condition of 100% hybrid mortality was far beyond what we’d expect from Razib’s cultural infertility effect, certainly in a place like Iceland, but then we don’t have to explain something like the near total cross-infertility of those fruit flies, either.