Human evolution goes on
I missed it when it first went up, but over at The Crux, Discover’s new group blog, Razib Khan has pointed to a couple of interesting papers on the heritability of fertility. As natural selection acts strongly on fertility and the traits that affect it, you might expect that the heritability of fertility would be low as variation is eliminated. But change the environment, and heritability can increase drastically. Razib writes:
Quebec is also an ideal “natural experiment.” It began as a frontier society with a small founding population on the order of thousands, but now has a population of over 8 million. …
One study http://www.pnas.org/content/108/41/17040.abstract focused on a 140-year period on an island in the Gulf of St. Lawrence. Over five generations, the island’s population increased by a factor of 10 through natural increase, while the average age of first reproduction declined from 26 to 22. Just as menarche and menopause are heritable (variation in genes explain much of the variation in the outcome of the trait), so too age at first reproduction seems heritable. …
Another group focused on the differences between core and frontier populations in the Saguenay–Lac-Saint-Jean region of Quebec. It turns out that the majority of the modern population descends from those in the frontier zone, not the core. Women on the frontier were ~20% more fertile, and fertility as a trait was heritable on the frontier but not in the core. (On the frontier, there was a connection between relatives and how many offspring they would have, in direct proportion to the amount of genes they shared. E.g., two sisters tended to have similar numbers of offspring, while there was no correlation between strangers.) …
Changes in the environment can dramatically change the power of selection on traits. While a trait may not be heritable in one environment, it may be in another. Considering the changes in environment to which humans have been exposed over the last couple of hundred years, there must be many traits now experiencing significant selective pressure. I recommend that you read the whole post by Razib.