The BBC reports on intriguing new research positing, on the basis of DNA evidence, that human evolution has sped up dramatically—perhaps by a factor of 100—over the last 5,000 years. The study authors offer growing population size as a possible explanation. This makes some intuitive sense: More organisms mean more opportunities for new genetic variations to arise, and a more spread out population means more opportunity for those variations to incubate somewhat independently without being totally isolated. But if that alone sufficed, we should expect to see evidence of even more pronounced discontinuities in other, still more numerous organisms.
For readers of Geoffrey Miller’s fascinating treatise The Mating Mind, another explanation springs readily to mind. Miller argues for the evolutionary centrality of mating selection. Miller contends that a hardwired sexual preference for a certain trait—like the peahen’s preference for peacocks with large, bright plumage—can trigger a kind of arms race or feedback loop. The outliers in one generation, precisely because they’re so reproductively successful, become the baseline above which the next generation’s mating-game winners must rise.
A process like this, says Miller, may account for our most advanced cognitive and artistic capacities. A preference for mates capable of performing complex tasks—composing a song or a sonnet, or deciphering ever changing cultural codes and fashions—may have emerged as a proxy for a sound genome, as the delicate and byzantine brain would be disrupted by mutations that might otherwise pass undetected by the naked eye. Once established, though, the preference would create a runaway effect detached from its original purpose.
The thing is, explanations of this kind require (for lack of a better term) some evolutionary slack. The peahen-peacock feedback loop can generate enormous, colorful tails that are highly maladaptive in terms of pure survival… but only if they aren’t so onerous that old fashioned natural selection preempts mate selection. If an area is so rife with predators that the supermodel peacocks become lunch before reaching reproductive age, the peahens will end up making do with drabber birds whatever their preference.
But with that background in place, it’s not hard to imagine why human evolution might have accelerated over the past five millennia. In fact, it’s precisely the reason why, on a cruder view of how evolution works, one might have expected evolution to halt: Our growing mastery over nature. For the most primitive humans—life nasty, brutish, short, and so on—skill in elaborate wooing rituals would have taken a distant second place to surviving long enough to woo. Survival adaptations would have priority, and the same sorts of adaptations would have been vital everywhere: Strength, stamina, visual acuity, a hardy immune system. The more we’re able to shelter ourselves from the elements, to treat wounds, to establish stable food supplies, the less central these features become: The sickly guy with bad eyesight makes it to adulthood too. That means, first, selection pressure for convergence on certain traits is weakened, which might in itself be a source of variety. Second, it allows mate selection—in which large long-run effects can be locked in by what amounts to genetic noise, the proverbial butterfly flapping a tsunami—to get in the driver’s seat.
All this is, obviously, armchair speculation, but I’m curious whether folks find it plausible… in particular, any folks more qualified than I to speculate.