Six generations

I have often wondered what is the fewest number of generations of selective evolutionary pressure before you can see a clear bifurcation between the current and the ancestral generation. The context was the discovery a few years ago that both lactose tolerance and blue eyes only emerged in Europeans within the past seven thousand years. Both those traits spread very rapidly in the scheme of things. But how fast can a trait spread?

This article, Scientists turn a brown butterfly purple—in just six generations by John Timmer, answers that question. Six Generations is the answer.

The researchers started by checking the absorption spectrum of their existing lab strain of butterflies. This showed a peak of reflection at 300nm wavelengths, well within the UV range of the spectrum. But the peak was broad and varied from individual to individual, so the researchers selected the males and females that had the peak shifted closer to the visible spectrum, then mated them to produce the next generation. They repeated the process of mating and measuring reflection for five additional generations.

After six generations of selective breeding, the peak reflection had shifted well into the purple at 400nm.

(Six generations in this species take less than a year. The experiment actually involved eight generations total, though, because two generations saw low numbers of offspring and were simply allowed to mate randomly to build up the numbers again.)

Obviously the determinants of attribute plasticity are the nature of the trait, the species involved, etc. Still. Six generations is a lot fewer than I would have expected.