Who says you can't watch evolution?

Scientists have just documented (another) inheritable change in a species that occurred in response to a change in the environment -- in this case a parasite. Hence they have observed the process of natural selection.

In the latest issue of Science, Charlat et al. observed that the sex ratio in a species of Polynesian butterfly -- Hypolimnas bolina -- changed from 99:1 favoring females to parity in less than 10 generations. The sex ratio favored females because there is a parasite -- a bacteria called Wolbachia -- that selectively kills the male embryos.

However, in the space of ten generations the scientists observed that some of the butterflies had restored parity between males and females. Clearly the butterflies had evolved some resistance to the parasite during that period. They also must have evolved it incredibly quickly in comparison to most known examples of natural selection.

To confirm that the restoration of parity was because of a inheritable change in the butterflies' genes, the researchers back-crossed the resistant butterflies with butterflies that were not resistant to the parasite. They found that they could restore susceptibility to the parasite within three generations and that the sex ratio returned to 99:1.

This data confirms that the switch in the sex ratio is the result of a inheritable genetic change in the butterflies that allowed the male embryos to resist to the parasite. In short, it confirms the process of natural selection.

Charlat has the money quote:

Natural selection typically moves very slowly, sometimes over hundreds of years, they said, but when under severe attack, this process was accelerated.

"It is the speed of the process that demonstrates the intensity of the selection," Charlat said.

"The take-home message is that evolution can be really, really fast." (Emphasis mine.)