Overnight Evolution

A year ago, the proportion of Blue Moon butterflies (Hypolimnas bolina) on the Samoan islands of Upolu and Savaii which are male dropped to approximately one percent, thanks to a parasitic bacteria which infected females and selectively killed male embryos. This put the species on the edge of extinction. A mere ten generations of butterflies, over less than a year's time, developed an immunity in male embryos, based on a "suppressor gene," that let them develop even when the parasite was present. This resulted in males reaching almost normal proportions (40 percent of the population).
NOTE: This is a more complex process that it may seem, and it's not clear from the media accounts I've read whether the suppressor gene was already present (but rare or unexpressed) in some butterflies or whether there was an actual change at the genetic level. Either way, it's a striking example of natural selection in action.