How did this happen, and why did the males disappear in the first place?
The culprit is probably a parasite called Wolbachia, which lives in the cells of female butterflies and is passed down to their offspring through the female’s eggs. Male butterflies are useless for helping Wolbachia reproduce, so the parasite kills male butterfly embryos.
Fortunately for the butterflies, they have some “suppressor genes” that counteract the parasite’s effects, although researchers don’t yet know exactly how the genes work.
Sylvain Charlat of University College London and University of California Berkeley and his colleagues studied male and female butterfly populations on the Samoan islands of Savaii and Upolu and compared their numbers, from 2006, with numbers from 2001, when the males were extremely rare. They found that, five years later, the numbers of males had either reached or were approaching those of females.
During this time, it seems that the butterflies had evolved rapidly, developing more suppressor genes that helped protect the males against the parasites. This change occurred in approximately 10 generations of butterflies, which is a surprisingly short time for natural selection to occur. -American Association for the Advancement of Science
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