Closely related butterflies all look so alike that they often confuse lepidopterists. In fact, they even confuse each other—and they have special adaptations to make sure they’re not actually breeding with closely related species, instead of their own species. Except when they don’t bother.
Butterflies are known for their ability to imitate each other. Looking like a relative with a special adaptation, like poison or just a nasty taste, is an advantage when a butterfly needs to save itself—but what exactly is it saving itself for? A butterfly can only pass down its genes if it mates with a member of its own species, instead of the close relative it’s imitating.
It’s tempting to assume that the butterflies themselves know the difference, but a look at butterfly anatomy shows that they don’t. Male butterflies have scent pads on their wings. These pads give off a faint odor that female butterflies of the same species can smell, and it’s this that guides them to the appropriate mate. Without the scent pads, a male either wouldn’t get chosen, or might get chosen by some cousin species with which he can’t produce any offspring. When it comes to evolutionary-necessary bits of equipment, the scent-pads should be the last thing to go.
A group studying the Oppia hairstreak butterflies noticed that they do not have scent pads—except they found two new representatives, Thereus lomalarga and Thereus brocki, which do have scent pads. This means that the butterflies have lost their scent pads twice over in their evolutionary history. What’s the difference between the butterflies with scent pads and the butterflies that have lost their scent? The ones without scent have moved to an area without any close relatives nearby. This means that, the moment a butterfly doesn’t absolutely have to stop its females going for its close relatives, it gives up its main countermeasure to prevent it. Twice, even.