How female choice creates new species

NATURAL SELECTION, as propounded in Charles Darwin’s master work, “On the Origin of Species”, explains how organisms evolve and adapt to their circumstances. Paradoxically, though, it is a bit hazy on the actual subject of its title, namely how parent species spin off new, daughter species. Darwin recognised that diverse ecological niches encourage speciation (though he did not use those as-yet-uninvented terms). But he did not ask how incipient daughter species were prevented from remixing before they had properly separated.

Somehow, barriers need to be erected against miscegenation. They might be geographical—fish in different lakes, for example. Or they might be ecological, such as a shift in food preferences causing insects formerly of the same species to feed in different types of tree. But sometimes species form in circumstances where no such barriers are apparent. That is puzzling.

One established exception to the rule that speciation requires separation is hybridisation. This is a common source of new plant species, but is rarer for animals. It is, though, thought to explain several brightly coloured animal groups, including the cichlid fish of Africa’s Great Lakes, the heliconid butterflies of South and Central America, and the southern capuchino seedeaters, a family of songbirds that also live in the New World...