BioEd Online

There are thousands of species of moths and butterflies, to the delight of lepidopterists everywhere. The most popular explanation for their exuberant diversity is coevolution: as plants evolve defenses against hungry larvae, each lepidopteran species evolves tricks to circumvent the defenses of a particular plant or moves to a new one. So, potentially there could be as many species of moths and butterflies as there are plants — and there are hundred of thousands of plant species. But a new analysis of an ancient family of moths suggests that such 'ecological speciation' isn't the whole story.

Yume Imada and her colleagues at the University of Kyoto in Japan travelled around the Japanese archipelago, collecting moths of the family Micropterigidae and identifying the liverworts that they feed on. They compared the moths' DNA, and constructed a family tree showing their likely relationships and evolutionary history. They found some 25 distinct species of moth, which had diverged from a common ancestor between 35 and 15 million years ago. And all of them eat the same species of liverwort — Conocephalum conicum — and appear to inhabit the same ecological niche. The team's findings appear in Proceedings of the Royal Society B this week.¹

It is possible that there is something subtly different about the ecological niches inhabited by the different moth species. But it cannot be the host plant, the most obvious ecological variable, and all the moths live in humid, mossy microenvironments.

More likely, say the researchers, is that the moths speciated because populations became geographically separated — what evolutionary biologists call 'allopatric speciation'. The moths are weak fliers and individual populations are more or less stuck in the humid streamside environment where they live. Over time, separated populations accumulate enough differences to become separate species.

A neglected option

Allopatric speciation has been somewhat neglected in the excitement over ecological speciation, according to Tommi Nyman of the University of Eastern Finland in Joensuu. Nyman has shown that another insect group, the sawflies, also tends to speciate without hopping to another niche². "It is so much more exciting when you find ecological speciation. There has been a huge focus on that. There is a tendency to forget that there are going to be lots of cases of normal, ordinary allopatric speciation, which tends to be kind of boring."

Nyman guesses that in plant-feeding insects, somewhere between 20% and 50% of speciation will turn out to have been ecological.

Imada and her colleagues suggest that species that currently specialize on different hosts didn't necessarily speciate because they moved into different niches. "A possible scenario is that an initial stage of allopatric speciation is followed by, or accompanied by, minor divergences in host choice," she says. Such a scenario might help to explain why closely related insects often specialize in closely related plants. The leaf-miner moth genus Ectoedemia, for example, comprises 26 species whose larvae all burrow into the leaves of various oak species.

Charles Mitter, an entomologist at the University of Maryland in College Park, agrees that it is time to move beyond the initial excitement over ecological speciation. "We are at the beginning of a new phase in insect–plant evolutionary studies," he says. The question is no longer whether ecological speciation happens, but "when does it happen, how often does it happen, and why does it happen".

To get at these questions, says Mitter, researchers will have to get out into the field and obtain the kind of detailed data that Imada and her team collected. "Lots of us like to sit back and compile data from the literature to ask questions like this," says Mitter. "This is one of the only groups that actually gets out there on their hands and knees on slippery rocks and collects life-history data."