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HOW does one species of animal split into two species without evolving in isolation for thousands of years? The problem has vexed biologists since Darwin, but now the curly shells of snails have suggested an answer.

Speciation is easy if two populations become separated and evolve in isolation for millions of years. But sometimes the split happens without a physical barrier. For instance, some crater lakes in East Africa are full of thousands of species of cichlid fish that evolved from a single ancestral species.

Now Mats Bj枚rklund and Jonathan Stone at Uppsala University in Sweden have found a quick trick for generating new species, at least in snails. Some snails have shells that coil in a right-handed or left-handed direction. 鈥淩ighties鈥� can鈥檛 mate with 鈥渓efties鈥� because their reproductive organs won鈥檛 meet.

The coil direction is controlled by a 鈥渕aternal effect鈥�. A single gene in the mother codes for a protein that she deposits in her eggs, and this protein makes an embryo鈥檚 early divisions happen in one of two orientations to make left or right-coiling shells.

Bj枚rklund says a righty snail population could generate a new lefty one when a rare mutation in a righty mother鈥檚 egg switches the coiling gene to the left. The mother鈥檚 egg protein still forces the embryo to coil to the right, resulting in a righty daughter who can mate with other righties鈥攔ather than a lone mutant who can鈥檛 mate with anyone. But all her offspring are lefties who can only mate with each other, producing a distinct species.

Michael Doebeli, a biologist at the University of British Columbia in Vancouver, says the model is neat because it relies on the simple mechanism of enforced mate choice, and could work in snails. 鈥淏ut it is another thing to make it more general,鈥� he says.

Bj枚rklund thinks it could apply to other animals. For instance, an insect that normally mates in the morning could acquire a mutation that makes it mate only with its siblings in the afternoon.