But if we look more closely at the greatest contrivance of snails, the protective shell, we can see that it is not only coiled, but also asymmetric. If we hold a shell with the opening facing us and the pointed tip facing up, the opening is usually on the right. These shells are referred to as dextral or right-handed. In rare individual snails from right-handed species, however, the shells will have the opening on the left and are referred to as sinistral, or left-handed. Some entire species are sinistral.

Biologists have known for almost a century that a simple genetic basis is behind shell handedness, at least in some species, such that the mutation of a single gene can cause right-left reversal. The mystery then is why, if both forms can occur and reversal is genetically easy, are most species either entirely right- or left-handed?

An answer may be found in one of the logistical challenges that living with most of the body inside a shell imposes on snails  mating. Here is where handedness matters a lot. The genital opening is behind the right tentacle in right-handed snails. When land snails mate, they face each other, which brings their genital openings side by side. Snails with opposite handedness are misaligned, making it physically difficult for them to mate.

Any rare reversed individual snail would obviously have difficulty mating. As Darwin knew so well, if one cannot mate, it is the end of the line. Once of a given handedness, a species and its descendants would tend to stay that way.

But what if there were some other reversed individuals around? They could mate with one another and might form a new, reversed population. If members of that population continued to fail to mate successfully with snails of opposite handedness, that population might eventually evolve into a new species. And that seems to be exactly what has happened several times in a group of Japanese snails  and perhaps very many times in land snails in general.

Image PREY Asymmetric jawbones of snakes that specialize in eating right-handed snail species. Credit... Cristina Grande and Nipam Patel

The genus Euhadra is unusual in that it contains multiple left-handed species, as well as right-handed species. By tracing species pedigrees through DNA, the biologists Rei Ueshima from the University of Tokyo and Takahiro Asami from Shinshu University found that several right-handed species of Euhadra snails appeared to have evolved from a left-handed ancestor and that closely related left- and right-handed species could not successfully mate.