Grove snails show fast evolution
Snails may be slow animals in our eyes, their evolution is extremely fast. This was discovered by the Dutch evolutionary biologist Menno Schilthuizen of Naturalis Biodiversity Center (Leiden, Netherlands) by studying the snails and comparing the time the ‘IJsselmeerpolders’ became dry land. This area used to be open sea. In the 20th century it was first closed off from the sea by building the ‘Afsluitdijk’, after which it became a freshwater lake, the IJsselmeer. After that big parts of this lake were made dry land, ‘polders’, starting in 1930, the last one in 1968. The ecosystems and populations developed on this land are young – and we know exactly how young. By comparing diversity in older and younger ecosystems it is possible to measure the speed of the evolution of species.
The snails inside forests show after 50 years a 4 times higher frequency of certain colour patterns than in grassland. This is one of the fastest speeds ever seen in the wild.
The grove snail Cepaea nemoralis is nearly as famous among evolutionary biologists as Darwins finches. Their shell comes in many colour patterns evolutionay adapted to their environment. Lighter colours do not get so hot in full sunshine, so they survive better in open grassland; in the forest it does not get hot anyway. Darker colours make the snail less visible there. But how strong this selection works – how fast differences evoluate in forest and open field – was not clear. Some scientists thought is would take centuries, others expected some years would be enough. Schilthuizen was the first to calculate the speed of the snail’s evolution in real life. He realised that the populations in the polders can not be older than the polder itself. The polders could be used as a ‘natural laboratory’ of the evolution of snails.
Analysis of his observations showed that snails in the forest are usually darker than in the open field, but this difference is bigger when the polder is older. The greatest difference was found in the Wieringermeerpolder from 1930. The least different were the snails in Southern Flevoland, from 1968. The populations were probably started by small numbers of animals coming by accident to the new land.
“This means that the snails quickly evoluate differences in the colours of their shells, but also that this process needs at least eighty years (maybe more) to reach maximum colour difference,” says Schilthuizen. His result shows a fast evolution, which can be compared with the beaks of the Darwin finches.
26-11-2012 in: geopersdienst