Butterfly restrains Darwin

In experiments with butterflies, evolutionary biologists from Leiden University have demonstrated that natural selection is not always the only factor which determines the appearance of an organism. Constraints also appear to play a role at times in determining the progress and outcome of the evolutionary process.

This research from Leiden provides evidence for the hypothesis of Stephen Jay Gould and Richard Lewontin. In 1979, these two scientists stated that Darwin`s theory of natural selection is often incorrectly seen as the sole factor which determines the outcome of evolution. Constraints also play an important role.

For example, constraints play a role in not allowing the development of eyes in the back of the head. Although decidedly advantageous, the evolution of eyes at this location will occur in few animals. This is due to limitations during the development and growth or because no variation for this trait is present in the genetic material. Another form of constraints occurs if two traits are linked to each other. A person with long arms always has long legs.

The evolutionary biologists from Leiden wanted to investigate, in the laboratory, whether they could rear butterflies with all possible combinations of characteristics or whether certain combinations were not possible. They used the tropical butterfly Bicyclus anynana for these experiments.

Two large selection experiments each gave a different outcome. In other words, the constraints of Gould and Lewontin sometimes do and yet sometimes do not play a role.

In one experiment the researchers tried to ensure that male butterflies developed less quickly than female butterflies. However, if the researchers succeeded in speeding up the development of the female butterflies in a new generation, the male butterflies also developed quicker. Even after eight generations and much effort on the part of the researchers, the difference remained unchanged.

Constraints were shown not to be decisive in what had up until now been considered the fixed combination of eyespot size and development time. Butterflies which grow quickly, do not necessarily have to have larger eyespots than butterflies which grow slowly. Although both characteristics are influenced by the same hormone, they can be disassociated from each other.