Rapid divergence in a recently isolated population of threespine stickleback (Gasterosteus aculeatus L.)

Abstract

In 1987, a fjord in the Snaefellsnes peninsula, north-west Iceland, was dammed and a freshwater lagoon formed. There is a large population of threespine stickleback in this lagoon. We compared morphological features of stickleback in the lagoon population to those of their marine ancestor, and morphological polymorphism within the lagoon in relation to mud and lava substrates. The freshwater stickleback have shorter spines and fewer armour plates than marine stickleback. There is also some morphological divergence between stickleback from the two substrates within the lagoon. Our results suggest that the threespine stickleback may adapt to a novel environment more rapidly than would be predicted from conventional models of biological differentiation.