Data for: Larger eyes are selected for in high competition environments

Intraspecific variation in vertebrate eye size is well known. Ecological
factors such as light availability are often correlated with shifts in
relative eye size. However, experimental tests of selection on eye size
are lacking. Trinidadian killifish (Anablepsoides hartii) are found in
sites that differ in predation intensity. Sites that lack predators are
characterized by lower light, high killifish densities, low resource
availability, and intense competition for food. We previously found that
killifish in sites that lack predators have evolved a larger relative eye
size than fish from sites with predators. Here we used transplant
experiments to test how selection operates on eye size when fish that are
adapted to sites with predators are translocated into sites where
predators are absent. We observed a significant ‘population x relative eye
size’ interaction; the relationship between relative eye size and a proxy
for fitness (rates of individual growth) was positive in the transplanted
fish. The trend was opposite for resident fish. Such results provide
experimental support that larger eyes enhance fitness and are favoured in
environments characterized by low light and high competition.