THE EVOLUTION OF CASTE POLYMORPHISM IN SOCIAL INSECTS - GENETIC RELEASE FOLLOWED BY DIVERSIFYING EVOLUTION

Caste polymorphism, defined as the presence within a colony of two or more morphologically differentiated individuals of the same sex, is an important character of highly eusocial insects both in the Hymenopter a (ants, bees and wasps) and in the Isoptera (termites), the only two groups in the animal kingdom where highly eusocial species occur. Freq uently, caste polymorphism extends beyond mere variations in size (alt hough the extent of variations in size can be in the extreme) and is a ccompanied by allometric variations in certain body parts. How such po lymorphism has evolved and why, in its extreme form, it is essentially restricted to the social insects are questions of obvious interest bu t without satisfactory answers at the present time. I present a hypoth esis entitled 'genetic release followed by diversifying evolution', th at provides potential answers to these questions. I argue that genetic release followed by diversifying evolution is made possible under a n umber of circumstances. One of them I propose is when some individuals in a species begin to rely on the indirect component of inclusive fit ness while others continue to rely largely on the direct component, as workers and queens in social insects are expected to do. Thus when qu eens begin to rely on workers for most of the foraging, nest building and brood care, and workers begin to rely increasingly on queens to la y eggs-when queen traits and worker traits do not have to be expressed in the same individual-I postulate the relaxation of stabilizing sele ction and new spurts of directional selection on both queen-trait gene s and worker-trait genes (in contrasting directions) leading to caste polymorphism.