R. Gadagkar, THE EVOLUTION OF CASTE POLYMORPHISM IN SOCIAL INSECTS - GENETIC RELEASE FOLLOWED BY DIVERSIFYING EVOLUTION, Journal of Genetics, 76(3), 1997, pp. 167-179
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.