The evolution of caste polymorphism in social insects:0 Genetic release followed by diversifying evolution |
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Authors: | Raghavendra Gadagkar |
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Institution: | (1) Centre for Ecological Sciences, Indian Institute of Science, 560012 Bangalore, India;(2) Animal Behaviour Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P. O., 560064 Bangalore, India |
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Abstract: | 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 Hymenoptera (ants, bees and wasps) and in the Isoptera
(termites), the only two groups in the animal kingdom where highly eusocial species occur. Frequently, caste polymorphism
extends beyond mere variations in size (although the extent of variations in size can be in the extreme) and is accompanied
by allometric variations in certain body parts. How such polymorphism has evolved and why, in its extreme form, it is essentially
restricted to the social insects are questions of obvious interest but without satisfactory answers at the present time. I
present a hypothesis entitled ‘genetic release followed by diversifying evolution’, that provides potential answers to these
questions. I argue that genetic release followed by diversifying evolution is made possible under a number of circumstances.
One of them I propose is when some individuals in a species begin to rely on the indirect component of inclusive fitness while
others continue to rely largely on the direct component, as workers and queens in social insects are expected to do. Thus
when queens begin to rely on workers for most of the foraging, nest building and brood care, and workers begin to rely increasingly
on queens to lay eggs—when queen traits and worker traits do not have to be expressed in the same individual—I postulate the
relaxation of stabilizing selection and new spurts of directional selection on both queen-trait genes and worker-trait genes
(in contrasting directions) leading to caste polymorphism. |
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Keywords: | Social evolution genetic release diversifying evolution eusociality social insects caste polymorphism |
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