Resource tracking: mechanism — polyphenism

Abstract

Aphid populations are made up of clones, the ‘evolutionary individuals’ in Janzen’s (1977) terminology. How a clone allocates resources to particular functions is likely to determine its fitness (Dixon, 1977). Individuals in each clone are involved to a varying degree in defence, dispersal, reproduction and aestivation/hibernation (Figure 6.1). However, specialization in one or other of these functions imposes constraints in terms of resource allocation, for carrying out the other functions. At certain times particular functions are more important than others for the overall fitness of a clone and this has resulted in the evolution of a division of labour within a clone that is reflected in its polyphenism. This implies that altruism is common in aphids. However, the general view is that only the soldier caste is altruistic. That most soldiers are sterile and are likely to die defending and safeguarding the survival of their clone mates supports the claim that they act altruistically. Winged forms similarly have a greatly reduced fecundity and a very low probability of surviving to reproduce, but in incurring this cost they benefit the overall fitness of the clone. It is also difficult to visualize how this benefit to the clone could be undermined by cheating, i.e. unrelated individuals profiting from the behaviour but making no contribution in return. Thus it would appear that winged forms also act altruistically. If this line of argument is accepted, then contrary to Hamilton’s (1987) claim altruism is common in aphids.