A POLYMORPHISM MAINTAINED BY OPPOSITE PATTERNS OF PARASITISM AND PREDATION

Although polymorphism is a widespread phenomenon that has been recogni zed for nearly two centuries, the basic mechanisms maintaining most po lymorphisms in nature are unknown(1,2). We present evidence that a pol ymorphism can be maintained exclusively by balanced selection from two predatory species. For field and laboratory experiments, we used the pea aphid, Acyrthosiphon pisum, which occurs as 'green' and 'red' colo ur morphs, and two species that attack pea aphids, the parasitoid Aphi dius ervi and the predator Coccinella septempunctata. We found that wh en parasitism rates in the field were high relative to predation rates , the proportion of red morphs increased relative to green morphs, whe reas the converse Tvas true when predation rates were high relative to parasitism rates. Detailed laboratory and field studies confirmed tha t green morphs suffer higher rate's of parasitism than red morphs, whe reas red morphs are more likely to be preyed on by predators than gree n morphs are. We present a mathematical model that demonstrates that b iased density-dependent parasitism and/or predation on different morph s is sufficient to maintain the colour polymorphism in the population. Our findings support an important role for predation in the maintenan ce of genetic diversity.