THE EVOLUTION OF DIMORPHIC TRAITS - PREDICTING THE GENETIC CORRELATION BETWEEN ENVIRONMENTS

Many traits vary in a dichotomous manner, although the underlying gene tic determination is polygenic. The genetic basis of such dimorphic tr aits can be analyzed using the threshold model, in which it is assumed that there is a continuously distributed underlying character and the phenotype is determined by whether the character is above or below a threshold. Threshold traits frequently vary with environmental variabl es such as photoperiod, temperature and density. This effect can be ac counted for using a threshold model in which (1) there is a critical v alue of the environmental variable at which a genotype switches to the alternate morph, and (2) switch (threshold) points are normally distr ibuted in the population. I term this the environmental threshold (ET) model. I show that the ET model predicts that across environments dif fering in only one factor the genetic correlation will be 1. This pred iction is supported by data from three wing dimorphic insects. Evidenc e is presented that the genetic correlation between environments diffe ring in two components (temperature and photoperiod) is less than 1.