The availability of metabolic energy provides a general measure of the environmental stress that can be tolerated by organisms, leading to the hypothesis that increased tolerance to a range of environmental stresses will be associated with a reduction in metabolic rate in Drosophila and many other organisms. This hypothesis makes three predictions about genetic variation for stress tolerance: (1) increased stress tolerance will tend to be associated with decreased metabolic rate; (2) genetic correlations between tolerance of different environmental stresses will tend to be positive; (3) stress tolerance and life-history traits will tend to be genetically correlated; in Drosophila correlations with life-history traits other than longevity will tend to be negative.
These predictions were tested by artificially selecting for increased desiccation tolerance in Drosophila melanogaster, using an 85% mortality level. The response to selection was rapid and the mean realized heritability was c. 0.65. The selection response was associated with a decreased rate of water loss, reduced activity and a decrease in metabolic rate in agreement with prediction (1). Selection did not alter body size. Selected lines were relatively more tolerant of starvation and a toxic concentration of ethanol in agreement with prediction (2), and had lower fecundities in agreement with prediction (3).