in biology, a factor in the evolution of characters indirectly connected with those characters that undergo natural selection. For example, an increase in fertility is an indirect result of more intensive selection, which is as a rule accompanied by intensified elimination. Indirect selection also refers to those cases in which characters change that are correlatively associated with characters that yield an advantage in selection. Thus, in sexual selection among vertebrates, the reactive thresholds of the tissues of the more active males to the effects of the male sex hormones are lowered, leading to the more intensive development of the sex characters in those animals.