Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

In the vast majority of cases, sexual selection acts as soft selection. This means that the sexual attractiveness of an individual depends not on the absolute degree of expression of the secondary sexual trait, but rather on its relative degree (related to the other individuals in the population). While, in hard selection, it can be expected that the degree of expression of a selected trait will attain certain values and will no longer increase in evolution; this need not be true in soft selection. The fact that, for secondary sexual traits, evolution also ends at certain values is a consequence partly of physiological or physical barriers but primarily by the action of environmental selection, acting against the action of sexual selection.

            In argus pheasants, females prefer males with the longest wing feathers and thus most of the chicks in the flock are progeny of the cocks with the longest feathers. Feather length is hereditary and thus cocks in the next generation should, on an average, have longer feathers. Feather length could have originally been an unimportant trait from the standpoint of viability; however, through sexual selection the feathers grew to such a degree that they are an obstacle to their bearers and reduce their chance of living to reproductive age. Thus, the feather length of contemporary pheasants is a certain compromise, a result of the action of two contrary forces, i.e. sexual selection, preferring cocks with longer feathers, and environmental selection, preferring cocks with shorter feathers.

            The contrary action of environmental and sexual selection has also been observed in drosophila. Here, females prefer males with the greatest content of cuticular hydrocarbons, while males with average content have the greatest viability (Hine et al. 2002).