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

In most cases, sexual selection does not act with the same intensity on the members of the two sexes. Females are usually more selective and males less selective in choice of a sexual partner. Consequently, males are usually subjected to more intense sexual selection and thus more obvious secondary sexual traits tend to emerge amongst them. The cause of this asymmetry lies primarily in the different cost of production of microgametes and macrogametes and secondarily in the different cost of the two parent roles. Because a female invests more energy in the production of macrogametes from the very beginning of reproduction than a male into the production of microgametes, she must generally accept the future thankless role of the parent that invests more into feeding and bringing up the offspring than her partner. In organisms with internal fertilization, this tendency is further reinforced by the fact that the greatest part of the cost of reproduction, nutrition of the embryo, is, in its biological nature, not transferrable to the male. Asymmetry in expenditure of energy thus attains extreme values here. The fitness of the individual depends both on the number of offspring produced in a lifetime and also on their quality. Males can increase their fitness through greater sexual activity, while females basically do not have any reserves in this area and the numbers of their offspring do not increase with an increasing number of sexual partners (Fig. XV.1). Thus, males need not be as discriminating in the quality of their sexual partners and can even reproduce with females that are of inferior quality phenotypically or genotypically. In contrast, females must carefully differentiate the quality of the males with whom they will reproduce. The quality of their future offspring depends to the same degree on the quality of their own genes as on the quality of the genes of the male.

            Thus, the population contains a constant excess supply of males that are willing to reproduce over the supply of females that are willing to reproduce. This leads to strong intrasexual competition and thus more intense sexual selection amongst males.