Two-fold ecological costs of sex
The emergence of sexual reproduction leads to sexual dimorphism – differentiation of individuals of a single species into males and females. Simple considerations indicate that an individual that would produce only asexually reproducing progeny (parthenogenetic female) would reproduced twice as fast, expending the same amount of energy, as an individual that would “squander” half its reproduction efforts in the production of males (Fig. XIII.1). This effect of sexual reproduction is generally denoted as the two-fold ecological costs of sex (the cost of males) (Maynard Smith 1978). The two-fold ecological cost of sex is, of course, not applicable to hermaphrodites which, during mating, exchange sex cells, to unicellular organisms, to organisms with external fertilization that produce male and female cells of the same size (isogametes) or to organisms where both parents must care for the young with comparable intensity. However, the two-fold ecological cost of sex does not apply even in a situation where reproduction of individuals in the population is limited by unfavourable external factors. It cannot be excluded that this could be the reason why, in species capable of both sexual and asexual reproduction, i.e. in species with facultative sexual reproduction, sexual reproduction usually occurs under circumstances where the population finds itself in unfavourable conditions (Burt 2000).