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Evolution of Gender and Sex

QUESTION: Evolution of Gender/Sex – Why is gender a problem for evolution?


There are two key aspects about sex that are contrary to the notion of evolution. First is the waste of resources in producing males. To appreciate this, compare a sexually reproducing female with one that reproduces asexually. Assuming that the sexual female produces an equal number of males and females, then only half of her offspring will be able to survive to produce further offspring, whereas all of the asexual offspring will be females which, in due course, may reproduce. Hence, if both types of female produce the same number of individuals (corresponding to a comparable economic cost of reproduction), the asexual female will proliferate at twice the rate of the sexual one. In other words, the species reproducing sexually has only 50% of the fitness of the asexual species, and being at a substantial selective disadvantage should -- by the operation of natural selection -- be ousted fairly quickly by the asexual species. To compensate for this selective disadvantage of sex, the male would be required to make a substantial contribution to the overall fitness of the species, for example by collecting food for the female and her offspring. But this occurs only in some sexual species.

Secondly, the whole point of natural selection is that those organisms which survive better reproduce preferentially, and hence tend to perpetuate their favorable genetic composition. However, for thriving sexually reproducing individuals, rather than perpetuating all of their successful genotype they pass on only half of it. It may be argued that individuals having advantageous genotypes will arise in both sexes and subsequently the fitter males will generally mate with fitter females. However, there is no getting away from the fact that sexually reproducing individuals are gambling with 50% of their ‘successful’ phenotype. In contrast, thriving parthenogenetic females pass on 100% of their successful genotype. Clearly, therefore, sexual reproduction conflicts with the standard Darwinian paradigm.

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