Criteria For Reproductive Success
The success of an organism is not only measured by the number of offspring left behind, but by the quality or probable fitness of the offspring: their reproductive fitness. Sexual selection increases the ability of organisms to differentiate one another at the species level: interspecies selection.
| “ | The grossest blunder in sexual preference, which we can conceive of an animal making, would be to mate with a species different from its own and with which hybrids are either infertile or, through the mixture of instincts and other attributes appropriate to different courses of life, at so serious a disadvantage as to leave no descendants. ... it is no conjecture that a discriminative mechanism exists, variations in which will be capable of giving rise to a similar discrimination within its own species, should such a discrimination become at any time advantageous. —Ronald Fisher, 1930 | ” |
The expansion of interspecies selection and intraspecies selection is a driving force behind species fission: the separation of a single contiguous species into multiple non-contiguous variants. Sexual preference creates a tendency towards assortative mating or homogamy, providing a system by which a group otherwise invaded by diverse genes is able to suppress their effects and diverge genetically.
| “ | Individuals in each region most readily attracted to or excited by mates of the type there favored, in contrast to possible mates of the opposite type, will, in fact, be the better represented in future generations, and both the discrimination and the preference will thereby be enhanced. It appears certainly possible that an evolution of sexual preference due to this cause would establish an effective isolation between two differentiated parts of a species, even when geographical and other factors were least favorable to such separation. —Ronald Fisher, 1930 | ” |
The general conditions of sexual discrimination appear to be (1) the acceptance of one mate precludes the effective acceptance of alternative mates, and (2) the rejection of an offer will be followed by other offers, either certainly, or at such high chance that the risk of non-occurrence will be smaller than the chance advantage to be gained by selecting a mate.
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