The Fisher’s runaway process for species development consists of the two phases which he outlines in his genetical theory of natural selection:
Female preferences initially evolve because the preferred traitis favored by natural selection and hence the offspring are more likely to carry the beneficial trait.
Whenever appreciable differences exist in a species, which are in fact correlated with selective advantage, there will be a tendency to select also those individuals of the opposite sex which most clearly discriminate the difference to be observed, and which most decidedly prefer the more advantageous type.
Once female preferences exist, males with the trait are even more fit (both a natural and a sexual selection advantage). There will then be an ever increasing selective force favouring stronger preferences and more extreme traits (Fisher’s runaway process).
The further development of the character trait will still proceed, by reason of the advantage gained in sexual selection, even after it has passed the point in development at which its advantage in Natural Selection has ceased.
- Males and females have different constraints on reproduction:
- Females tend to be limited by fertility,
- Males limited by mating success;
- This disparity leads to:
- Males having a much larger variance in reproductive success;
- Female “choosiness” (the bases for which are not always clear);
- This choosiness leads to extreme male traits, often at odds withmale viability fitness.