Fitness Spreading

One effect of fitness matching is to increase the variation in fitness in the next generation. In fact, it creates the widest possible fitness differences between babies. Fitness matching by parents leads to fitness spreading among offspring. Consider the extremes of the fitness spread. The only way to produce a baby of the highest possible fitness given the parents available, would have been for the highest-fitness male to mate with the highest-fitness female. That is exactly what happened, through the mating market. And the only way to produce a baby of the lowest possible fitness would have been for the lowest-fitness male to mate with the lowest-fitness female. Again, that is exactly what happened. Fitness matching does notjust increase the variation in fitness a little bit. It increases that variation as much as any mate choice process could, with or without monogamy.

The fitness-spreading effect is important because it creates a very tight link between sexual selection and natural selection. The power of natural selection is proportional to the fitness spread that is available in a population. Bigger fitness differences between babies lead to faster evolution. By creating the largest possible fitness spread, fitness matching gives natural selection the greatest diversity of raw material to work on. Psychologists Aaron and Steven Sloman emphasized the importance of this effect in an important paper they published in 1988.

From a genetic point of view, fitness matching concentrates harmful mutations from low-fitness parents in their low-fitness babies. When those babies die, they take a lot of harmful mutations with them. Fitness matching also concentrates helpful mutations (which are much rarer) in high-fitness babies. When those babies thrive at the expense of lower-fitness competitors, the helpful mutations increase their share of the gene pool. This is a heartlessly unromantic view of sexual selection's effects, but evolution is heartless.

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