Now, if the goal of sexual reproduction is to keep at least some of your offspring safe from your harmful mutations, it would be foolish to pick your sexual partners at random. Any sex partner will carry his or her own load of mutations. You should pick the partner with the lowest number of harmful mutations: that will give your offspring the highest expected fitness, which means the best chance of surviving and reproducing. If your choice of sexual partner is very good indeed, your genes may hitch a ride to evolutionary stardom on the genetic quality of your mate. Many biologists are coming to the view that mate choice is a strategy for getting the best genes you can for your offspring.
Because of genetic dominance, many mutations are hidden from view. They do not affect body or behavior, so they cannot be used in mate choice. However, dominance is often incomplete, and a lot of genetic variation between individuals does show up in body and behavior. Some traits reveal more genetic information than others. Complex traits such as peacock tails that vary conspicuously between individuals may be especially informative. Their complexity means that their development depends on many genes interacting efficiently. They summarize more genetic information by being more complicated. And their variation at the visible level of body and behavior means that genetic variation can be perceived during mate choice. With sexual selection there is a big incentive to pay very close attention to traits like these.
Such traits are called "fitness indicators." A fitness indicator is a biological trait that evolved specifically to advertise an animal's fitness. Fitness means the propensity to survive and reproduce successfully. It is determined mainly by an individual's genetic quality, which boils down to their mutation load.
There is a close connection between mutations and fitness. If a species has been living in its present environment for many generations, its average genes are probably very well adapted to that environment. Because they have already been tested again and again by natural selection, the average genes in the species are already optimal. If they weren't, they would already have been replaced by different genes. This suggests that any deviation from the genetic norm is a deviation from optimality. Mutations are deviations from the genetic norm. If a set of mutations makes an individual unable to grow an optimal body and unable to produce optimal behavior, then they impair that individual's ability to survive and reproduce. Since fitness means the ability to survive and reproduce, mutations almost always lower fitness; conversely, high fitness implies freedom from harmful mutations. If fitness indicators advertise high fitness, they are also advertising freedom from mutations, which is what mate choice wants. Normal genes are tried and tested, whereas mutations are shots in the dark.
Sexual selection needs some way to connect the sensory abilities of animals to the mutation levels of the potential mates they are choosing between. Fitness indicators are the connection, for they are the traits that make fitness visible. What they make visible can be favored by mate choice, and what is favored by mate choice can evolve through sexual selection. Fitness indicators are the genetic sieve that lets sexual selection sift out harmful mutations. In this mutation-centered view of sex, sexual ornaments and courtship behaviors evolve as fitness indicators.
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