Sexual selection is a special kind of natural selection in which mating preferences influence the traits of the organism (SF Fig. 1.5). SF Fig. 1.5 shows some examples of secondary sex characteristics produced by sexual selection. The number of offspring that are produced makes sexual selection a strong force in nature, and is a measure of the organism’s success. In fact, harmful traits can still persist in a population if potential mates find them favorable. Peacock tails, for example, slow down male birds, but they are so attractive to peahens (the females) that the huge tails have persisted in the population because birds with huge tails get more mating opportunities (even before they are picked off by predators that can catch the slow moving males more easily). Sometimes natural selection and sexual selection are in direct opposition to one another. For example, populations of wild guppies that live in streams in Trinidad in the Caribbean have different color patterns depending on where they live. Male guppies living in streams that have a crayfish predator are drab green in color, but males living in streams that lack the crayfish predator have bright red tails. Apparently females prefer to mate with males with red tails, but in streams with a predator (the crayfish) that has good color vision, not enough red-tailed males survive to reproduce.