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Compare-Contrast-Connect: Natural and Sexual Selection

NGSS Disciplinary Core Ideas

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.


SF Fig. 1.5. (A) A male wild guppy (Poecilia reticulata) swims above two females.

Image courtesy of Per Harald Olsen, Wikimedia Commons

SF Fig. 1.5. (B) Resplendent quetzal (Pharomachrus mocinno) female (left) and male (with yellow beak on right) are found in the trees of Costa Rica.

Images courtesy of Joseph C. Boone, Wikimedia Commons


Question Set
  1. Use your own words to define the term sexual selection.
  2. Among the animal species where mate choice exists, it is very common for females to choose male mates rather than vice versa. Explain why this phenomenon is so common.
  3. The following organisms have some strange adaptations. Which features do you hypothesize might be the result of natural selection to avoid predation, and which might be the result of natural selection for sexual competition? Explain your reasoning.
    1. Flashy (large, colorful) flowers
    2. Quetzel with long tail (SF Fig. 1.5 B)
    3. Camouflaged fish
    4. Frog with very loud croak
    5. Urchin with long spines
    6. Running cheetah
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.