How does evolution work?

Learning Goals Recognize the importance of sexual reproduction for variability in organisms Understand the process of natural selection and how it accounts for adaptations in organisms Recognize that populations, not individuals, evolve Quick Clicks Teacher's Guide Web Resources Access the Web resources referenced in this guide—from handouts to video segments to Web features—by going to pbs.org/evolution and clicking on Teachers and Students, and then going to the Evolution Teacher's Guide, where the material is presented by unit. F or more than 25 years, Robert Vrijenhoek has been returning to the remote hills of the Sonoran Desert in Mexico to study unique populations of minnows. Living side-by-side in the small hillside pools are two different species of minnows, one an asexual reproducer and the other a sexual reproducer. Vrijenhoek has been trying to understand which conditions might favor the sexual minnows and which favor the asexuals. He has noticed that the sexual species tends to predominate in the river where they are 60 percent to 80 percent of the total fish population. Early on Vrijenhoek discovered that 40 percent of all minnows were infected with a parasite that causes black spot disease. Upon closer investigation, he observed an interesting pattern—most of the parasitized fish were asexual reproducers. " Why should they be more parasitized than the sexual reproducers they were living right beside? " he wondered. At first Vrijenhoek didn't have an answer. Finally it hit him; he was looking at a real world demonstration of the value of sexual reproduction related to an evolutionary hypothesis called the " Red Queen. " This hypothesis, suggested by scientist Leigh Van Valen, asked " Does evolution stop when things get perfectly well adapted to their environment? " The answer is no. Evolution is a race like the one in Alice in Wonderland. Alice and the Red Queen are running as fast as they can and Alice says, " Isn't this curious, as fast as we run, nothing seems to change. We're staying in the same place. " The Red Queen answers, " Yes, you have to run just as fast as you can to stay in the same place. " Evolution is like that too. We live in a complex world full of parasites, viruses, bacteria, predators, and competitive species—all of them evolving. At the moment any species stops evolving in response to these challenges and threats, it is doomed.