These individuals will then contribute more offspring to the next generation.
Directional selection produced resistance to tetrodotoxin (TTX) in garter snakes.
• Disruptive selection favors individuals that vary in both directions from the mean of the population.
Individuals at both extremes of a population are simultaneously favored. Increases variation in the
Ex: the bill sizes of black-bellied seedcrackers
Sexual Selection influences reproductive success
• Sexual selection was Darwin’s explanation for the evolution of apparently useless but conspicuous traits in males
of many species, such as bright colors, long tails, horns, antlers, and elaborate courtship displays.
• He hypothesized that these traits either improved the ability of their bearers to compete for access to members
of the other sex (intrasexual selection) or made them more attractive to the other sex (intersexual selection).
• Hypothesis: having well-developed ornamental traits signals vigor and health
• Zebra finch bills are bright red because of carotenoids in their diet.
• Carotenoids are antioxidants and part of the immune system. Males in good health will have brighter bills
because they need to allocate fewer carotenoids to immune function.
• Zebra finch males were fed diets with and without carotenoids. The diet with carotenoids enhanced
22.4 – How Is Genetic Variation Maintained within Populations?
• Genetic drift, stabilizing selection, and directional selection all tend to reduce genetic variation within an animal
• However, most species have considerable genetic variation.
• Things that help maintain genetic variation:
populations in different (varying) environments
• Sexual Production Pros & Cons:
Pros: • Permits elimination of deleterious mutations
• Existing genetic variation is amplified. Cons:
• Sexual recombination generates new • Recombination can break up adaptive
combinations of alleles on which natural combinations of genes.
selection can act. • Reduces rate at which females pass genes to
• It expands variation in a trait influenced by offspring.
alleles at many loci by creating n