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:
• Existing genetic variation is amplified.
• Sexual recombination generates new
combinations of alleles on which natural
selection can act.
• It expands variation in a trait influenced by
alleles at many loci by creating new genotypes.
• Sexual reproduction facilitates repair of
damaged DNA. Damage on one chromosome
can be repaired by copying intact sequence on
the other chromosome.
• Permits elimination of deleterious mutations
• Recombination can break up adaptive
combinations of genes.
• Reduces rate at which females pass genes to
• Dividing offspring into genders reduces the
overall reproductive rate.
• An allele that does not affect the fitness of an organism is called a neutral allele.
• Neutral alleles tend to accumulate in a population of organisms over time, increasing genetic variation.
Frequency-dependent selection maintains genetic variation within populations.
• A polymorphism is the coexistence of two or more alleles at a locus at frequencies greater than mutations can
• A polymorphism may be maintained when the fitness of a genotype (or phenotype) varies with its frequency
relative to that of other genotypes (or phenotypes).
• This process is known as frequency-dependent selection.
• Ex: Fish with right- and left-mouthed individuals in Lake Tanganyika
• Subpopulations vary genetically because they are subjected to different selective pressures in different