Natural Selection n Selection is the only evolutionary mechanism that leads to nonrandom changes in allele frequencies. n Natural selection can increase or decrease the amount of genetic diversity in a population.

Natural Selection n Types of natural selection: Directional selection decreases diversity by favoring one extreme phenotype in the population. Stabilizing selection decreases diversity by favoring the average phenotype over both extremes. Disruptive selection increases diversity by favoring both extremes, and selecting against the average.

After selection During selection Number of individuals Before selection Normal distribution Directional selection changes the average value of a trait. Value of a trait Figure 22.7a

Body size class Percentage of birds Difference in average Survivors N = 1027 Nonsurvivors N = 1853 For example, directional selection caused overall body size to increase in a Cliff Swallow population Figure 22.7b

Normal distribution High fitness Value of a trait Number of individuals After selection During selection Before selection Stabilizing selection reduces the amount of variation in a trait. Figure 22.8a

Birthweight (pounds) Percentage of mortality Percentage of Population Heavy mortality on extremes Mortality For example, very small and very large babies are most likely to die, leaving a narrower distribution of birthweights. Figure 22.8b

Goldenrod Spherical Gall

The gall fly lays an egg near the apical meristem of a goldenrod n The larva hatches from the egg and burrows into the goldenrod at the apical meristem n Secretions from the larva induce the formation of the gall by the goldenrod plant

The larva overwinters inside the gall, emerging the following spring

The Spot-winged Fly larvae have two types of enemies Parasitoid wasps These wasps oviposit an egg through the wall of the gall The wasp larva hatches and then consumes the gall fly larva

Birds are also enemies of the gall fly larvae

Stabilizing selection is observed for the diameter of the galls n Galls that are thin-walled are subject to wasp parasitism n Galls that are thick-walled are subject to bird predation n Selection favors the galls of intermediate diameter Thick enough to discourage wasp oviposition Thin enough to make bird predation difficult

Value of a trait Low fitness Normal distribution Before selection During selection After selection Number of individuals Disruptive selection increases the amount of variation in a trait. Figure 22.9a

Beak length (mm) Number of individuals For example, only juvenile Black-bellied Seedcrackers with very long or very short beaks survived long enough to breed. Figure 22.9b

Figure 16.18

Song Thrush

Cepaea nemoralis