KEY CONCEPT Populations, not individuals, evolve.

Natural selection acts on distributions of traits. A normal distribution graphs as a bell-shaped curve. highest frequency near mean value frequencies decrease toward each extreme value Traits not undergoing natural selection have a normal distribution. Synthesis: What other types of data might follow a normal distribution?

Natural selection can change the distribution of a trait in one of three ways. Microevolution is evolution within a population. observable change in the allele frequencies can result from natural selection

Natural selection can change the distribution of a trait in one of three ways. 1. Directional Selection Before antibiotics were developed in the 1940’s, a trait for varying levels of drug resistance existed among bacteria. At the time there was no advantage to having drug resistance. But once antibiotics came into use, the resistant bacteria had a great advantage. The overuse of antibiotics to control infectious disease let to even more resistant phenotypes.

1. Directional selection Fig. 2.2

Natural selection can take one of three paths. 2. Stabilizing selection The gall flies lay their eggs in developing shoots of the goldenrod plant. The larvae produce chemicals that cause the plant tissue to swell into a mass called a gall. There is a range of phenotypes for the body size of the gall-fly. Each body size causes the size of the gall vary. There are 2 main predators of gall flies based on the specific gall size. Downy woodpeckers attack the largest gall and feed on those larvae inside. The parasitic wasp lays its own effs inside the small galls. After the wasp larvae emerge from the eggs, they eat the gall-fly larvae.

2. Stabilizing Selection Selective pressure form predators works against fly phenotypes that produce galls at both extremes, large and small. As a result, flies that produce middle-sized galls become more common. Fig. 2.4

What is the difference between stabilizing and directional selection? What type of selection would occur if only the woodpeckers were putting pressure on the gall fly population? What would the resulting graph look like? How is the dashed line being used in the graph?

Natural selection can change the distribution of a trait in one of three ways. 3. Disruptive selection Young male lazuli buntings vary widely in the brightness of their feathers, ranging from dull brown to bright blue. Dominant adult males are those with the brightest feathers on their heads and back. These birds have their pick of the best territories. They are also the most successful at attracting females. However, for young buntings, the brightest blue and dullest brown males are more likely to win mates than males with bluish brown feathers. Research suggests that dominant adult males are aggressive toward young buntings that they see as a threat, including the bright blue and bluish brown males. The dullest brown birds can therefore win a mate because the adult males leave them alone. Meanwhile, the bright blue birds attract mates simply because of their color. Both extreme phenotypes are favored in this situation, while intermediate forms are selected against. The bluish brown males are not as well adapted to compete for mates because they are too blue to be left alone by adult males, but not blue enough to win a mate based on color alone.

Natural selection can take one of three paths. 3. Disruptive selection – By favoring both extreme phenotypes, disruptive selection can lead to the formation of new species. Apply: If bluish brown coloring became advantageous for young males, what type of selection would likely occur in a lazuli bunting population?