Download 11.2 Natural Selection in Populations

11.2 Natural Selection in Populations
KEY CONCEPT
Populations, not individuals, evolve.
11.2 Natural Selection in Populations
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?
11.2 Natural Selection in Populations
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
11.2 Natural Selection in Populations
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.
11.2 Natural Selection in Populations
– 1. Directional selection
Fig. 2.2
11.2 Natural Selection in Populations
• 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.
11.2 Natural Selection in Populations
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
11.2 Natural Selection in Populations
What is the difference between stabilizing and
directional selection?
1. What type of selection would occur if only the
woodpeckers were putting pressure on the gall fly
population?
2. What would the resulting graph look like?
3. How is the dashed line being used in the graph?
11.2 Natural Selection in Populations
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.
11.2 Natural Selection in Populations
• 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?