Download 11.3 Other Mechanisms of Evolution

11.3 Other Mechanisms of Evolution
KEY CONCEPT
Natural selection is not the only mechanism through
which populations evolve.
11.3 Other Mechanisms of Evolution
Gene flow is the movement of alleles between
populations.
• Gene flow occurs when
individuals join new
populations and reproduce.
• Gene flow keeps neighboring
populations similar.
• Low gene flow increases the
chance that two populations
will evolve into different
species.
Predict: How does gene flow
affect neighboring populations?
bald eagle migration
Fig. This map shows the locations where young banded eagles were found during the first
summer after hatching.
11.3 Other Mechanisms of Evolution
Genetic drift is a change in allele frequencies due to
chance.
• Genetic drift causes a loss of genetic diversity.
• It is most common in small populations.
• A population bottleneck can lead to genetic drift.
– It occurs when an event
drastically reduces
population size.
– The bottleneck effect is
genetic drift that occurs
after a bottleneck event.
Fig. The bottleneck effect
describes the effect of a destructive
event that leaves only a few
survivors in a population.
Initial
population
Bottleneck effect
Surviving
population
11.3 Other Mechanisms of Evolution
• The founding of a small population can lead to genetic drift.
– It occurs when a few individuals start a new population.
– The founder effect is genetic drift that occurs after start
of new population.
A bird carries a few seeds to a
new location. These seeds
“found” a new population.
A. The gene pool for a population of
flowers has genetic diversity that results in
red, yellow and blue phenotypes.
B. Alleles for yellow flower color increase in the
new small population through genetic drift.
Apply: Why is genetic drift more likely to occur in smaller populations?
11.3 Other Mechanisms of Evolution
• Genetic drift has negative effects on a population.
– less likely to have some individuals that can adapt to a
changing environment due to loss of genetic variation
– harmful alleles can become more common due to
chance
Example of Genetic Drift
Cheetahs exhibit much lower levels of
variation than other mammals. In most
species, related individuals share about 80
percent of the same genes. With cheetahs, this
figure rises to approximately 99 percent. The
genetic inbreeding in cheetahs has led to low
survivorship (a large number of animals
dying), poor sperm quality, and greater
susceptibility to disease. Inbred animals suffer
from a lack of genetic diversity.
11.3 Other Mechanisms of Evolution
Implications of the Founder Effect in National Parks
• What are some population management strategies
National Parks can use to prevent genetic drift from
occurring in reintroduced species?
– Yellowstone National Park wolf reintroduction.
– Cherokee Nation American Bison reintroduction.
11.3 Other Mechanisms of Evolution
Sexual selection occurs when certain traits increase
mating success.
• Sexual selection occurs
due to higher cost of
reproduction for females.
– males produce many
sperm continuously
– females are more
limited in potential
offspring each cycle
Apply: Male Irish elks, which are now extinct, had
12-foot wide antlers. Describe how sexual selection
could have caused such an exaggerated trait to evolve.
Fig.
Widowbird
11.3 Other Mechanisms of Evolution
• There are two types of sexual selection.
– intrasexual selection: competition among males
– Ex: Head-butting of bighorn sheep
– intersexual selection: males display certain traits to females
– Ex: peacocks fanning out their tail feathers
Fig. Male frigate birds inflate an air sac
in their chest to attract females. This trait
has evolved through sexual selection.
This trait is not always adaptive for the
survival of the individual. The bright red
air sac likely make the male frigate bird
easy to spot by predators.
11.3 Other Mechanisms of Evolution
Identifying patterns
Graph 1: Tail feather length and reproductive success
• Biologist were trying
to determine the
relationship between
tail feather length and
reproductive success in
male widowbirds.
Interpret: According to the
data, what gives a male
widowbird greater reproductive
success?
11.3 Other Mechanisms of Evolution
Identifying patterns
• Three experiments
were run to
determine if female
guppies prefer males
with specific tail
sizes.
Analyze: What is the
relationship between tail
size in male guppies and
female preference for
mates?
Infer: Why might the
difference in preference
be larger in Experiment 1
than in Experiment 2?
Graph 2: Sexual selection in guppies