Download Mechanisms for 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.
• How?
bald eagle migration
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.
– How will the resulting
population be different?
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. How will the new population be
different?
11.3 Other Mechanisms of Evolution
• Genetic drift has negative effects on a population. Why?
– Some important alleles that help in survival disappear
from the population
– less likely to have some individuals that can adapt
– harmful alleles can become more common due to
chance
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
• Name some species that
use sexual selection when
mating.
11.3 Other Mechanisms of Evolution
• There are two types of sexual selection.
– intrasexual selection: competition among males
– intersexual selection: males display certain traits to females
11.4 Hardy-Weinberg Equilibrium
KEY CONCEPT
Hardy-Weinberg equilibrium provides a framework for
understanding how populations evolve.
11.4 Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Biologists use models to study populations. Why?
• Hardy-Weinberg equilibrium is a type of model.
11.4 Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Genotype frequencies stay the same if five conditions are
met.
– very large population: no genetic drift
– no emigration or immigration: no gene flow
– no mutations: no new alleles added to gene pool
– random mating:
no sexual selection
– no natural selection:
all traits aid equally
in survival
11.4 Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Real populations rarely meet all five conditions.
– Real population data is
compared to a model.
– Models are used to
studying how populations
evolve.
11.4 Hardy-Weinberg Equilibrium
The Hardy-Weinberg equation is used to predict genotype
frequencies in a population.
• Predicted genotype frequencies are compared with actual
frequencies.
– used for traits in simple dominant-recessive systems
– must know frequency of recessive homozygotes
– p2 + 2pq + q2 = 1
"The Hardy-Weinberg equation
is based on Mendelian genetics.
It is derived from a simple
Punnett square in which p is the
frequency of the dominant allele
and q is the frequency of the
recessive allele."
11.4 Hardy-Weinberg Equilibrium
There are five factors that can lead to evolution.
11.4 Hardy-Weinberg Equilibrium
• Genetic drift changes allele frequencies due to chance
alone. How can this lead to evolution?
11.4 Hardy-Weinberg Equilibrium
• Gene flow moves alleles from one population to another.
How can this lead to evolution?
11.4 Hardy-Weinberg Equilibrium
• Mutations produce the genetic variation needed for
evolution. How can this lead to evolution?
11.4 Hardy-Weinberg Equilibrium
• Sexual selection selects for traits that improve mating
success. How can this lead to evolution?
11.4 Hardy-Weinberg Equilibrium
• Natural selection selects for traits advantageous for
survival. How can this lead to evolution?
11.4 Hardy-Weinberg Equilibrium
• In nature, populations evolve.
– expected in all populations
most of the time
– respond to changing
environments
11.5 Speciation Through Isolation
KEY CONCEPT
New species can arise when populations are isolated.
11.5 Speciation Through Isolation
The isolation of populations can lead to speciation.
• Populations become isolated when there is no gene flow.
– Isolated populations adapt to their own environments.
– Genetic differences can add up over generations.
– How could this lead to a new species?
11.5 Speciation Through Isolation
• Reproductive isolation can occur between isolated
populations.
– members of different
populations cannot
mate successfully
– final step to
becoming separate
species
• Speciation is the rise of two or more species from one
existing species.
11.5 Speciation Through Isolation
Populations can become isolated in several ways. Give
one example.
• Behavioral barriers can cause isolation.
– called behavioral isolation
– includes differences in courtship or mating behaviors
11.5 Speciation Through Isolation
• Geographic barriers can cause isolation.
– called geographic isolation
– physical barriers divide population – what could happen
to cause a new barrier to form?
• Temporal barriers can cause isolation.
– called temporal isolation
– timing of reproductive periods prevents mating
11.5 Speciation Through Isolation
How will a showy trait in males become established in a population?
• A. Males with a showy trait will mate more often and be more likely to
pass the allele for the trait to offspring.
• B. Males with a showy trait are more likely to survive.
• C. Showy traits are dominant, so they become established in the
gene pool.
• D. Alleles for showy traits are not lost due to genetic drift.
11.5 Speciation Through Isolation
How will a showy trait in males become established in a population?
• A. Males with a showy trait will mate more often and be more likely to
pass the allele for the trait to offspring.
• B. Males with a showy trait are more likely to survive.
• C. Showy traits are dominant, so they become established in the
gene pool.
• D. Alleles for showy traits are not lost due to genetic drift.
• Correct Answer = A
11.5 Speciation Through Isolation
How are the bottleneck effect and founder effect similar?
• A. Both add genetic variation to their gene pools.
• B. Both result in loss of certain alleles from their gene pools.
• C. Both result in their gene pools becoming larger.
• D. In both, a small population with a varied gene pool forms from a
large population with little variation.
11.5 Speciation Through Isolation
How are the bottleneck effect and founder effect similar?
• A. Both add genetic variation to their gene pools.
• B. Both result in loss of certain alleles from their gene pools.
• C. Both result in their gene pools becoming larger.
• D. In both, a small population with a varied gene pool forms from a
large population with little variation.
• Correct Answer = B
11.5 Speciation Through Isolation
How might gene flow benefit a small population?
• A. Harmful alleles could leave the population.
• B. The population might evolve into two species.
• C. The flow of alleles into a population can decrease variation.
• D. The flow of alleles into a population can increase variation.
11.5 Speciation Through Isolation
How might gene flow benefit a small population?
• A. Harmful alleles could leave the population.
• B. The population might evolve into two species.
• C. The flow of alleles into a population can decrease variation.
• D. The flow of alleles into a population can increase variation.
• Correct Answer = D
Review
• Gene flow is the mixing of genes among populations
• Low gene flow increases the chances of a population becoming two
species
• Genetic drift causes a loss of diversity – usually occurs after a significant
event lowers the population dramatically – has a negative affect on the
population
• The founder effect can cause genetic drift if a small number of individuals
found a new area
• Sexual selection occurs when a characteristic increases the chance of
mating
– Intrasexual – members of the same sex compete
– Intersexual – members of one sex “show off” for the other
Review
• Hardy-Weinberg Evolution – a model to explain a population that is not
evolving – used to predict gene frequency
• There are five factors that can lead to evolution
– Genetic drift changes allele frequencies due to chance
– Gene flow moves alleles from one population to another
– Mutations produce the genetic variation needed for evolution
– Sexual selection selects for traits advantageous for mating
– Natural selection selects for traits advantageous for survival
• Isolated populations adapt to their environments and genetic differences
add up over generations
• Final step to speciation is inability to reproduce
• Behavioral, geographic, and temporal barriers can all result in speciation