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 eventdrastically reducespopulation size.

– The bottleneck effect isgenetic drift that occursafter 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 equallyin 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 iscompared to a model.

– Models are used tostudying how populationsevolve.

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

"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."

– must know frequency of recessive homozygotes– p2 + 2pq + q2 = 1

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