Evolution

11.1 KEY CONCEPT

A population shares a common gene pool.

Why it’s beneficial:

• Genetic variation leads to phenotypic variation.

• It increases the chance that some individuals will survive

• Phenotypic variation is necessary for natural selection.

How it’s stored in a population…

• Genetic variation is stored in a population’s gene pool.

– made up of all alleles in a population

– Different allele combinations form when organisms have offspring

• Allele frequencies measure genetic variation.

– measures how common allele is in population

– can be calculated for each allele in gene pool

How it’s measured:

Genetic variation comes from two main sources.

• Mutation is a random change in the DNA of a gene.

• Recombination forms new combinations of alleles.

– can form new allele

– can be passed on to

offspring if in

reproductive cells

– usually occurs during meiosis

– parents’ alleles

arranged in new

ways in gametes

Genetic variation comes from several sources.

• Hybridization is the crossing of two

different species.

– occurs when individuals can’t find mate of

own species

– topic of current scientific research

11.2 KEY CONCEPT

Populations, not individuals, evolve.

Natural selection acts on distributions of traits.

• A normal distribution graphs as a bell-shaped curve.

• A population follows a

normal distribution when:

that population is not under

natural selection for the trait

– highest frequency near

mean value

– frequencies decrease

toward each extreme

value

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Natural selection can change the distribution of a trait

in one of three ways.

This is called microevolution. It is evolution within a

population.

– There is an observable change in the allele frequencies

over time

– can result from natural selection

Natural selection can take one of three paths.

– Directional selection favors phenotypes at one

extreme.

– Example: drug-resistance in bacteria

Draw the solid line in your notes

– Stabilizing selection favors the

intermediate phenotype.

– Example: size of gall flies

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– Disruptive selection favors both

extreme phenotypes.

– Example: body color in bunting

birds

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11.3 KEY CONCEPT

Natural selection is not the only mechanism

through which populations evolve.

Gene flow

Definition: movement of alleles between populations.

How it works:

• When animals migrate from one

population to another and breed

in the new population.

• immigration & emigration

Lots of gene between populations

keeps populations (genetically)

similar.

Limited gene flow results in

• Genetically different populations

that could evolve into diff species

Genetic drift

Definition: change in allele frequencies due to chance.

• How it works:

– causes a loss of genetic diversity and is most common in

small populations.

Key Terms:

• Bottleneck Effect

occurs when an event

drastically reduces

population size.

• Founder effect

– It occurs when a few individuals start a new population.

– The founder effect is genetic drift that occurs after the

start of new population.

• Genetic drift has negative effects on a population.

– some individuals can’t adapt

– harmful alleles can become more common due to chance

Sexual selection

Definition: occurs when certain traits increase mating

success.

How it works:

• 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

• Females preferentially

mate with males that

display certain traits

• There are two types of sexual selection.

– intrasexual selection: competition among males for a

female

– intersexual selection: males display certain traits to attract

females

11. 5 KEY CONCEPT

New species can arise when populations are

isolated.

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.

• 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.

Populations can become isolated in several ways.

1. Behavioral barriers can cause isolation.

– called behavioral isolation

– includes differences in courtship or mating behaviors

2. Geographic barriers can cause isolation.

– called geographic isolation

– physical barriers like mountains and rivers divide

population

3. Temporal barriers can cause isolation.

– called temporal isolation

– timing of reproductive periods prevents mating

11.6 KEY CONCEPT

Evolution occurs in patterns.

Evolution through natural selection is not random.

• Natural selection can have direction.

• The effects of natural selection add up over time.

• Convergent evolution describes

evolution toward similar traits in

unrelated species.

• Divergent evolution describes evolution toward

different traits in closely related species.

How do convergent and divergent

evolution illustrate the directional

nature of natural selection?

ancestor

Kit fox red fox

LE 24-6

A. harrisi A. leucurus

Species can shape each other over time.

• When two or more species evolve together, this is called

coevolution.

– evolutionary paths become connected

– species evolve in response to changes in each other

• Coevolution can occur in beneficial relationships.

• Both species receive benefits from the other as a result of

adaptations that each species evolved over many

generations

The ant lives inside hollow

thorns and protects the acacia

by stinging any potential

predators

• Coevolution can occur in competitive relationships,

sometimes called evolutionary arms races.

• Both species respond to competitive pressure from

the other through adaptations over many

generations

Species can become extinct.

• Extinction is the elimination of a species from Earth.

• Background extinctions occur randomly but at a

very low rate.

– usually affects a few species

in a small area

– caused by local changes in

environment

• Mass extinctions are rare but much more intense.

– destroy many species at global level

– thought to be caused by catastrophic events like

earthquakes, volcanic eruptions, etc.

– at least five mass extinctions in last 600 million years

Speciation often occurs in patterns.

• A pattern of punctuated equilibrium exists in the fossil

record.

– theory proposed by Eldredge and Gould in 1972

– episodes of speciation occur suddenly in geologic time

followed by long periods of little evolutionary change

– revised Darwin’s idea that species arose through

gradual transformations

• Many species evolve from one species during

adaptive radiation.

– ancestral species diversifies into many descendent species

– descendent species

usually adapted to

wide range of

environments