Evolution & Speciation

Evolution

Evolution is the change in living things over long periods of time.

It may occur through the processes of natural selection and genetic drift.

Natural Selection

Natural selection explains why organisms are adapted to their environment.

This theory describes a natural process that can produce new adaptations and new species.

Natural Selection

4 conditions:1. Reproduction2. Variation in traits between individuals3. Heredity some of this variation must

be heritable (genetic)4. Individuals differ in fitness (the number

of offspring they produce) some individuals are more likely to reproduce than others

Natural Selection

If these 4 conditions are met, then natural selection will result.

Let’s look at an example…

Natural SelectionA cryptic moth:1. These moths reproduce2. There is variation in their wing color pattern 3. This color pattern variation is heritable (offspring resemble

their parents)

http://www.bio.umass.edu/biology/kunkel/catocala.html

Natural Selection4. Because they use their color pattern as

camouflage from bird predators, if individuals do not match their background they will get eaten only moths that look cryptic will survive and reproduce

http://www.bio.umass.edu/biology/kunkel/catocala.html

Natural SelectionOther examples:• Pesticide resistant insects:

- only those that can tolerate the pesticide will survive and reproduce- if this pesticide tolerance is heritable, then their offspring will also be tolerant to the pesticide

e.g. DDT used to be a very effective pesticide and initially wiped out ~90% of mosquitoes in India. After about 10years, mosquitoes evolved resistance to DDT and now only ~20% of mosquitoes are wiped out by DDT.

pesticide

Pesticide resistant insectsSurviving insects

Natural SelectionOther examples:2. Antibiotic resistant bacteria:

- only those that can tolerate the antibiotic will survive and reproduce- if this antibiotic tolerance is heritable, then their offspring will also be tolerant to the antibiotic- have become increasingly prevalent as the use of antibiotics has become widespread

e.g. About 70% of bacteria that cause infections in hospitals are resistant to at least one of the drugs most commonly used to treat infections.

Add antibiotic

Surviving bacteria Antibiotic resistant strains

Natural SelectionTypes of selection:

• Directional selection– Favors one extreme and shifts the population mean

• Stabilizing selection– Favors the mean and decreases variation around the mean

• Disruptive selection– Favors the extremes and selects against the mean

Responses to Selection

Directional selection

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Responses to Selection

Directional selection Stabilizing selection

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Responses to Selection

Directional selection Stabilizing selection Disruptive selection

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Genetic Drift

Genetic drift is the fluctuation of genotypes in the population due to chance.

In small populations, genetic drift can result in non-adaptive evolution.

Genetic Drift

Population 11:1

Population 31:1

Population 26:1

Population 41:6

Genotype AGenotype B

For the population as a whole, the frequency of the 2 genotypes is 1:1

Genetic Drift

Population 11:1

Population 31:1

Population 26:1

Population 41:6

Genotype AGenotype B

For the population as a whole, the frequency of the 2 genotypes is 6:1 the frequency of genotype A has increased due to chance

Genetic Drift

Population 11:1

Population 31:1

Population 26:1

Population 41:6

Genotype AGenotype B

For the population as a whole, the frequency of the 2 genotypes is 1:1 the frequency of the 2 genotypes has remained the same due to chance

Genetic Drift• Endangered species are often reduced to dangerously small numbers and restricted to small, isolated habitats.• In small populations, genetic drift erodes genetic variation which limits a species ability to adapt to environmental change (e.g. disease, global warming, etc) and increases its probability of going extinct due to chance events.

Speciation

What is a species?• A group of interbreeding individuals of

common ancestry that are reproductively isolated from other groups

SpeciationHow might speciation (formation of new

species) occur?

• Allopatric speciation– A barrier divides a continuous population into 2

parts• Parapatric speciation

– A hybrid zone divides a continuous population into 2 connected parts

• Sympatric speciation– A population splits in 2 without any geographic

separation

Speciation

Allopatric speciation

A physical barrier divides the ancestral population. Over time, each isolated population diverges due to natural selection or drift

Ancestral population

Speciation

Allopatric speciation

Parapatric speciation

There is some kind of habitat gradient that results in locally adapted individuals.

hybrid zone

A physical barrier divides the ancestral population. Over time, each isolated population diverges due to natural selection or drift

Ancestral population

Speciation

Allopatric speciation

Parapatric speciation

Sympatric speciation

There is some kind of habitat gradient that results in locally adapted individuals.

hybrid zone

A physical barrier divides the ancestral population. Over time, each isolated population diverges due to natural selection or drift

The extremes in the population are selected for and there is assortative mating.

Ancestral population

Allopatric Speciation

http://cas.bellarmine.edu/tietjen/images/speciation.htm

Blue-headed wrasse

Cortez rainbow wrasse

Isthmus of

Panama

Parapatric Speciation

http://cas.bellarmine.edu/tietjen/images/speciation.htm

Sympatric Speciation

http://www.cichlidnewsmagazine.com/issues/2001apr/nicaragua.html

Is thought to have occurred in Cichlids in a lake in Central America (also in Africa).

The diversity of the animals we see today is the result of processes such as natural selection and genetic drift that lead to speciation over time.

Hawaiian honey-creepers