Hardy-Weinberg Genetic Hardy-Weinberg Genetic EquilibriumEquilibrium

Flamingo Flamingo populationpopulation

Remember: to maintain Remember: to maintain equilibriumequilibrium

Tule Elk population at Pt Tule Elk population at Pt ReyesReyes

1.1. No change due to No change due to mutations.mutations.

2.2. Individuals do Individuals do not move in or not move in or out of the out of the population.population.

3.3. The population is The population is and remains and remains large.large.

4.4. Random mating.Random mating.5.5. No selectionNo selection

Disruption of Genetic EquilibriumDisruption of Genetic Equilibrium

Tule Elk population at Pt. ReyesTule Elk population at Pt. Reyes

• If any of the 5 If any of the 5 factors occur, factors occur, they can cause they can cause the gene the gene frequency to frequency to change in a change in a population.population.

• If the gene If the gene frequencies frequencies change evolution change evolution has occurred.has occurred.

MutationsMutations

• Mutations occur normally but at extremely low Mutations occur normally but at extremely low rates.rates.

• Mutations do not cause gene frequencies to Mutations do not cause gene frequencies to change.change.

MigrationMigration• Immigration (movement in) or emigration Immigration (movement in) or emigration

(movement out) can effect gene (movement out) can effect gene frequencies.frequencies.

• Movement of individuals from one Movement of individuals from one population to another results in gene flow.population to another results in gene flow.

Genetic DriftGenetic Drift• Small populations Small populations

are subject to are subject to genetic drift.genetic drift.

• Random events Random events lead to changes in lead to changes in gene frequencies.gene frequencies.

Each line represents a different population. Each starts with the same number of allele A. over time the number of individuals with that allele changes. If a population is small enough over several generation the allele will either be 0 or 100% present.

Nonrandom MatingNonrandom Mating

• Random mating Random mating throughout a throughout a population does not population does not occur.occur.– Geographical Geographical

regions.regions.– Sexual selectionSexual selection

• Ex: birds of Ex: birds of paradiseparadise

– Assortive matingAssortive mating• Ex: Fruit flies.Ex: Fruit flies.

Directional SelectionDirectional Selection

Cactus and wild pigsCactus and wild pigs

• Wild pigs impose Wild pigs impose selection upon the selection upon the cactus population.cactus population.

– Cactus that have Cactus that have fewer needles do not fewer needles do not survive to survive to reproduce.reproduce.

– Cactus with more Cactus with more needles survive to needles survive to reproduce.reproduce.

• Result of directional Result of directional selection is a shift in selection is a shift in the population the population towards more towards more spines.spines.

• One extreme is One extreme is selected for and one selected for and one against.against.

• Ex: cliff swallows Ex: cliff swallows after a cold snap.after a cold snap.

Directional Selection resultsDirectional Selection results

I’ve posted a short article on how cliff swallow wing length has shorted due to new selective pressures. Read it and refer to it in your summary.

• Both extremes are Both extremes are selected against.selected against.

• The average is The average is selected for.selected for.

• Population becomes Population becomes more similar due to more similar due to average being average being selected for.selected for.

• Ex: human birth Ex: human birth weightweight

Stabilizing SelectionStabilizing Selection

• Extremes are Extremes are selected for. selected for.

• Results in two Results in two distinctly different distinctly different populations.populations.

• Disruptive selection Disruptive selection can lead to two can lead to two species.species.

• Average is selected Average is selected against.against.

• Ex: finches in west Ex: finches in west Africa, eat different Africa, eat different sized seeds. sized seeds.

Disruptive SelectionDisruptive Selection

Calculating allele frequencies

• Use the Hardy-Weinberg equation: – p² + 2pq + q² = 1 – “q” is the recessive trait and q² is homozygous

recessive– “p” is the dominate trait and p² is homozygous

dominant. – pq is heterozygous – added together you have 100% of the

individuals in a population

Disruption of Genetic Equilibrium - Disruption of Genetic Equilibrium - Summary of SelectionSummary of Selection

Summary Review1. Describe the 3 types of selection.2. What is genetic drift, what causes it, an what

results from it?3. What can cause new genes to enter a

population? 4. What do these two pictures illustrate?