3 Causes of Deviation from Equilibrium 1. Natural Selection

○ Stabilizing selection

○ Disruptive selection

○ Directional selection

2. Genetic Drift○ Founder Effect

○ Bottleneck Effect

3. Gene flow

1. Natural Selection

When natural selection occurs, variation of traits in the populations change over time

3 patterns of change due to natural selectionA. Stabilizing selectionB. Disruptive selectionC. Directional selection

Bell Curve

All of the patterns are compared to the standard bell curve.

Shows that most members of the population share similar values for a certain trait.

What do you observe?

A. Stabilizing Selection Individuals with average (intermediate)

forms of a trait has the highest fitness and the extreme traits have a lower fitness

Extremes- selected against Intermediate- selected for

Ex: Human birth weight

A. Stabilizing Selection

What do you observe?

What do you observe?

B. Disruptive Selection

Selection where both extreme traits are considered more “fit” for the environment than the intermediate trait

Both Extremes- selected for Intermediate- selected against

Ex: African Swallowtail

B. Disruptive Selection

What do you observe?

C. Directional Selection

C. Directional Selection

Individuals in a population with one extreme trait have higher fitness than the individuals with the average trait

One extreme is selected for The average trait and the other extreme

trait are selected against

Ex: Beak size in finchesGuppy experiment

Guppy Experiment

To Summarize…

2. Genetic Drift

Causes deviation from equilibrium

Genetic drift- change in gene pool due to chanceThe smaller the population, the greater the

impact it has on the population○ Ex: Founder effect○ Ex: Bottleneck effect

Founder Effect

When alleles occur at a higher frequency in a population isolated from the general population

The founding individuals could only contain a fraction of the total genetic diversity of the original gene pool.

So the new population has only the alleles that the founders can provideEx: The Amish

Founder Effect

Polydactyly in the Amish•The Amish in Lancaster County are an isolated population that was begun by German founders.

•Some of the founders carried the recessive allele which causes polydactylism (extra fingers)

•Today, as many as 1 in 14 individuals carries that recessive allele.

Bottleneck Effect Occurs when a species is subjected to

near extinction due to natural disaster, overhunting, disease, or habitat loss

Only a few survivors are left from the event

Bottleneck effect prevents the majority of the original genotypes from participating in the next generation.Have much less genetic variation than the

original population

Bottleneck Effect

3. Gene Flow Gene flow- gaining or losing alleles

when individuals move into or out of a populationImmigration- moving intoEmigration- moving out of

Could be moving for new breeding ground, place to hibernate, or more food sourcesEx: Salmon migration

Salmon Migration

Concluding Questions

What are the three instances where populations will deviate from equilibrium?

Natural selection, genetic drift and gene flow

What is the name for the changes in gene pool due to chance?Genetic drift

What is this picture an example of?

Founder Effect

What types of selection do each of these curves represent?

Directional Disruptive Stabilizing