Deviations from Hardy Weinberg Equilibrium

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 selection A. Stabilizing selection B. Disruptive selection C. 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? What are we measuring here? What’s the red line represent? What does the green line represent? So at which birth weight is the infant mortality the lowest? Which birth weights are favored? Why would these birth weights be favored?

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 is being selected here? Notice how the peak of the curve narrows compared to the original bell curve.

What do you observe? These are two types of British land snails. These snails have a wide habitat range which can include forests and low- vegetation areas like grass fields and hedgerows. Why do you think their shells are different patterns?

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? What do you notice about this picture? Which beak depth is favored? Why would this type of beak be favored for finches over time?

C. Directional Selection What is being selected for in this picture? How does it compare to the original population?

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 finches Guppy experiment

Guppy Experiment - Endler and Reznick performed an experiment on guppies in Trinidad, who were generally very brightly colored. They noticed that when there was predation, these guppies tended to be drab in color and a small size. In areas with low predation, the guppies were brightly colored and larger. - They took a supply of small drab guppies and placed some in a high-predation area (below a waterfall) and placed some in a low-predation (above the waterfall) area. The waterfall prevented the predators from entering the low predation area. - After 12 months, they found that the guppy population above the waterfall underwent directional selection- they became brightly colored and large. The guppies below the waterfall with the predators remained small and drab.

To Summarize… Yellow represents the traits that are selected for.

2. Genetic Drift Causes deviation from equilibrium Genetic drift- change in gene pool due to chance The 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 provide Ex: 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 population Immigration- moving into Emigration- moving out of Could be moving for new breeding ground, place to hibernate, or more food sources Ex: 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