Do Now

Answer the following questions in your notebooks:

1. What causes variation in populations?

2. What is a gene pool?

What have we learned about evolution so far?

• Evolution is change in the traits of a population over time

• Natural selection is the mechanism of evolution, and it acts on an organism’s phenotype

• Evidence of evolution comes from the fossil record, DNA and proteins, comparative anatomy, and embryology

• Isolation can lead to speciation. The major types of isolation are geographic, behavioral, temporal, and reproductive

• Variation within a species increases the probability that some members of the species will survive

• A great diversity of species increases the chance that at least some organisms will survive great changes in the environment

Review: MutationsMutations are changes in DNA sequence or chromosome structure that can change the proteins produced by an organism. Mutations can result in different traits.

How do mutations occur? Do mutations ever stop happening? Write the answers in your notes now.

Mutations produce variation within species.

Vocabulary1. Alleles: different forms of a gene2. Frequency: how often something happens3. Allele frequency: the percentage of alleles for a specific trait within a gene pool3. Immigration: to enter into a new population4. Emigration: to exit a population5. Genetic drift: change in allele frequency in a population6. Founder effect: when allele frequencies change as a result of the migration of a small subset of a population out of the group7. Population bottleneck: when a population experiences a large decrease in the number of individuals

Genetic DriftIn small populations, an allele can become more or less common in the gene pool simply by random chance, rather than by natural selection. The smaller a population is, the greater the chance that this kind of random change in allele frequency will happen. This random change in allele frequency is called genetic drift.

Genetic Drift: Key PointIn small populations, individuals that carry a particular allele may have more offspring than other individuals just by random chance. Over time, several random chance events like this can cause an allele to become common in a population.

Answer in your notes now: After genetic drift occurs, will there be more or less variation in the population? Why?

The Founder Effect

The founder effect is a specific type of genetic drift. The founder effect can be seen when a few individuals migrate out of a population and start a new population with a different allele frequency than the original population.

The Founder Effect: Ellis-van Creveld Syndrome

The founder effect can be seen in the Amish population in Eastern Pennsylvania. This population was founded by about 200 German immigrants. One couple in this founding population, Samuel King and his wife, each carried an allele for a rare recessive genetic disorder called Ellis-van Creveld syndrome and passed it onto their children.

The Founder Effect: Ellis-van Creveld Syndrome

This syndrome causes short stature, polydactly, and several other abnormalities. Since the members of this population only have children with other members of the Amish population, no new genetic variation is being introduced. Now, the percentage of people in this Amish community with Ellis-van Creveld syndrome is much higher than the percentage of people with the disease in the rest of the United States.

Population Bottlenecks

A population bottleneck is when a population becomes much smaller due to random events (earthquakes, fires, floods, droughts, etc) or due to human activities such as excessive hunting, habitat destruction, and poaching. When a population becomes much smaller, the amount of genetic variation is also reduced. This reduction in variation also reduces the chances that the population will survive. Some populations that experience bottlenecks go extinct.

Population Bottleneck in Cheetahs

About 10,000 years ago - because of climate changes - all but one species of the cheetah became extinct. With the drastic reduction in their numbers, close relatives were forced to breed, and the cheetah became genetically inbred, meaning all cheetahs are closely related.

Population Bottleneck in CheetahsWhen geneticists looked at the amount of genetic variation in cheetahs, they found that they have much less variation than other mammals. The inbreeding in cheetahs has led to low survival rates, and greater susceptibility to disease. Inbred animals suffer from low genetic diversity. This means cheetahs do not have the ability to adjust to sudden changes in the environment, such as disease epidemics, and have unusually high susceptibility to certain viruses. For example, if a virus gets into a healthy population of leopards, not every animal dies; only some do, because leopards are genetically diverse. But if every animal is genetically the same, like the cheetah, and one gets infected, all of them may become infected and die. Because of their low genetic diversity, a deadly virus could kill all of the worlds' wild cheetahs instead of just the susceptible animals.

Question for YOU!

Write the answer in your notes now:

What is the major difference between the founder effect and the bottleneck effect?

Directions for Genetic Drift Activity

In this activity, you will examine the effects of genetic drift on the phenotypes of a population of the organism Colored pompomius. You may work with a partner on the activity, but each person must complete their own handout and answer questions in their own words.

Let’s read the instructions and the questions together before we get started.