#7 Genetic Drift and Natural Selection

Question 1

List 3 factors that affect genetic drift.

Answer 1

• Changes in population size and unequal umbers of reproducing males/females
• Driven by Chance or randomness
• Meiosis causes random changes in allele frequencies

Question 2

Define effective population size.

Answer 2

Effective population size: number of individuals in an ideal population (in which every adult reproduces) where the rate of genetic drift (measured by the rate of decline of heterozygosity) would be the same as the rate of drift in the actual population under study

Question 3

What is a population bottleneck?

Answer 3

Population Bottleneck: is a dramatic decreases in population size that results in intense genetic drift for a time

Question 4

What is a founding event? Describe an example (be specific and focus on the genetic changes that can occur).

Answer 4

Founding event: when a new population is started from a small number of individuals and reduces genetic variation
Example 1: zebra finch from Australia colonized some islands offshore and they showed a lot less genetic variation than their mainland counterparts
Example 2: Pennsylvania Amish all intermarried and started from a few hundred people which resulted in a random set of rare alleles to become more common in that population which was polydactyly

Question 5

5. For a diploid population, N (or Ne)=10000. What is probability of two gene lineages coalescing 1 generation ago, 2 generations ago and three generations ago? What is the mean coalescence time for two gene lineages?

Answer 5

Heterozygosity depends on time so the longer time goes on the less heterozygosity. A drop in population leads to a drop on heterozygosity

Question 6

For a diploid population, N (or Ne)=10000 and n=10, what is the expected time to the most recent common ancestor of all gene lineages in the sample?

Answer 6

Average time to the most recent common ancestor is two times the effective population size

Question 7

Define natural selection. What are two examples of natural selection?

Answer 7

Natural selection: the process by which certain inherited traits make it easier for some individuals to thrive and multiply
ex 1: positive selection
ex 2: balancing selection

Question 8

When traits are under selection, what is happening at a genetic level? Explain using an example of selection.

Answer 8

One example is positive selection where a gene is beneficial for the population so after a long time everyone has that specific gene which results in decreases in genetic diversity where the only gene on that locus is the positive selected one.

Question 9

What are the 4 basic conditions for natural selection? Give an example of each condition.

Answer 9

1. There is variation in reproductive success
   Ex: # of offspring
2. Variation in the trait of interest
   Ex: height, intelligence
3. There is a Correlation between the trait and reproductive success
   Ex: individuals who better resist disease may reproduce more successfully than those who do not resist as well
4. The trait is heritable
   Ex: phenotype and genotype are inherited