BIOL108 Lecture Notes - Lecture 6: Allele Frequency, Genetic Drift, Frequency Distribution

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mauvesquirrel844

21 Jun 2018

School

University of Alberta

Department

Biology (Biological Sciences)

Course

BIOL108

Professor

Lien Luong

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EVOLUTION of a POPULATION (lecture 06)

Gene: combination of two alleles – in diploid (two characters) individuals

oDiploid = two chromosomes

One allele per chromosome (usually one chromosome from mom and one from dad)

oHeterozygous: two different alleles

oHomozygous: same alleles

Evolution and Allele Frequencies

Evolution = the change in frequencies of alleles in a population (gene pool) between generations

op = A gene frequency

oq = a gene frequency

Factors that can result in evolutionary change:

o1) natural selection

o2) mutations

o3) gene flow (migration)

o4) genetic drift

Fitness = the extent to which an individuals genotype is represented in in the next generation

Natural Selection can disrupt the frequency of allele frequencies in several ways:

o1) DIRECTIONAL SELECTION

Extreme phenotype is favoured (highest fitness)

Response to steady change in the environment

Frequency distribution of alleles shifts

Directional shift is the mean of the population (variance stays the same)

o2) DISTRUPTIVE SLECTION

Two extreme phenotypes that are favoured

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BIOL108 Full Course Notes

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Document Summary

Gene: combination of two alleles in diploid (two characters) individuals: diploid = two chromosomes. One allele per chromosome (usually one chromosome from mom and one from dad: heterozygous: two different alleles, homozygous: same alleles. Evolution = the change in frequencies of alleles in a population (gene pool) between generations: p = a gene frequency, q = a gene frequency. Factors that can result in evolutionary change: 1) natural selection, 2) mutations, 3) gene flow (migration, 4) genetic drift. Fitness = the extent to which an individuals genotype is represented in in the next generation. Natural selection can disrupt the frequency of allele frequencies in several ways: 1) directional selection. Directional shift is the mean of the population (variance stays the same: 2) distruptive slection. Results in polymorphisms (2 or more divergent phenotypes) Variation is maintained within a population: 3) stablizing selection. The mean of the population stays the same = variation is reduced.

Related Questions

Q1. What are alleles?Different forms of the same gene.Different genes.The different characteristics of an organism.Genes found only in humans.Q2. If you add up the frequency of homozygotes + the frequency of heterozygotes for one gene (i.e. at one locus) in a population, what number do you get?Q3. People who are heterozygous for the sickle-cell allele:Are susceptible to malaria but not sickle-cell anemia.Are susceptible to sickle-cell anemia but not malaria.Are not susceptible to either sickle-cell anemia or malaria.Are susceptible to both sickle-cecell anemia and malaria.Q4. In the case study in this lab, which genotype is represented by "2pq" in the Hardy-Weinberg equation?Homozygous HbAHomozygous HbSHeterozygous HbA/HbSNo specific genotypeQ5. Suppose that, in an isolated village, the proportion of individuals that have sickle-cell anemia is 0.1 (or 1 of every 10 people). What proportion of the population should be sickle-cell carriers, according to Hardy-Weinberg expectations?Q6. When you simulated the elimination of malaria at the beginning of the lab, once the allele frequencies stabilized, what was the frequency of the HbA allele?Q7. When you investigated regional differences, what was the frequency (over many generations) of the HbS allele in the village in the "slightly wet" part of Africa?Q8. What is the number of deaths due to sickle-cell anemia in a region without mosquitoes expected to be?Very lowVery highModerateRelated to the frequency of malaria allelesQ9. As long as there are multiple alleles of a gene in a population, why will the frequencies of the alleles always change over time?Because natural selection changes all allele frequencies in populations over time.Because genetic drift causes random fluctuations of allele frequencies in populations.Because diseases will eliminate some of the alleles, generating fluctuations in their frequencies.Because allele frequencies fluctuate before the alleles eventually become fixed.Q10. Imagine a huge population with a gene that has ten functionally equivalent, neutral alleles. A small but genetically representative group of individuals from the big population is transported to an island, forming a small population. Which of the statements below best describes what will likely happen over time, and why?Because natural selection doesn't act on neutral alleles, the frequencies in the two populations should remain the same over time.Genetic drift will cause one allele to first become fixed in the island population, and later to become fixed in the original, large population.Genetic drift will result in the allele frequencies of the two populations diverging over time, with alleles being lost sooner in the island population.The allele frequencies of the two populations will change similarly due to a combination of genetic drift natural selection.

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In this first exercise, we are going to look for evidence of evolutionary change in a population in the absence of natural selection by looking at the change in allele frequencies over time in a simulated population. We will start with a population of 50 individuals in which there are two alternate alleles (H and h) in equal proportions (each at a frequency of 0.5 or 50%). Individuals have the possible genotypes: HH, Hh or hh. These two alleles do not offer any selective advantage, so neither is selected for or against, meaning they are neutral. We will record the frequency of these alleles over 10 generations. Prior to advancing on to the next generation, six alleles (= three individuals) will be removed at random.

Before you begin, answer the following:

Question

What is your prediction as to what will happen to the frequencies (note that this is different than the number) of these two alleles over 10 generations? Keep in mind that there is no selective advantage of one allele over the other. Be sure to word your prediction as an âif-thenâ statement based on the experiment design. (1 pts).

Q1. What is the founder effect?

Sampling error that occurs during the establishment of a newpopulation by a small number of migrants.

Strong natural selection acting on the founders of a newpopulation because the environment they are now living in is sodifferent from the environment they came from.

A phenomenon named after William Founder, who observed that toppredators generally have low genetic diversity.

Rapid population growth following a bottleneck.

Q2. The effects of genetic drift are more pronounced in largerpopulations.

True

False

Q3. Inbreeding tends to increase the proportion of homozygousindividuals in a population (i.e. individuals that have two copiesof the same allele).

True

False

Q4. The effective size of a population is:

The size of an idealized randomly-mating population that is notunder selection and has the same heterozygosity as the actualpopulation.

The size of an idealized randomly-mating population that has thesame heterozygosity as the actual population, but does not loseheterozygosity over time.

The size of an idealized randomly-mating population losinghomozygosity at the same rate as the actual population.

The size of an idealized randomly-mating population losingheterozygosity at the same rate as the actual population.

Q5. Which of the following tends to reduce the effective size ofa population?

Check all that apply:

Increasing the population size.

A biased sex ratio.

Non-random mating.

Q6. If gametes from a gene pool combine randomly to make only asmall number of zygotes, the allele frequencies among the zygotesmay be different than they were in the gene pool because:

The effects of natural selection are more pronounced in smallpopulations.

The effects of genetic drift over several generations are morepronounced with small numbers of gametes.

The effects of differences in frequencies for different allelesare more pronounced with small numbers of zygotes.

The effects of sampling error are more pronounced with smallsamples.

Q7. Genetic drift is different from natural selectionbecause:

In natural selection allele frequencies change because somealleles confer higher fitness, whereas in genetic drift allelefrequencies change because of chance sampling error.

Natural selection acts primarily in large populations, whereasgenetic drift acts primarily in small ones.

Natural selection is a mechanism of evolution, whereas geneticdrift is an outcome of evolution.

Natural selection tends to cause rapid evolution, whereasgenetic drift tends to cause slow evolution.

Q8. Imagine a population evolving by genetic drift in which thefrequency of allele K is 0.65. What is the probability that at somepoint in the future allele K will drift to a frequency of 1?

Q9. A dwindling population of 1000 frogs occupies an isolatedwatershed in Costa Rica. To help preserve the species, scientistscaught 20 frogs to start a new population in a nearby watershed.This species has a gene that affects eye shape. The 1000-memberwild population has two alleles for this gene: R and r, withfrequencies 0.7 and 0.3, respectively. What will be the allelefrequencies of R and r in the 20-member founder population?

The frequencies will be 0.7 for R and 0.3 for r.

The frequencies will be 1.0 for R and 0 for r.

The expected frequencies are 0.7 for R and 0.3 for r. The actualfrequencies could be different.

The founder populations's allele frequencies will necessarily bedifferent than the source population's frequencies.

Q10. For a population containing 90 females and 10 males, whatis the effective population size, N_e ?

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BIOL108 Lecture Notes - Lecture 6: Allele Frequency, Genetic Drift, Frequency Distribution

BIOL108 Full Course Notes

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