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BISC 102 Lecture Notes - Lecture 13: Horizontal Gene Transfer, Allele Frequency, Genetic Drift

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School
Simon Fraser University
Department
Biological Sciences
Course
BISC 102
Professor
Erin Barley

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Document Summary

Population genetics study of allele frequencies and changes to allele frequencies in a population. Hardy weinberg principle provides model for detecting and understanding changing allele frequencies: 2 questions. If a population is not evolving (in hwe), we can predict genotype frequencies from allele frequencies and vice versa. Allele frequency = number of an allele out of a population. Evolution = change in allele frequency in a population over time. Change caused by: natural selection, genetic drift, gene flow. New ones arise: mutation, horizontal gene transfer. Hwe assumptions + microevolution if hwe is violated = population evolving. No mutations violated by mutations, large population by genetic drift. Can result in 3 modes: directional selection, disruptive selection, stabilizing selection: directional = one extreme phenotype is better or worse, disruptive = both extreme phenotypes are better, stabilizing = both extreme phenotypes are worse. Genetic drift = random changes in allele frequencies in a population (especially in a small population)

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Related Questions

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, Ne ?

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.

What must occur in order for a new trait to appear in a population and then reach a steady, high frequency within that population?

a

New traits appear through gene mutations or through genetic drift. In order to reach a steady, high frequency in the population, there must be many mutagens, such as UV radiation, in the environment to produce many mutations.

b

New traits appear through gene mutations or through genetic drift. In order to reach a steady, high frequency in the population, there must be a consistent source of immigrant individuals with the allele.

c

New traits appear through gene mutations or through evolution. In order to reach a steady, high frequency in the population, the allele must code for a favorable adaptation.

d

New traits appear through gene mutations or through gene flow. In order to reach a steady, high frequency in the population, the trait associated with the gene must be favored by either natural or sexual selection.