22. Evolution of Populations

Hardy-Weinberg Model

The Evolution of Populations: Natural Selection, Genetic Drift, and Gene Flow

There are 25 individuals in population 1, all with genotype AA, and there are 40 individuals in population 2, all with genotype aa. Assume that these populations are located far from each other and that their environmental conditions are very similar. Based on the information given here, the observed genetic variation most likely resulted from
a. genetic drift.
b. gene flow.
c.nonrandom mating.
d. directional selection.

 No. The frequency of the recessive allele is 0.2. 

 No. This is the frequency of homozygous recessive individuals. 

 No. The frequency of the dominant allele is 0.8. 

No. This is pq, so you are close to the solution.

You will need to apply the Hardy-Weinberg equation.

In a certain group of African people, 4% are born with sickle-cell disease (homozygous recessive). If this group is in Hardy-Weinberg equilibrium, what percentage of the group has the selective advantage of being more resistant to malaria (heterozygous) than those individuals who are homozygous for normal hemoglobin or for sickle-cell disease?

In a population of 2500, how many babies would you expect to have cystic fibrosis, a homozygous recessive condition, if the frequency of the dominant allele is 0.9 and the population is at Hardy–Weinberg equilibrium?

a. 0.9×2500=2250
b. 2×0.9×0.1×2500=450
c. 0.9×0.1×2500=225
d. 0.1x0.1x2500=25

Hardy Weinberg Equation and Equilibrium

 No. This is one mechanism for altering allele frequencies, but there is a better answer. 

 None of the listed responses is correct. 

 All of the listed responses are correct. 

Don't forget about chance.

Allele frequencies can be altered by __________.

Determine what is incorrect in the following statement: Red deer mutated their genes so that they could have larger antlers to compete for mates.

 A large population, no mutations, with natural selection 

 No. A population in which natural selection is acting is not in Hardy-Weinberg equilibrium. 

 A large population, random mutations, and no migration of alleles in or out of the population 

 No. Mutations would negate Hardy-Weinberg equilibrium. 

 No mutations, no natural selection, with sexual selection 

 No. Hardy-Weinberg equilibrium requires random mating. A population with sexual selection would not be randomly mating. 

 Random mating, a small population, and no mutations 

 No. A small population would alter Hardy-Weinberg equilibrium. 

 Random mating, no natural selection, and a large population 

Does the Hardy-Weinberg principle describe a population that is real or hypothetical?

Which of the following sets of conditions is required for Hardy-Weinberg equilibrium?

In a population with two alleles, B and b, the allele frequency of b is 0.4. B is dominant to b. What is the frequency of individuals with the dominant phenotype if the population is in Hardy-Weinberg equilibrium?
a. 0.16
b. 0.36
c. 0.48
d. 0.84

Mechanisms of Natural Selection Part 1: Types of Sexual Selection

No. p (the frequency of the dominant allele) = 1 &#8722; q (the frequency of the recessive allele), or 0.9.

Apply the Hardy-Weinberg equation.

In a large population of bonobos, the frequency of the recessive allele is initially 0.1. There is no migration and no selection. What is the frequency of the dominant allele? Assume that there are two alleles of this gene.

 No. This would be the percentage of individuals who show the dominant trait. 

No. This would be the value of p2.

 The answer cannot be determined from the information given. 

 No. One of the other choices is correct. 

Apply the equation for Hardy-Weinberg equilibrium.

In a population in Hardy-Weinberg equilibrium, 1% of the individuals in a population show the recessive trait of a certain characteristic. In this situation, what is the value of p?

 the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will go up, and the frequency of the homozygous recessives will remain the same 

 No. Heterozygotes express the dominant trait. 

 the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will remain the same, and the frequency of the homozygous recessives will go up 

 the frequency of the homozygous dominants will go up, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go down 

 No. The dominant phenotype is being selected against. 

 the frequency of the homozygous dominants will go up, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go up 

 the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go up 

Is the human intervention random or selective?

Consider a gene that exists in two allelic forms in a simple Mendelian dominant/recessive pair. In a large population of randomly breeding organisms, the frequency of a recessive allele is initially 0.3. There is no migration and no selection. Humans enter this ecosystem and selectively hunt individuals showing the dominant trait. When the gene frequency is reexamined at the end of the year, __________.

Hardy-Weinberg Equilibrium

In humans, albinism is caused by loss-of-function mutations in genes involved in the synthesis of melanin, the dark pigment in skin. Only people homozygous for a loss-of-function allele (genotype aa) have the albino phenotype. In Americans of northern European ancestry, albino individuals are present at a frequency of about 1 in 10,000 (or 0.0001). Assuming that genotypes are in Hardy–Weinberg equilibrium, what is the predicted frequency of Caucasians in the United States who carry a single allele for albinism?

No. You forgot that the number of hybrids is 2pq, not pq.

 No. This would be the answer if the allele for cystic fibrosis were dominant. 

 No. You are not using the Hardy-Weinberg equation. 

No. That would be correct if q equaled 0.0004 (1/2,500); however, q2 equals 0.0004&mdash;it is the frequency of the recessive genotype, not the recessive allele.

Carriers are heterozygotes.

Approximately 1 out of every 2,500 Caucasians in the United States is born with the recessive disease cystic fibrosis. According to the Hardy-Weinberg equilibrium equation, approximately what percentage of people are carriers?

Solving Hardy Weinberg Problems

A fruit fly population has a gene with two alleles, A1 and A2. Tests show that 70% of the gametes produced in the population contain the A1 allele. If the population is in Hardy-Weinberg equilibrium, what proportion of the flies carry both A1 and A2?
a. 0.7
b. 0.49
c. 0.42
d. 0.21

 Evolution is the tendency for some individuals in a population to leave more offspring than others. 

 No. This tendency was one of Darwin's major inferences, but it is not measurable. 

 Evolution is the way in which sexual reproduction can rapidly spread advantageous traits throughout a population. 

 No. Sexual reproduction is not necessary for populations to evolve. 

 Evolution is the explanation for how organic molecules formed from inorganic molecules. 

 No. Abiotic synthesis is the explanation of how organic molecules formed from inorganic molecules. 

 Evolution is always caused by natural selection. 

 No. Gene flow and genetic drift can have significant effects on allele frequencies from generation to generation. 

 Evolution is a change in a population's allele frequencies over generations. 

How can we actually measure evolution?

In the context of populations, how do we define evolution?

BioFlix: Allele Frequencies

Population Genetics: When Darwin Met Mendel - Crash Course Biology #18

 No. The total number of alleles in the gene pool depends on the number of individuals in the population. 

 the frequency of heterozygous individuals in the population 

No. Heterozygotes are represented by 2pq.

 the frequency of homozygous recessive individuals in the population 

No. The frequency of homozygous recessives is q2.

 the frequency of dominant alleles in the population 

No. The frequency of dominant alleles in the population is p, not p2.

 the expected frequency of homozygous dominant individuals in the population 

Review the equation for Hardy-Weinberg equilibrium.

In the Hardy-Weinberg equation, p2 represents __________.

Sickle-cell disease is caused by a recessive allele. Roughly one out of every 400 African Americans (0.25%) is afflicted with sickle-cell disease. Use the Hardy-Weinberg equation to calculate the percentage of African Americans who are carriers of the sickle-cell allele. (Hint: q2 = 0.0025.)

 the frequency of heterozygous individuals in the populations 

No. In a situation in which there are only two alleles of a gene, the 2pq term in the equation for Hardy-Weinberg equilibrium represents the frequency of heterozygotes.

No. In a situation in which there are only two alleles of a gene, q2 represents the frequency of homozygous recessive individuals.

 all the possible phenotypes in the population 

 No. The Hardy-Weinberg equation is concerned with genotype, not phenotypes. 

 the frequency of homozygous dominant individuals in the population 

No. In a situation in which there are only two alleles of a gene, p2 represents the frequency of homozygous dominant individuals.

 the sum of the frequencies of the genotypes for a particular gene locus 

What does each term in the equation for Hardy-Weinberg equilibrium represent?

In the equation for Hardy-Weinberg equilibrium, 1 represents __________.

In what sense is the Hardy–Weinberg principle a null hypothesis?

If the nucleotide variability of a locus equals 0%, what is the gene variability and number of alleles at that locus?
a. gene variability=0%; number of alleles=0
b. gene variability=0%; number of alleles=1
c. gene variability=0%; number of alleles=2
d. gene variability>0%; number of alleles=2

 No. Homozygous dominant individuals will exist in the population. 

 No. Review the Hardy-Weinberg equation. 

Apply the Hardy-Weinberg equation, remembering that we are interested in the next generation.

Assume a population in Hardy-Weinberg equilibrium with these genotypic frequencies: AA = 0.25, Aa = 0.50, and aa = 0.25. If you remove all the homozygous dominants and allow the remaining population to reproduce (again under Hardy-Weinberg conditions), what will be the frequency of homozygous dominants in the next generation?

 The best traits for the current environment arise in the population and are selected for. 

 No. Natural selection can only act on variations that exist or arise in a population by chance. There is no guarantee that any of the existing alleles provide the best traits. 

 Brand-new body plans arise often, giving natural selection many new possibilities to work with. 

 No. Descent with modification has a long history. Existing body plans have been extensively modified for different environments, but new ones rarely arise. 

 Sexual selection acts to make organisms better adapted to their environments. 

 No. Sexual selection often leads to compromise. Traits that increase an organism's reproductive success, such as colorful plumage in birds, may make them more susceptible to predation. 

 Neutral traits are often selected for because they will eventually produce alleles that confer an advantage. 

 No. Natural selection is not predictive. Although neutral alleles sometimes become advantageous, natural selection only selects for the best available traits in the current environment. 

 Chance and the environment interact with natural selection, so that the best available traits are selected for. 

Does natural selection always produce perfectly adapted organisms?

How does natural selection fashion organisms?

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