Chapter 20 - Evolution

14. How is a molecular clock used to determine the time of divergence of two species?

A molecular clock is a region of DNA or protein that has a known rate of accumulation of mutations over time. The more differences we observe when comparing these sequences from two different species, the longer it has been since the species diverged from each other. This relative timescale of divergence can be further clarified using chronological information from the fossil record.

A researcher is studying the color of grasshopper exoskeletons. Assume that this phenotype is caused by the action of two alleles. Green grasshoppers have the genotype AA and brown grasshoppers have the genotype aa. If the frequency of the A allele in this population is 0.2, what is the frequency of the a allele? A. 0.8 B. 0.2 C. 0.1 D. 0.4 E. 0.6

A. 0.8 p+q=1 0.2+q=1 q=0.8

In a population of Mendel's garden peas, the frequency of the dominant A (yellow flower) allele is 80%. Assuming that the population is in Hardy-Weinberg equilibrium, what are the genotype frequencies? A. 64% AA, 32% Aa, 4% aa B. 50% AA, 25% Aa, 25% aa C. 16% AA, 40% Aa, 44% aa D. 75% AA, 15% Aa, 10% aa E. 80% AA, 10% Aa, 10% aa

A. 64% AA, 32% Aa, 4% aa 1 - 0.8 = 0.2 p^2 + 2pq + q^2 (0.8)^2 + 2(0.8)(0.2) +(0.2)^2 = 1

12. Why, of all the evolutionary mechanisms, is selection the only one that can result in adaptation?

Adaptation is the fit between an organism and its environment. Of all the evolutionary mechanisms, only selection causes allele frequencies to change based on how they contribute to the success of an individual in terms of survival and reproduction. This means that allele frequencies in the next generation are ultimately governed by the environment. As an example, suppose that running speed in zebra is at least in part genetically determined. Then, a zebra population's environment—such as the presence of lions—will have an impact on which alleles are passed on to the next generation, with alleles that result in slow running being eliminated. Natural selection therefore involves feedback with the environment, whereas genetic drift and other evolutionary forces involve no such feedback process.

6. What does it mean to say that an allele is "fixed" in a population?

An allele is fixed if there is only one allele for that gene in the population. In other words, its frequency is 1 (or 100%).

You are given the following information about a population: - There are two alleles: C and c. - C codes for green hair and c codes for white hair. - C is dominant over c. - The frequency of the c allele is 0.3. - The population is comprised of 100 individuals. Assuming the population is in Hardy-Weinberg equilibrium, how many individuals have green hair? A. 49% of the population will have green hair. B. 91% of the population will have green hair. C. 9% of the population will have green hair. D. 51% of the population will have green hair.

B. 91% of the population will have green hair. 1 - 0.3 = 0.7 p^2 + 2pq + p^2 (0.7)^2 + 2(0.7)(0.3) +(0.3)^2 = 1 (0.7)^2 + 2(0.7)(0.3) = 0.91 -> 91%

You can use Hardy-Weinberg to calculate allele frequencies, but not genotype frequencies. A. true B. false

B. false

You find that a wild population of antelope is not in Hardy-Weinberg equilibrium. From this information alone, can you determine the mechanism of evolution operating on the population? A. yes B. no

B. no

If the allele frequency for the recessive single allele that causes a particular rare fur color is 0.02, how frequently would you expect the fur color to be present in a population? A. 1 in every 50 individuals B. 1 in every 2000 individuals C. 1 in every 2500 individuals D. 1 in every 5000 individuals

C. 1 in every 2500 individuals

If there are 100 individuals in a population and 20 are homozygous dominant, 60 are heterozygous, and 20 are homozygous recessive, what is p? A. 20% B. 45% C. 50% D. 80 E. p cannot be calculated with the provided information.

C. 50%

Mutations are the ultimate source of genetic variation. However, they usually occur at very low frequencies. Assume a mutation resulting in a novel allele occurs in a gene in one individual in a population of 500 diploid individuals. What is the frequency of the new allele? A. 0.05 B. 0.005 C. 0.01 D. 0.001

D. 0.001 500 x 2 = 1000 1/1000 = 0.001

In a hypothetical population of 1000 frogs there exists a gene with two alleles. In that population, 280 of the frogs are homozygous dominant, and 220 are homozygous recessive. What is the frequency of heterozygotes in the population? A. 0.0 B. 0.28 C. 0.22 D. 0.50

D. 0.50

The diagram below represents a protein electrophoresis gel with samples from six individuals tested for variants (alleles A and B) of a single protein. The darkness of the band is a reflection of the intensity of staining in the gel and corresponds to the amount of enzyme present at that band. What are the allele frequencies? A. 33.3% A, 66/6% B B. 75% A, 25% B C. 75% B, 25% A D. 66.6% A, 33.3% B E. The frequency cannot be determined from the information provided.

D. 66.6% A, 33.3% B For 6 individuals, there will be 12 total alleles in the population. Single, dark bands represent homozygotes for that allele. Two, light bands represent heterozygotes for the alleles. Three individuals are homozygous for allele A, and two individuals are heterozygous for allele A) 3(2) +2(1) = 8/12 for the frequency of the A allele. Since the frequency of all alleles must equal 1, 4/12 is the frequency of allele B in the population.

If a population is not in Hardy-Weinberg equilibrium, we can conclude that: A. nonrandom mating has occurred. B. natural selection has occurred. C. one of the assumptions of the Hardy-Weinberg equilibrium has been violated. D. evolution has occurred because one or more of the assumptions of the Hardy-Weinberg equilibrium has been violated. E. All of these choices are correct.

D. evolution has occurred because one or more of the assumptions of the Hardy-Weinberg equilibrium has been violated.

In general, in a sample of n individuals, the frequency of an allele is: A. the number of occurrences of the allele. B. the number of occurrences of the allele divided by N. C. N. D. the number of occurrences of the allele divided by twice the number of individuals in the sample (2N). E. twice the number of occurrences of the allele divided by N.

D. the number of occurrences of the allele divided by twice the number of individuals in the sample (2N).

9. How would you calculate genotype frequencies of a population in Hardy-Weinberg equilibrium, given the allele frequencies for that trait?

For a population that is under Hardy‒Weinberg equilibrium, you can use known allele frequencies to determine the genotype frequencies in that population using the Hardy‒Weinberg equation. In this equation, p represents the allele frequency of one allele and q represents the frequency of the second allele. The frequency of homozygous dominant individuals is determined by calculating p2, the frequency of homozygous recessive individuals is determined by calculating q2, and the frequency of heterozygous individuals is determined by calculating 2pq. For example, in a population with a dominant allele frequency of 0.6 (p = 0.6) and a recessive allele frequency of 0.4 (q = 0.4), the genotype frequencies for homozygous dominant, homozygous recessive, and heterozygous individuals is 0.36 (or 36%), 0.16 (or 16%), and 0.48 (or 48%), respectively.

13. A female lizard floats on a log to an island where she lays her previously fertilized eggs and starts a new population on the island, where there are no other lizards. How do you think the genetic variation and allele frequencies of the island population will compare to those of the mainland population? Which mechanism of evolution is at work here?

Genetic variation is likely to be less extensive on the island compared to the mainland, and allele frequencies will be different between the two. This is an example of genetic drift (specifically, a founder event).

3. What is genetic variation, and how is it measured?

Genetic variation refers to the differences that exist between individuals within the nucleotide sequences of their genomes. Genetic variation can be measured by counting the number of individuals with observable differences (phenotypes) for a given trait, by using gel electrophoresis to detect differences in the properties of enzymes encoded by variable nucleotide sequences, or by performing direct sequencing of regions of DNA, which is the current method for measuring genetic variation.

1. Why are germ-line mutations more important in evolution than somatic ones?

Germ-line mutations are passed on to the next generation but somatic mutations are not. Evolution is a change in the genetic makeup of populations from generation to generation, so germ-line mutations are key to this process. Somatic mutations, especially if they result in disorders like cancer, can be important in the life of an individual, but they are not passed on to that individual's offspring.

Fixed

In genetics, describes the situation in which all individuals in a population are homozygous for the same allele of a particular gene; the noun form is fixation. In metabolism, refers to the processes by which simple inorganic molecules are converted into biologically available forms, especially carbon fixation and nitrogen fixation

5. Data on genetic variation in populations have become ever more precise over time, from phenotypes determined by a single gene to results obtained through gel electrophoresis that looks at variation among genes that encode for enzymes, to data generated directly from the DNA sequence. Has this increase in precision resulted in the discovery of more genetic variation or less?

More. Take, for example, a protein variant that was previously identified through protein gel electrophoresis. Now that we can sequence the DNA of multiple copies of that allele, we may find that there are several other amino acid differences that do not affect the mobility of the protein on a gel. In addition, there may be additional DNA differences that, because of the redundancy of the genetic code, do not change the amino acid sequence of the protein.

10. What is natural selection, and how is it different from other mechanisms of evolution?

Natural selection is an evolutionary mechanism that causes a change in allele or genotype frequencies within a population over time based on the relative fitness of each genotype in a particular environment. Due to competition for limited resources, those individuals with alleles that allow them to survive and reproduce better than individuals without the same alleles are more likely to pass on their genes to the next generation, thereby enriching subsequent generations of the population for these alleles and allowing for adaptation of the population to its environment over time. Natural selection is unlike other forms of evolution in that it consistently results in populations that are better suited for their environment, whereas other evolutionary mechanisms, such as genetic drift, result in changes in allele or genotype frequencies that usually do not lead to adaptation of the population.

2. Why is recombination critical to generating genetic variation?

Recombination shuffles mutations into new permutations. Imagine three genes along a chromosome, A, B, and C. There is only one allele at A: A1. There are two alleles at B and C: B1 and B2, and C1 and C2, respectively. Now imagine a mutation that creates a new allele, A2, at A and that this mutation occurred on a chromosome with B1 and C2 alleles. In the absence of recombination, A2 will always be associated with B1 and C2. Recombination reconfigures these associations into new permutations: from the original A2 B1 C2 to A2 B1 C1, A2 B2 C1, and A2 B2 C2.

11. Sexual selection tends to cause bigger size, more elaborate weaponry, or brighter colors in males. Is this an example of stabilizing, directional, or disruptive selection?

Sexual selection for bigger size, more elaborate weaponry, or brighter colors in males is an example of directional selection.

4. Using the example of pea color in Mendel's pea plants, devise equations to determine the allele frequencies of A and a from the genotype frequencies of aa, Aa, and AA.

The allele frequency of a was calculated as follows: Frequency(a) = [2 × (number aa) + 1 × (number Aa)] / [2 × (total number of individuals)] This equation can be rewritten as Frequency(a) = [(number aa) + ½ × (number Aa)] / (total number of individuals) Note that Number aa/total number of individuals = frequency(aa) and ½ × (number Aa)/total number of individuals = ½ frequency(Aa) Therefore, Frequency(a) = frequency(aa) + ½ frequency(Aa) equals the frequency of aa homozygotes plus half the frequency of Aa heterozygotes. By similar logic, Frequency(A) = frequency(AA) + ½ frequency(Aa) These equations are very useful for determining allele frequencies directly from genotype frequencies. Let's apply them to the pea color data we have already looked at: 50% aa, 25% Aa, and 25% AA. Now we can compute allele frequencies: Frequency (A) = 0.25 + 0.25/2 = 0.375 Frequency (a) = 0.5 + 0.25/2 = 0.625

8. What can and can't we conclude about a population whose allele frequencies are not in Hardy-Weinberg equilibrium?

We can conclude that the population is evolving. What we cannot tell is which mechanism—selection, genetic drift, migration, mutation, or nonrandom mating—is causing it to evolve. To determine which mechanisms are driving the process requires more detailed population genetics analysis.

7. Can evolution occur without allele frequency changes? If not, why not? If so, how?

Yes, evolution is a change within a population over time in the frequency of alleles or genotypes. The frequency of a genotype in a population can change without changing the frequency of the alleles through nonrandom mating. In nonrandom mating, individuals of one genotype may preferentially mate with individuals of the same genotype or a different genotype, maintaining the same frequencies of alleles from generation to generation but configuring them preferentially in certain genotypes.

heterozygote advantage

a form of balancing selection in which the heterozygote's fitness is higher that that of either of the homozygotes, resulting in selection that ensures that both alleles remain in the population at intermediate frequencies

Artificial selection

a form of directional selection similar to natural selection, but with selection done intentionally by humans, usually with a specific goal in mind, such as increased milk yield in cattle

Disruptive selection

a form of selection that operates in favor of extremes and against intermediate forms, selecting against the mean

Sexual selection

a form of selection that promotes traits that increase an individual's access to reproductive opportunities

Directional selection

a form of selection that results in a shift of the mean value of a trait in a population over time

Stabilizing selection

a form of selection that selects against extremes and therefore maintains the status quo

Intersexual selection

a form of sexual selection involving interaction between males and females, as when females choose from among males

Intrasexual selection

a form of sexual selection involving interactions between individuals of one sex, as when members of one sex compete with one another for access to the other sex

Pseudogene

a gene that is no longer functional

Species

a group of individuals that can exchange genetic material through interbreeding to produce fertile offspring

Fitness

a measure of the extent to which an individual's genotype is represented in the next generation

Germ-line mutations

a mutation that occurs in eggs and sperm or in the cells that give rise to these reproductive cells and therefore is passed on to the next generation

Somatic mutations

a mutation that occurs in somatic (nonreproductive) cells

Genetic drift

a random change in the frequency of an allele due to the statistical effects of finite population size

interbreeding depression

a reduction in fitness resulting from breeding among relatives causing homozygosity of deleterious recessive mutations

Hardy-Weinberg equilibrium

a state in which allele and genotype frequencies do not change over time, implying the absence of evolutionary forces (such as natural selection). It also specifies a mathematical relationship between allele frequencies and genotype frequencies

Founder event

a type of genetic drift that occurs when only a few individuals establish a new population

Gene pool

all the alleles present in all individuals in a population or species

Populations

all the individuals of a given species that live and reproduce in a particular place; one of several interbreeding groups of organisms of the same species living in the same geographical area

Bottleneck

an extreme, usually temporary, reduction in population size that may result in marked loss of genetic diversity and, in the process, genetic drift

Molecular evolution

evolution at the level of DNA, which in time results in the genetic divergence of populations

Deleterious

genetic changes that are harmful to an organism

Neutral

genetic changes that have no effect or negligible effects on the organism, or whose effects are not associated with differences in survival or reproduction

Advantageous

genetic changes that improve their carriers' chances of survival or reproduction

Nonrandom mating

mate selection biased by genotype or relatedness

Balancing selection

natural selection that acts to maintain two or more alleles of a given gene in a population

Positive selection

natural selection that increases the frequency of a favorable allele

Negative selection

natural selection that reduces the frequency of a deleterious allele

Modern Synthesis

the current theory of evolution, which combines Darwin's theory of natural selection and Mendelian genetics

Gene flow

the movement of alleles from one population to another through interbreeding between members of each population

Migration

the movement of organisms from one place to another, including the movement of individuals from one population to another

Molecular clock

the observation of rate constancy in molecular evolution. The extent of genetic divergence at a gene in two taxa is thus a reflection of the time since the taxa last shared a common ancestor

Allele frequencies

the proportion of a specific allele among all the alleles of a gene in a population

Genotype frequency

the proportion of a specified genotype among all the genotypes for a particular gene or set of genes in a population

Selection

the retention or elimination of mutations in a population of organisms