hlw_ch21

CHAPTER SUMMARY

21.1 GENETIC VARIATION IS THE RESULT OF DIFFERENCES IN DNA SEQUENCES.

Visible differences among members of a species (phenotypic variation) are the result of differences at the DNA level (genetic variation) as well as the influence of the environment.
Mutation, a change in DNA sequence, is the ultimate source of genetic variation. This variation is reconfigured by recombination.
Mutations can be somatic (in body tissues) or germ-line (in gametes), but germ-line mutations are the only ones that can be passed on to the next generation.
When a mutation occurs in a gene, it creates a new allele.

21.2 INFORMATION ON ALLELE FREQUENCIES IS KEY TO UNDERSTANDING PATTERNS OF GENETIC VARIATION.

An allele frequency is the number of occurrences of a particular allele divided by the total number of occurrences of all alleles of that gene in a population.
In the past, population geneticists relied on observable traits determined by a single gene to measure genetic variation, but this method is not generally useful as most traits are influenced by many genes and by the environment.
Allele frequencies can be measured by protein gel electrophoresis.
DNA sequencing is now the standard technique for measuring allele frequencies.

21-17

21.3 EVOLUTION IS A CHANGE IN THE FREQUENCY OF ALLELES OR GENOTYPES OVER TIME.

Allele frequencies change in response to selection, migration, mutation, and genetic drift. These are the primary mechanisms of evolution.
The Hardy–Weinberg equilibrium describes situations in which allele frequencies do not change. In this way, it provides a null hypothesis against which to test whether or not evolution is occurring in a population.
The Hardy–Weinberg equilibrium makes five assumptions. These assumptions are no selection, no migration, no mutation, no sampling error due to small population size, and random mating.
The Hardy–Weinberg equilibrium allows allele frequencies and genotype frequencies to be calculated from each other.

21.4 NATURAL SELECTION LEADS TO ADAPTATION, WHICH ENHANCES THE FIT BETWEEN AN ORGANISM AND ITS ENVIRONMENT.

Independently conceived by Charles Darwin and Alfred Russel Wallace, natural selection is the differential reproductive success of genetic variants.
Under natural selection, a harmful allele is eliminated over time, and a beneficial one increases in frequency. Natural selection does not affect the frequency of neutral mutations.
Natural selection can maintain alleles at intermediate frequencies by balancing selection.
Changes in phenotype show that natural selection can be stabilizing, directional, or disruptive.
In artificial selection, a form of directional selection, a breeder governs the selection process.
Sexual selection involves the evolution of traits that increase an individual’s access to members of the opposite sex.

21.5 MIGRATION, MUTATION, AND GENETIC DRIFT ARE NON-ADAPTIVE MECHANISMS OF EVOLUTION.

Migration involves the movement of alleles between populations (gene flow) and tends to have a homogenizing effect.
Mutation is the ultimate source of variation, but it also can change allele frequencies on its own.
Genetic drift is a kind of sampling error, which is more marked in small populations.

21.6 MOLECULAR EVOLUTION LOOKS AT CHANGES AT THE LEVEL OF DNA OR AMINO ACID SEQUENCES.

The extent of sequence difference between two species is a function of the time they have been genetically isolated from each other.
Correlation between sequence differences among species and time since common ancestry of those species is known as the molecular clock.
The rate of the molecular clock varies among genes because some genes are more selectively constrained than others.

Self-Assessment Question 1

Differentiate between a phenotype and a genotype.

Show Model Answer

Model Answer:

A phenotype refers to the characteristics or traits of an organism that can be observed, such as the color of a flower’s petals or a person’s height. Phenotypes are determined both by an organism’s underlying composition of alleles, also known as its genotype, and by the environment.

Self-Assessment Question 2

Name three types of mutations in terms of their effect on an organism.

Show Model Answer

Model Answer:

Mutations can increase the ability of an organism to survive or reproduce (beneficial), decrease the organism’s fitness (harmful), or have no effect on the organism (neutral).

Self-Assessment Question 3

Define genetic variation.

Show Model Answer

Model Answer:

Genetic variation refers to the differences that exist between individuals within the nucleotide sequences of their genomes.

Self-Assessment Question 4

Describe three ways to measure genetic variation, and state which one is used today.

Show Model Answer

Model Answer:

Measuring genetic variation can be done by counting the number of individuals with observable differences (phenotypes) for a given trait, using gel electrophoresis to detect differences in the properties of enzymes encoded by variable nucleotide sequences, or by direct sequencing of regions of DNA, which is the current method for measuring genetic variation today.

Self-Assessment Question 5

Given a set of genotype frequencies, calculate allele frequencies.

Show Model Answer

Model Answer:

The frequency of each allele at a given locus can be determined by tallying the number of each allele that is contributed by each genotype present in a given population. For example, if 30% of a population is homozygous dominant for a particular trait (AA), 45% are homozygous recessive (aa) and 25% are heterozygous (Aa), the frequency of each of the two alleles, ‘A’ and ‘a’, can be determined. Each homozygous dominant individual contributes two identical ‘A’ alleles (30% or 0.3 x 2 = 0.6) and each heterozygote contributes one dominant allele (45% or 0.45 x 1 = 0.45). If we assume the population in question is diploid, the total number of alleles in the population will be two times the number of genotypes (200% or 2.0). Thus, the frequency of the ‘A’ allele is 0.6 + 0.45 / 2 = 0.525 or 52.5%. The allele frequency of the recessive ‘a’ allele can then be determined in a similar manner ((0.25 x 2) + (0.45 x 1) = 0.95 / 2 = 0.475 or 47.5%).

Self-Assessment Question 6

Define evolution.

Show Model Answer

Model Answer:

Any change in the frequency of alleles or genotypes within a population over time is considered evolution.

Self-Assessment Question 7

Describe what happens to allele and genotype frequencies under the Hardy–Weinberg equilibrium.

Show Model Answer

Model Answer:

When a population is under Hardy-Weinberg equilibrium, allele and genotype frequencies do not change over time, and thus no evolution occurs within that population.

Self-Assessment Question 8

Name and describe the five assumptions of the Hardy–Weinberg equilibrium.

Show Model Answer

Model Answer:

For a population to be under Hardy-Weinberg equilibrium, the following five assumptions must be met: (1) there can be no selection, meaning that all genotypes must be equally likely to survive and reproduce within the population, (2) there can be no migration of individuals into or out of the population, (3) there can be no mutations in the DNA sequence of any individuals in the population, (4) there must be a sufficiently large population to avoid chance events altering the allele or genotype frequencies, and (5) individuals within the population must mate randomly with one another regardless of their genotype.

Self-Assessment Question 9

Given a set of allele frequencies, calculate genotype frequencies if the population is in Hardy–Weinberg equilibrium.

Show Model Answer

Model Answer:

For a population that is under Hardy-Weinberg equilibrium, known allele frequencies can be used to determine the genotype frequencies in that population using the Hardy-Weinberg equation. According to this equation, where ‘p’ represents the allele frequency of one allele and ‘q’ represents the frequency of the second allele, the frequency of homozygous dominant individuals is given by calculating p², the frequency of homozygous recessive individuals is given by calculating q² and the frequency of heterozygous individuals is given by calculating 2pq. For example, in a population with a dominant allele frequency of 60% (p = 0.6) and a recessive allele frequency of 40% (q = 0.4), the genotype frequencies for homozygous dominant, homozygous recessive, and heterozygous individuals would be 0.36 or 36%, 0.16 or 16% and 0.48 or 48%, respectively.

Self-Assessment Question 10

Name and describe the five mechanisms of evolution.

Show Model Answer

Model Answer:

Changes in allele or genotype frequencies can occur through natural selection (the preferential survival and reproduction of individuals with certain alleles based on their fitness for their environment), gene flow (the migration of individuals into or out of a population), mutation (changes in a DNA sequence), genetic drift (random shifts in allele frequency due to chance events within a small population), or non-random mating (either by choice, based on the organism’s mate preferences, or by force, as in artificial selection where a breeder determines which individuals will mate).

Self-Assessment Question 11

Define natural selection and indicate how it is different from other mechanisms of evolution.

Show Model Answer

Model Answer:

Natural selection is an evolutionary process 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 those alleles will be more likely to pass on their genes to the next generation, thus preferentially maintaining these alleles in subsequent generations of the population and allowing for adaptation of the population to their environment over time. Natural selection is unlike other forms of evolution because it consistently results in populations that are better suited for their environment whereas non-adaptive evolutionary mechanisms, such as genetic drift, result in random changes in allele or genotype frequencies, which usually do not lead to adaptation of the population.

Self-Assessment Question 12

Explain how a molecular clock can be used to determine the time of divergence of two species.

Show Model Answer

Model Answer:

A molecular clock is a region of DNA that has a known rate of mutation over time, so the more differences observed when comparing these sequences from different species, the longer it has been since they diverged from one another. The relative timescale of divergence generated through the use of a molecular clock can be further clarified using chronological information from the fossil record.