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21.1 Genetic variation refers to 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. page 426
Mutation and recombination are the two sources of genetic variation, but all genetic variation ultimately comes from mutation. page 427
Mutations can be somatic (in body tissues) or germ line (in gametes), but germ-
When a mutation occurs in a gene, it creates a new allele. Mutations can be deleterious, neutral, or advantageous. page 427
21.2 Patterns of genetic variation can be described by allele frequencies.
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. page 427
In the past, population geneticists relied on observable traits determined by a single gene and protein gel electrophoresis to measure genetic variation. page 428
DNA sequencing is now the standard technique for measuring genetic variation. page 428
21.3 Evolution is a change in the frequency of alleles or genotypes over time.
The Hardy–
The Hardy–
The Hardy–
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. page 432
Under natural selection, a harmful allele decreases in frequency, and a beneficial one increases in frequency. Natural selection does not affect the frequency of neutral mutations. page 434
Natural selection can maintain alleles at intermediate frequencies by balancing selection. page 434
Changes in phenotype show that natural selection can be stabilizing, directional, or disruptive. page 435
In artificial selection, a form of directional selection, a breeder governs the selection process. page 436
Sexual selection involves the evolution of traits that increase an individual’s access to members of the opposite sex. page 437
In intrasexual selection, individuals of the same sex compete with one another, resulting in traits like large size and horns. page 437
In intersexual selection, interactions between females and males result in traits like elaborate plumage in male birds. page 438
21.5 Migration, mutation, genetic drift, and non-
Migration involves the movement of alleles between populations (gene flow) and tends to have a homogenizing effect. page 438
Mutation is the ultimate source of variation, but it also can change allele frequencies on its own. page 438
Genetic drift is a kind of sampling error, which acts more strongly in small populations than in large ones. page 438
Non-
21.6 Molecular evolution is a change in DNA or amino acid sequences over time.
The extent of sequence difference between two species is a function of the time they have been genetically isolated from each other. page 440
Correlation between sequence differences among species and time since common ancestry of those species is known as the molecular clock. page 440
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The rate of the molecular clock varies among genes because some genes are more selectively constrained than others. page 441
Why does the relation between phenotype and genotype matter in evolution?
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 genotype—
What is a neutral mutation and what evolutionary mechanism causes it to change in frequency over time?
A neutral mutation is one that has no effect on the fitness of the organism. It might, for example, lie in a region of the genome that has no function. Neutral mutations increase and decrease in frequency by genetic drift, in which case they can be lost or fixed.
Define genetic variation and explain how it can be 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, 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.
How would you calculate the allele frequencies for a two-
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 × 2 = 0.6), and each heterozygote contributes one dominant allele (45% or 0.45 × 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 × 2) + (0.45 × 1) = 0.95/2 = 0.475 or 47.5%].
Can evolution occur without allele frequency changes? If not, why not? If so, how?
Yes, we define evolution as 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 or different genotype, maintaining the same frequencies of alleles from generation to generation but configuring them preferentially in certain genotypes.
Describe what happens to allele and genotype frequencies under the Hardy–
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.
Name the five assumptions of the Hardy–
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; if selection is operating, certain alleles or genotypes will be overrepresented in the next generation. (2) There can be no migration of individuals into or out of the population; if migration occurs, alleles from outside the population will change the population’s allele frequencies. (3) There can be no mutations in the DNA sequence of any individuals in the population; mutation, like migration, will change allele frequencies. (4) There must be a sufficiently large population to avoid chance events altering the allele or genotype frequencies; in small populations, sampling error—
How would you calculate genotype frequencies of a population in Hardy–
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.
Define natural selection and explain how it is different from other mechanisms of evolution.
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 nonrandom 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).
Explain how a molecular clock can be used to determine the time of divergence of two species.
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 the same 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 nonadaptive 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.