BIOA01H3 Chapter 21: Summary
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Core Concepts Summary
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.
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-line mutations
are the only ones that can be passed on to the next generation. page 427
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–Weinberg equilibrium describes situations in which allele frequencies do not change.
By seeing if a population is in Hardy–Weinberg equilibrium, we can determine whether or not
evolution is occurring in a population. page 430
The Hardy–Weinberg equilibrium makes five assumptions. These assumptions are that the
population experiences no selection, no migration, no mutation, no sampling error due to small
population size, and random mating. page 430
The Hardy–Weinberg equilibrium allows allele frequencies and genotype frequencies to be
calculated from each other. page 431
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
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. 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-random mating are non-adaptive
mechanisms of evolution.
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.
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