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AP Biology
Mrs. Farish
Name ______________________________
Chapter 23: The Evolution of Populations
Study Questions
1. Predict how microevolutionary change can affect a gene pool.
2. Demonstrate that a population not at equilibrium requires only one generation of random
mating to establish Hardy-Weinberg equilibrium.
3. Describe the conditions that a population must meet in order to maintain Hardy-Weinberg
equilibrium.
4. Distinguish between the bottleneck effect and the founder effect.
5. Explain why even though mutation can be a source of genetic variability, it contributes a
negligible amount to genetic variation in a population.
6. Explain the concept of relative fitness and its role in adaptive evolution.
7. Why does the rate of decline for a deleterious allele depend upon whether the allele is
dominant or recessive to the more successive allele?
8. Distinguish among stabilizing selection, directional selection, and diversifying selection.
9. How can sexual dimorphism influence evolutionary change?
10. In your own words, explain why natural selection cannot breed perfect organisms.
1. Predict how microevolutionary change can affect a gene pool.
Microevolution is evolutionary change below the species level; change in the genetic
makeup of a population from generation to generation.
 Genetic Drift – unpredictable fluctuations in allele frequencies from one
generation to the next because of a population’s finite size
i. Bottle neck effect – gen. Drift resulting from the reduction of a
population, typically by a natural disaster, such that the surviving
population is not longer genetically representative of the original
population
ii. Founder effect – gen. Drift that occurs when a few individuals become
isolated from a larger population, with the result that the new population’s
gene pool is not reflective of the original population
Gene flow- genetic additions to or subtractions from a population resulting from the
movement of fertile individuals or gametes.
Mutation- change in DNA of a gene creating genetic diversity
Non-random mating:
 Inbreeding- mating of genetically similar individuals, which tends to increase the
number of individuals that are homozygous for a trait and therefore increase the
appearance of recessive traits.
 Assortive matingi. (+)  mate with individuals that are genetically similar
ii. (-)  mate with individuals that are different
Natural Selection- differential success in the reproduction of different phenotypes resulting
from the interaction of organisms with their environment. Evolution occurs when natural
selection causes changes in relative frequencies of alleles in the gene pool.
2. Demonstrate that a population not at equilibrium requires only one generation of
random mating to establish Hardy-Weinberg equilibrium.
3. Describe the conditions that a population must meet in order to maintain HardyWeinberg equilibrium.
 Extremely large population size – The smaller the population, the greater the
role played by chance fluctuations in allele frequencies from one generation to the
next (genetic drift)
 No gene flow – Gene flow (transfer of alleles between populations) can alter
allele frequencies
 No mutations – By introducing or removing genes from chromosomes or by
changing one allele into another, mutations modify the gene pool
 Random mating – If individuals preferentially choose mates with certain
genotypes, including close relatives (inbreeding), random mixing of gametes does
not occur.
 No natural selection – Differential survival and reproductive success of
individuals carrying different genotypes will alter allele frequencies.
4. Distinguish between the bottleneck effect and the founder effect.
i. Bottle neck effect – gen. Drift resulting from the reduction of a
population, typically by a natural disaster, such that the surviving
population is not longer genetically representative of the original
population
ii. Founder effect – gen. Drift that occurs when a few individuals become
isolated from a larger population, with the result that the new population’s
gene pool is not reflective of the original population
5. Explain why even though mutation can be a source of genetic variability, it
contributes a negligible amount to genetic variation in a population.
Most mutations occur in somatic cells and are lost when the individual dies. Only
mutations in cell lines that produce gametes can be passed to offspring, and only a small
fraction of these spread through populations. Organisms reflect thousands of generations
of past selection, and a single mutation is not very likely to improve the genome.
6. Explain the concept of relative fitness and its role in adaptive evolution.
Relative fitness is the contribution of one genotype to the next generation compared to
that of alternative genotypes for the same locus. Adaptive evolution involves adaptations
(such as a higher reproductive rate in wildflowers due to brighter colors or fragrance to
attract pollinators) that serve as genetic contributions to the gene pool of the next
generation.
7. Why does the rate of decline for a deleterious allele depend upon whether the allele
is dominant or recessive to the more successive allele?
8. Distinguish among stabilizing selection, directional selection, and diversifying
selection.
Stabilizing selection – natural selection that favors intermediate variants by acting
against extreme phenotypes.
Directional selection – natural selection that favors individuals at one end of the
phenotypic range.
Disruptive selection – natural selection that favors individuals on both extremes of a
phenotypic range over intermediate phenotypes.
9. How can sexual dimorphism influence evolutionary change?
Sexual dimorphism is a case of polymorphism based on the distinction between the
secondary sex characteristics of males and females. For example, if females choose their
mates (intersexual selection), she will choose based on a certain appearance or behavior.
As a result, she perpetuates the alleles that influenced her to make that choice, allwing a
male with an especially showy phenotype to pass on his alleles to offspring.
10. In your own words, explain why natural selection cannot breed perfect organisms.
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Evolution is limited by historical constraints.
Adaptations are often compromises.
Chance and natural selection interact.
Selection can edit only existing variations.