Download Population Evolution

Biology, Seventh Edition
Solomon • Berg • Martin
Chapter 18
Evolutionary Change
in Populations
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• A population’s gene pool
• Includes all the alleles for all the loci
present in the population
–Diploid organisms have a maximum of
two different alleles at each genetic
locus
–Typically, a single individual therefore
has only a small fraction of the alleles
present
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Evolution of populations is
best understood in terms of
frequencies:
• Genotype
• Phenotype
• Allele
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
Genotype frequencies for all 1000 individuals
of a hypothetical population
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
Phenotype frequencies for all 1000 individuals
of a hypothetical population
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
Allele frequencies for all 1000 individuals
of a hypothetical population
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Hardy-Weinberg Principle
• Explains stability of successive
generations in populations at
genetic equilibrium
• Essential to understanding
mechanisms of evolutionary
change
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Genetic equilibrium requires
• Random mating
• No net mutations
• Large population size
• No migration
• No natural selection
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Hardy-Weinberg principle
• Shows that if population is large,
process of inheritance alone does
not cause changes in allele
frequencies
• Explains why dominant alleles
are not necessarily more
common than recessive alleles
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Hardy-Weinberg equation
• p = frequency of dominant allele
• q = frequency of the recessive
allele:
p+q=1
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• The genotype frequencies of a
population are described by the
relationship p2 + 2pq + q2 = 1
• p2 is frequency of homozygous
dominant genotype
• 2pq is frequency of heterozygous
genotype
• q2 is frequency of homozygous
recessive genotype
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
(a) Genotype and allele frequencies
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
(b) Segregation of
alleles and random
fertilization
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Microevolution
• Intergenerational changes in
allele or genotype frequencies
within a population
• Often involves relatively small or
minor changes, usually over a
few generations
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Changes in allele frequencies of
a population caused by
microevolutionary processes:
•
•
•
•
•
Nonrandom mating
Mutation
Genetic drift
Gene flow
Natural selection
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Nonrandom mating
• Inbreeding
–Inbreeding depression
• Assortative mating
• Both of these increase frequency
of homozygous genotypes
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Mutation
• Source of new alleles
• Increases genetic variability
acted on by natural selection
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Genetic drift
• Random change in allele
frequencies of a small population
• Decreases genetic variation
within a population
• Changes it causes are usually
not adaptive
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Genetic drift
• Bottleneck is a sudden decrease
in population size caused by
adverse environmental factors
• Founder effect is genetic drift
occurring when a small
population colonizes a new area
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Gene flow
• Movement of alleles caused by
migration of individuals between
populations
• Causes changes in allele
frequencies
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Natural selection
• Causes changes in allele
frequencies leading to adaptation
• Operates on an organism’s
phenotype
• Changes genetic composition of
a population favorably for a
particular environment
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Modes of selection
• Stabilizing
–Favors the mean
• Directional
–Favors one phenotypic extreme
• Disruptive
–Favors two or more phenotypic
extremes
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
Modes of selection
(a) No selection
(b) Stabilizing selection
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
Modes of selection
(c) Directional selection (d) Disruptive selection
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Genetic variation in
populations caused by
• Mutation
• Sexual reproduction
–Allows new phenotypes
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Methods of evaluating
genetic variation
• Genetic polymorphism
–Balanced polymorphism
• Neutral variation
• Geographic variation
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
• Balanced polymorphism: two
or more alleles persist in a
population over many
generations
• Heterozygote advantage
• Frequency-dependent selection
Copyright © 2005 Brooks/Cole — Thomson Learning
Biology, Seventh Edition
CHAPTER 18 Evolutionary Change in Populations
Clinal variation in yarrow (Achillea millefolium)
Copyright © 2005 Brooks/Cole — Thomson Learning