Evolutionary Change in Populations

Copyright © 2005 Brooks/Cole — Thomson Learning

Biology, Seventh Edition Solomon • Berg • Martin

Chapter 18Evolutionary Change Evolutionary Change

in Populationsin Populations


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



• Evolution of populations is best understood in terms of frequencies:• Genotype• Phenotype• Allele



Genotype frequencies for all 1000 individuals of a hypothetical population



Phenotype frequencies for all 1000 individuals of a hypothetical population



Allele frequencies for all 1000 individuals of a hypothetical population



• Hardy-Weinberg Principle• Explains stability of successive

generations in populations at genetic equilibrium

• Essential to understanding mechanisms of evolutionary change



• Genetic equilibrium requires• Random mating• No net mutations• Large population size• No migration• No natural selection



• 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



• Hardy-Weinberg equation• p = frequency of dominant allele• q = frequency of the recessive

allele:p + q = 1



• 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



(a) Genotype and allele frequencies



(b) Segregation of alleles and random fertilization



• Microevolution• Intergenerational changes in

allele or genotype frequencies within a population

• Often involves relatively small or minor changes, usually over a few generations



• Changes in allele frequencies of a population caused by microevolutionary processes:• Nonrandom mating• Mutation• Genetic drift• Gene flow• Natural selection



• Nonrandom mating• Inbreeding

–Inbreeding depression• Assortative mating• Both of these increase frequency

of homozygous genotypes



• Mutation• Source of new alleles• Increases genetic variability

acted on by natural selection



• Genetic drift• Random change in allele

frequencies of a small population• Decreases genetic variation

within a population• Changes it causes are usually

not adaptive



• 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



• Gene flow• Movement of alleles caused by

migration of individuals between populations

• Causes changes in allele frequencies



• 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



• Modes of selection• Stabilizing

–Favors the mean• Directional

–Favors one phenotypic extreme• Disruptive

–Favors two or more phenotypic extremes



Modes of selection

(a) No selection (b) Stabilizing selection



Modes of selection

(c) Directional selection (d) Disruptive selection



• Genetic variation in populations caused by• Mutation• Sexual reproduction

–Allows new phenotypes



• Methods of evaluating genetic variation• Genetic polymorphism

–Balanced polymorphism• Neutral variation• Geographic variation





• Balanced polymorphism: two or more alleles persist in a population over many generations• Heterozygote advantage• Frequency-dependent selection



Clinal variation in yarrow (Achillea millefolium)

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Evolutionary Change in Populations