Chapter 15

Essentials of Biology
Sylvia S. Mader
Chapter 15
Lecture Outline
Prepared by: Dr. Stephen Ebbs
Southern Illinois University Carbondale
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
15.1 Microevolution
• A population is defined as all the members of a
single species occupying a particular area and
reproducing with one another.
• Microevolution involves the evolutionary
changes within a population.
• While variation within a population is important
to evolution, it is not the only factor.
15.1 Microevolution (cont.)
15.1 Microevolution (cont.)
Evolution in a Genetic Context
• Evolution at the population level can be
studied using population genetics.
• In population genetics, the various alleles
at all the gene loci in all the individuals
make up the gene pool of that population.
• The gene pool of a population can be
described in terms of gene frequencies.
Evolution in a Genetic Context
• Consider the following example for a
population of 100 Drosophila fruit flies.
– 36% of flies are homozygous dominant for
long (L) wings (36 flies)
– 16% are homozygous recessive for short (l)
wings (16 flies)
– 48% are heterozgous (48 flies)
Evolution in a Genetic Context
• So how many L and l alleles are in the
Number of L alleles
Number of l alleles
LL (2 L x 36)
= 72
LL (0 l)
= 0
Ll (1 L x 48)
= 48
Ll (1 l x 48)
= 48
= 0
= 32
(0 L)
120 L
(2 l x 16)
80 l
Evolution in a Genetic Context
• To determine the frequency of an allele in
the population, calculate the percentage of
that allele from the total number of alleles
in the population.
• For the dominant allele, L = 120/200 = 0.6
• For the recessive allele, l = 80/200 = 0.4
Evolution in a Genetic Context
• These percentages represent the
frequency of each allele in the gametes of
this population.
• If the mating in this population is assumed
to be random, then the genotypes in the
subsequent generation can be determined
using a Punnett square.
Evolution in a Genetic Context
• In this Punnett square, the cross indicates the
possible alleles contributed by the population,
not an individual.
0.6 L
0.4 l
0.6 L 0.36 LL
0.24 Ll
0.4 l
0.16 ll
0.24 Ll
Genotype frequencies
0.36 LL + 0.48 Ll + 0.16 ll
• Note that the frequencies of L and l remain the
same in the subsequent generation.
Evolution in a Genetic Context
• Hardy-Weinberg equilibrium is expressed
as a simple binomial equation.
p2 + 2 pq + q2
• The letters p and q are used to represent
the frequency of the two alleles in the
Evolution in a Genetic Context
Evolution in a Genetic Context
Evolution in a Genetic Context
• Hardy-Weinberg equilibrium is maintained
in a population of sexual reproducing
individuals if five conditions are met.
– No net change in frequency due to mutations
– No gene flow (migration of alleles in or out of
the population)
– Random mating must occur
– No genetic drift
– No natural selection
Evolution in a Genetic Context
• These conditions are rarely if ever met in
the real world.
• Thus allele frequencies continually change
and microevolution occurs.
• The value of the Hardy-Weinberg principle
is that it describes the factors that cause
Evolution in a Genetic Context
• In order for natural selection to act on
allele frequencies, the change must affect
the phenotype associated with the gene.
• A classic example of microevolution is
industrial melanism.
Evolution in a Genetic Context
Evolution in a Genetic Context
Causes of Microevolution
• Deviations from the conditions of HardyWeinberg equilibrium cause the allelic
changes associated with microevolution.
– Mutations
– Gene flow
– Nonrandom mating
– Genetic drift
– Natural selection
Genetic Mutations
• Mutations are the raw material of
evolutionary change.
• Mutation introduces new variation into a
• This variation is adaptive if it helps
members of a population adjust to specific
environmental conditions.
Gene Flow
• Gene flow, or gene migration, occurs when
breeding members of a population leave a
population or new members enter.
• Gene migration can introduce new alleles to
• However continual gene flow between
populations decreases differences in allele
frequencies, preventing speciation.
Gene Flow (cont.)
Nonrandom Mating
• When males and females reproduce together
strictly by chance it is called random mating.
• Any behavioral activity that fosters the selection
of specific mates is nonrandom mating.
– Assortive mating occurs when organisms select
mates with a similar phenotype.
– Sexual selection favors traits that increase the
likelihood of securing a mate.
Genetic Drift
• Chance events that cause the allele
frequency to change is called genetic drift.
• The effect of genetic drift becomes
increasingly important as the size of the
population decreases.
Genetic Drift (cont.)
Genetic Drift (cont.)
• Another example of genetic drift is the bottleneck
• A bottleneck occurs when an event or a
catastrophe drastically reduces the number of
organisms in a population.
• The variation in that population may also be
reduced, changing the allele frequencies within
the population.
Genetic Drift (cont.)
Genetic Drift (cont.)
• The founder effect is another example of
genetic drift.
• The founder effect occurs when
combinations of alleles occur at a higher
frequency in a population that has been
isolated from a larger population.
Genetic Drift (cont.)
15.2 Natural Selection
• Natural selection is the process that
adapts populations to the environment.
• Some aspects of the environment can
involve biotic (living) components.
– Competition for limiting resources
– Predation
– Parasitism
15.2 Natural Selection (cont.)
• Some aspects of the environment can
involve abiotic (nonliving) components.
– Weather and climate
– Temperature
– Moisture
15.2 Natural Selection (cont.)
Types of Selection
• The variation within a population creates
different phenotypes for a given trait.
• The distribution of those phenotypes
typically forms a normal distribution.
• The effect of the three types of natural
selection have different effects on this
normal distribution.
Directional Selection
• When one extreme phenotype is favored
by natural selection, the distribution of the
phenotype shifts in that direction.
• This type of selection is therefore called
directional selection.
Directional Selection (cont.)
Directional Selection (cont.)
Stabilizing Selection
• Stabilizing selection occurs when the
intermediate, or most common, phenotype is
• This type of selection tends to narrow the
variation in the phenotype over time.
• This is the most common type of selection
because it is associated with the adaptation of
an organism to the environment.
Stabilizing Selection (cont.)
Disruptive Selection
• In disruptive selection, natural selection
acts upon both extremes of the phenotype.
• This creates a increasing division within
the population which may ultimately lead
to two different phenotypes.
• Disruptive selection is the process that
leads to speciation.
Disruptive Selection (cont.)
Disruptive Selection (cont.)
Maintenance of Variations
• The preservation of variation in a population is
important because it provides a foundation on
which natural selection can act.
• Variation is preserved by a variety of processes.
Mutations and genetic recombination
Gene flow
Natural selection
Polymorphisms (differences in form)
Diploidy and the heterozygotes
Diploidy and the Heterozygote
• Natural selection can only cause evolution if the
different alleles produce different phenotypes.
• Because many organisms are diploid,
heterozygotes are carriers of recessive alleles,
preserving them in the population.
• This also creates another phenotype that may
contribute to ratio of balanced polymorphisms.
Diploidy and the Heterozygote
• Sickle cell disease is an example of balanced
• Under normal conditions, the different
phenotypes provide different fitness levels.
– HbSHbS genotypes suffer from sickle cell and usually
die young.
– HbAHbA genotypes are normal and usually the most
– HbAHbS genotypes are affected by sickle cell disease
only when oxygen levels are low.
Diploidy and the Heterozygote
• Surprisingly, the recessive allele (HbS)
occurs at a higher than expected
frequency in regions where malaria is
present, such as in Africa.
• This occurs because the heterozygote
phenotype is favored under these
conditions and homozygotes are selected
Diploidy and the Heterozygote
Diploidy and the Heterozygote