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The Evolution of Populations
CHAPTER 23
Overview: The Smallest Unit of Evolution
 One common misconception about evolution is that
individual organisms evolve, in the Darwinian sense,
during their lifetimes
 Natural selection acts on individuals, but
populations evolve!!
Genes, Populations, and Evolution
 A population is all the members of a single
species occupying a particular area at the same
time.
 Diversity exists among members of a population.
 Population genetics is the study of this diversity
in terms of allele differences.

Evaluates the diversity of a population by studying
genotype and phenotype frequencies over time
Microevolution
 In the 1930s, population geneticists began to
describe variations in a population in terms of
alleles
 Microevolution pertains to evolutionary changes
within a population.
Various alleles at all the gene loci in all individuals make
up the gene pool of the population.
 The gene pool of a population can be described in terms
of:

Genotype frequencies
 Allele frequencies

Genes, Populations, and Evolution
 Allele Frequencies

The proportion of each allele within a population’s gene pool.

Frequencies of the dominant and recessive allele must add up to 1.


This relationship is described by the expression p + q = 1

p is the frequency of one allele and q is the frequency of the other
Microevolution involves a change in these allele frequencies within
populations over time.

If the gene frequencies do not change over time, microevolution has not
occurred.
Genes, Populations, and Evolution
 The Hardy-Weinberg Equilibrium states
that:
 Allele
frequencies in a population will remain
constant assuming:
 No
Mutations
 No
Gene Flow
 Random
Mating
 No
Genetic Drift
 No
Natural Selection
Genes, Populations, and Evolution
 Hardy-Weinberg Equilibrium:

Required conditions are rarely (if ever) met

Deviations from a Hardy-Weinberg equilibrium indicate
that evolution has taken place

Analysis of allele changes in populations over time determines the
extent to which evolution has occurred.
Genes, Populations, and Evolution
 Frequencies of the phenotypes will equal one and
follow the equation:

p2+2pq+q2=1
 The genotypes in a Hardy-Weinberg population are
indicated by each term on the left side of the
equation above
Hardy-Weinberg Equilibrium
 Learning to solve the problem:
 If the question states “frequency of alleles in the a population”,
then you should start with p+q=1 and solve for p or q

If the question says frequencies of organisms that express the
trait (dominant or recessive) then you start with p2+2pq+q2=1
Your turn....
 If 9% of the population has blue eyes, what percent
of the population is hybrid for brown eyes?
Homozygous for brown eyes?
Practice
 Determine the percent of the population that is
homozygous dominant if the percent of the
population that is homozygous recessive is 16%.
Practice
 Determine the percent of the population that is
hybrid if the allele frequency of the recessive trait is
0.5.
Causes of Evolution
 Mutations
 Gene Flow
 Nonrandom mating
 Natural selection
 Genetic Drift
Mutations
 Genetic
 The
mutations
raw material for evolutionary change
 Provide
new alleles
 Increase
 Some
diversity
mutations might be more adaptive than others
 Ex:
Genetic mutations affecting pigment color in
peppered moths have provided the variation needed for
natural selection to occur

Average about 1 mutation every 100,000 genes per generation
Gene Flow

Gene Flow (gene migration)
 Increases diversity
 Movement of alleles between populations when:
 Gametes or seeds (in plants) are carried into another
population
 Breeding individuals migrate into or out of
population
 Continual gene flow reduces genetic divergence
between populations
Nonrandom Mating
 Nonrandom Mating

When individuals do not choose mates randomly
 Assortative
mating:
 Individuals
select mates with the same phenotype
with respect to a certain characteristic
 Individuals
 Increases
loci
reject mates with differing phenotype
the frequency of homozygotes for certain
Genetic Drift

Genetic Drift
Changes in the allele frequencies of a population due to change
rather than selection by the environment
 Tends to limit diversity
 Does not necessarily lead to adaptation to the environment
 Occurs by disproportionate random sampling from population




Can cause the gene pools of two isolated populations to become
dissimilar
Some alleles are lost and others become fixed (unopposed)
Likely to occur:



After a bottleneck
When severe inbreeding occurs, or
When founders start a new population
Genetic Drift
 Bottleneck effect

A random event prevents a majority of individuals from
entering the next generation

Fire, earthquake, floods and human hunting

The next generation is composed of alleles that just happened
to make it

Example: Tay-Sachs disease among Eastern European Jews
Genetic Drift
 Founder effect

When a new population is started from just a few individuals

The alleles carried by population founders are dictated by
chance

Formerly rare alleles will either:

Occur at a higher frequency in the new population, or

Be absent in new population

Example: polydactyly in Old Order of Amish in Lancaster, PA
Original population
gene pool = 3,800 alleles*
Remnant population
gene pool = 90 alleles*
11%
13%
26%
8%
44%
53%
45%
Natural Selection
 Results in:

A change in allele frequencies of the gene pool

Improved fitness of the population
 Major cause of microevolution
Natural Selection
 Natural selection can alter the frequency of inherited
traits in a population in five different ways,
depending on which phenotype in a population are
favored





Stabilizing
Disruptive (diversifying)
Directional
Sexual
Artificial
Natural Selection
 Stabilizing Selection:
 Eliminates the extremes and favors the more common
intermediate forms
 Example: Human birth weights (6-8 lbs)
Natural Selection
 Disruptive (diversifying) selection:
 Increases the extreme types in a population at the expense of
intermediate forms.
 May result in balanced polymorphism


One population divided into two distinct types
Example: British land snails in different habitat ranges
Disruptive Selection
Initial
Distribution
Number of
Individuals
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After
Time
Number of
Individuals
Banding Pattern
After
More Time
Number of
Individuals
Banding Pattern
Banding Pattern
a.
b.
b: © Bob Evans/Peter Arnold/Photolibrary
Natural Selection
 Directional selection:
 Changing environmental conditions causes one phenotype to
replace another in the gene pool
 Example: Peppered moths and antibiotic resistance bacteria
Directional Selection
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
After More Time
Number of
Individuals
After Time
Number of
Individuals
Number of
Individuals
Initial Distribution
Body Size
Body Size
Body Size
a.
Hyracotherium
Merychippus
Equus
b.
Natural Selection
 Sexual selection:
adaptive changes in males and females lead to an
increased ability to secure a mate.
 Select for traits that enhance an individual’s chance of
mating
 Males - increased ability to compete with other males
for a mate
 Females choose to select a male with the best fitness
(ability to produce surviving offspring).

Natural Selection
 Artificial selection:
 Humans breed plants and animals by seeking individuals with
desired traits as breeding-stock.
Racehorses are bred for speed
 Dogs for desired traits

Maintaining Variations
 Variation in a population is necessary in order for a
population to evolve as the environment changes.
 The more variation, the more capacity for evolution







Balanced polymorphism
Geographic variation
Sexual reproduction
Outbreeding
Diploidy
Heterozygote advantage
Frequency-dependent selection
Maintaining Variations
 Balanced polymorphism:
 The presence of two or more phenotypes in a single population

Freckles and no freckles in humans
 Geographic variation:
 Results from differences in phenotypes or genotypes between
populations that inhabit different areas
Rabbits in cold vs. warm climates
 Graded variation in the phenotype is known as a cline

Maintaining Variations
 Diploidy (2n):
 Prevents the elimination of recessive alleles
 At certain times the recessive allele may not be favored but at
other times it might be the advantage
 Heterozygote advantage
 Preserves multiple alleles in a population
 The hybrid individual is selected for because of greater
reproductive success

Sickle cell anemia in West Africa
Sickle Cell Disease
malaria
sickle-cell
overlap of both
Maintaining Variations
 Frequency-Dependent selection:
 Decreases the frequency of the more common phenotypes and
increase the frequency of the less common ones

A predator develops a vision for a specific characteristic and
therefore the less desired characteristic ends up becoming more
frequent