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The Evolution of Populations
Chapter 23
What is the difference between a gene and an
Quick refresherA gene is the basic instruction, a sequence of DNA, an
allele is one variant of that instruction.
1. The cat fell from the roof.
2. The cat fell off the roof.
3. The snake fell from the roof.
All 3 sentences are the equivalent of a gene, they
represent different alleles.
Sentences 1 &2 have nearly identical meanings, however
sentence 3 has a very different meaning.
Genetic variation makes evolution possible
Mutations are the only source of new genes and new alleles.
Only mutations in gametes can be passed to offspring.
Point mutations are changes in one base in a gene. They can have significant
impact on phenotype, as in sickle-cell disease.
Chromosomal mutations delete, disrupt, duplicate, or rearrange many loci
at once. They are usually harmful.
Most of the genetic variations within a population are due to the sexual
recombination of alleles that already exist in a population.
Sexual reproduction rearranges alleles into new combinations in every
generation. Recall there are three mechanisms for this shuffling of alleles:
Crossing over during prophase I of meiosis.
Independent assortment of chromosomes during meiosis
Fertilization (223 X 223 different possible combinations for human sperm and
Population Genetics
Population genetics is the study of how
populations change genetically over time.
Population: A group of individuals of the same
species that live in the same area and
interbreed, producing fertile offspring.
Gene pool: All of the alleles at all loci in all the
members of a population.
In diploid species, each individual has two
alleles for a particular gene, and the individual
may be either heterozygous or homozygous.
If all members of a population are homozygous
for the same allele, the allele is said to be fixed.
Gene Pools
A gene pool is the combined genetic information of
all the members of a particular population
Recall that a population is a collection of individuals of the
same species in a given area which share a common
group of genes
A gene pool typically contains 2 or more alleles (or
forms of certain genes)
The relative frequency of an allele is the number of
times that allele occurs in a gene pool compared to
the number of times other alleles occur.
Population Genetics
Hardy-Weinberg Principle – states that allele
frequencies tend to remain constant in populations
unless something happens OTHER THAN Mendelian
segregation and sexual recombination.
This situation in which allele frequencies remain constant is
called genetic equilibrium
 If allele frequencies do not change, the population will not
Hardy-Weinberg is a mathematical model that
describes the changes in allele frequencies in a
Allows us to predict allele and genotype frequencies in
subsequent generations (testable)
G.H. Hardy
Hardy-Weinberg Theorem
Model conditions required to maintain genetic
equilibrium from generation to generation:
Random mating population
Large population size
No emigration or immigration (no movement into or out of
the population)
No mutations
No natural selection (all genotypes have an equal chance
of survival and reproduction)
If all 5 conditions are met, there should be NO
Hardy-Weinberg Equation
Let p= frequency of allele A
Let q= frequency of allele a
Let p2= frequency of genotype AA
Let 2pq= frequency of genotype Aa
Let q2= frequency of genotype aa
p2+2pq+q2=1 (genotype frequencies)
p+q=1 (allele frequencies)
W. Weinberg
The Hardy-Weinberg Equation can show that
evolution IS OCCURING within a population
Causes of Microevolution
Natural selection, genetic drift, and gene flow can
alter allele frequencies in a population and cause
MOST evolutionary changes.
Microevolution –Evolution on its smallest scale.
Three main causes:
Generation to generation change in a population’s allele
genetic drift
natural selection
gene flow
Genetic Drift
Genetic drift is the unpredictable fluctuation
in allele frequencies from one generation to
the next. The smaller the population, the
greater the chance is for genetic drift. This is
a random, non- adaptive change in allele
Can and does lead to allele fixation
Allele fixation means that a population changes
(evolves) from many alleles represented, to only 1
allele represented
Consequences of Genetic Drift
Consequences of genetic drift:
Fixation of alleles
Effect of chance is different from population to
Small populations are effected by genetic drift more
often than larger ones
Given enough time, even in large populations genetic
drift can have an effect
Genetic drift reduces variability in populations by
reducing heterozygosity
The Bottleneck Effect
The Founder Effect
Real World Examples of Genetic Drift
The Bottleneck Effect
Occurs when only a few
individuals survive a random
event, resulting in a shift in
allele frequencies within the
Small population sizes
facilitate inbreeding and
genetic drift, both of which
decrease genetic variation
Reduces genetically
variability because at least
some alleles are likely to be
lost from gene pool
Figure 23.5 The bottleneck effect: an analogy
Real World Examples of Genetic Drift
Founder effect: A few individuals become isolated
from a larger population and establish a new
population whose gene pool is not reflective of the
source population
This small population size means that the colony may
• reduced genetic variation from the original
• a non-random sample of the genes in the original
For example, the Afrikaner population of Dutch
settlers in South Africa is descended mainly from a
few colonists. Today, the Afrikaner population has an
unusually high frequency of the gene that causes
Huntington’s disease, (which causes nerve cells in
certain parts of the brain waste away, or degenerate.)
because those original Dutch colonists just happened
to carry that gene with unusually high frequency. This
effect is easy to recognize in genetic diseases.
The Founder Effect
Founding Population A
Founding Population B
Gene Flow
Gene flow can also change allele frequencies
Gene flow is the physical flow of alleles into or
out of a population.
Immigration – alleles coming in (added)
Emigration – alleles moving out (lost)
Gene flow counteracts differences that arise
through mutation, natural selection, and genetic
Gene flow helps keep separated populations
genetically similar – reduces differences between
Natural selection is the only mechanism that
consistently causes adaptive evolution
Relative fitness refers to the contribution an
organism makes to the gene pool of the next
generation relative to the contributions of other
members. Fitness does not indicate strength or
size. It is measured only by reproductive
Natural selection acts more directly on the
phenotype and indirectly on the genotype and
can alter the frequency distribution of heritable
traits in three ways.
1. Directional selection Example: Large black bears survived periods
of extreme cold better than smaller ones, and so became more
common during glacial periods.
2. Disruptive selection Example: A population has individuals with either large beaks or small beaks, but few with the intermediate beak
size. Apparently the intermediate beak size is not efficient in cracking
either the large or small seeds that are common.
3. Stabilizing selection Example: Birth weights of most humans lie in
a narrow range, as those babies who are very large or very small have
higher mortality
Directional selection
Shifts the overall
makeup of the
population by
favoring variants
that are at one
extreme of the
In this case, darker mice are favored because they live
among dark rocks, and a darker fur color conceals them
from predators.
Disruptive selection
variants at
both ends of
the distribution.
These mice have colonized a patchy habitat made up of light
and dark rocks, with the result that mice of an intermediate
color are at a disadvantage.
Stabilizing selection
Removes extreme
variants from the
population and
intermediate types.
If the environment consists of rocks of an intermediate color,
both light and dark mice will be selected against
Sexual Selection
Sexual selection is a form of natural selection
in which individuals with certain inherited
characteristics are more likely than other
individuals to obtain mates. It can result in
sexual dimorphism, a difference between the
two sexes in secondary sexual characteristics
such as differences in size, color,
ornamentation, and behavior.
Figure 23.16x1 Sexual selection and the evolution of male
Types of Sexual Selection
Intrasexual Selection means selction within the
same sex – typically males
Individuals of one sex compete directly for mates of the
opposite sex
Often it is based on rituals and displays that don’t risk injury
Intersexual Selection is also called “mate choice” –
typically females
Female choice is typically based on showiness of the
male’s appearance and/or behavior
Males will often weight the attraction of predators versus
the attraction of mates
How is genetic variation preserved in a
Diploidy: (refers to organisms carrying genes in pairs)
Because most eukaryotes are diploid, they are capable of
hiding genetic variation (recessive alleles) from selection.
Heterozygote advantage: Individuals who are
heterozygous at a certain locus have an advantage for
Example: sickle-cell disease, individuals homozygous for
normal hemoglobin are more susceptible to malaria, whereas
homozygous recessive individuals suffer from the
complications of sickle-cell disease. Heterozygotes benefit
from protection from malaria and do not have sickle-cell
disease, so the mutant allele remains relatively common.