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Chapter 16
Population Genetics and
Section 1: Genetic Equilibrium
Section 2: Disruption of Genetic Equilibrium
Section 3: Formation of Species
Leopard Seal, Antarctica
Section 1: Genetic Equilibrium
Population Genetics
Study of evolution from a genetic point of view
1. Every population has some genetic variation that
influences fitness
- Evolution is potentially a continuing process in all populations
2. Changes in selective factors in the environment
will almost always be met by evolutionary
- leads to shifts in the frequencies of genotypes in a population
3. Rapid changes will often exceed the capacity of a
population to respond by evolution
- decline could lead to extinction
Darwin’s finches
• Peter and Rosemary Grant
• The Galapagos Islands: normally dry
• El Nino: increase rainfall, vegetation
• La Nina: periods of drought
• Reproductive success and survival of
individuals differed between El Nino and La
Nina years
• Caused dramatic evolutionary change
• Medium ground finch
• Seeds, cracks with beak
• La Nina (drought):
– amount of seeds dropped, seeds became tougher
– population dropped
• 1400 in 1975 to 200 at end of 1977
– larger beaks could crack the larger seeds and
survived better than those with smaller beaks
– Average beak size increased
Within a population
individuals vary in
observable traits.
Traits cover a range
that can be
represented by a
bell curve
• El Nino (wet) 1983
– Small seeds in abundance
– Those with smaller beaks handled the
smaller seeds better, able to survive,
produce more offspring than those with
larger beaks
– Average beak size returned to a lower
Population genetics studies the ways in
which populations respond to such
selective pressures with changes in
allele frequencies
Genetic Variation
1. Mutation: random change in a gene,
passed on to offspring
2. Recombination: reshuffling of genes
 Independent assortment
 Crossing-over
3. Random Pairing of gametes
Gene Pool: total genetic information available in a
When mate at random: all combinations of different
alleles are possible
Peccaries are small,
tough relatives of the
modern pig, whose
lineage diverged about
40 million years ago. They
live in southern Texas, Arizona, and New Mexico.
The forces of evolution
shape and change the
composition of this gene
pool and thus the nature
of the population.
Allele Frequency = # of a
certain allele / total number of
B: Long bristles on the bodies
b: short bristles
15 Individual peccaries in the population 30 alleles
If 6 alleles in this population are b, and 24 are B
Then the frequencies of these alleles are:
6/30 of the genes in the gene pool are b – a frequency of 0.2
24/30 of the gene in the gene pool are B – a frequency of 0.8
Phenotype Frequency = # of individuals with a
particular phenotype / total number of
individuals in the population
Hardy-Weinberg Equilibrium
Frequencies of alleles and
genotypes remain constant
from generation to
generation in a population
(No evolutionary change occurs
through the process of sexual
Changes can result only from
the action of additional
forces on the gene pool of
a population
Ideal hypothetical population
that is not evolving
Set of Assumptions
1. No net mutations occur
2. Individuals neither enter
nor leave the population
3. The population is large
4. Individuals mate
5. Selection does not occur
*any exception to these can
result in evolution
Section 2: Disruption of Genetic Equilibrium
To be at
No Net Mutations Occur
• Spontaneous mutations occur constantly at very low rates
• If exposed to mutagens, rates can increase significantly
• Mutations can produce totally new alleles for a trait
To be at
Individuals can neither
enter nor leave a population
• Size of the population must remain constant
• If individuals move, genes move too
Immigration: movement into a population
Emigration: movement out of a population
Gene flow: process of genes moving from one population to another
To be at
Large Population
Genetic Drift: phenomenon by which allele
frequencies in a population change as a result of
random events, or chance
To be at
Individuals Mate Randomly
Nonrandom Mating
• Influenced by geographic proximity
• Select a mate with similar traits:Assortative Mating
• Sexual selection: females tend to choose males based on
certain traits (Planet Earth: Jungles, Birds of Paradise)
Birds of Paradise-Planet Earth
QuickTime™ and a
are needed to see this picture.
To be at
No Natural Selection
Ongoing process in nature
Some members of a population are more likely than
others to survive and reproduce and thus contribute
their genes to the next generation
Stabilizing Selection: individuals with the average form of a
trait have the highest fitness
Disruptive Selection: individuals with either extreme
variation of a trait have greater fitness than those with the
average form of that trait
Directional Selection: individuals that display a
more extreme form of a trait have a greater fitness
than those with an average form
Black dots represent individuals that die before passing on their genes.
Hardy-Weinberg Conditions Animations
Hardy-Weinberg Equation
Used to discover the probable genotype frequencies in a
population and to track their changes from one generation
to the next
p2 + 2pq + q2 = 1
p = frequency of the dominant allele (A)
q = frequency of the recessive allele(a)
p2 is the predicted frequency of homozygous dominant
people (AA)
2pq is the predicted frequency of heterozygous people (Aa)
q2 is the predicted frequency of homozygous recessive
people (aa)
Section 3: Formation of Species
Speciation: process of species formation
Species: a single kind of organism. Members
are morphologically similar (external
structure and appearance) and can interbreed
to produce fertile offspring. (biological
species concept)
How do species give rise to other ones?
1. Geographic isolation: physical separation
of members of a population.
No longer experience gene flow, so the gene pools of each separate
population may begin to differ due to genetic drift, mutations and
natural selection
Allopatric Speciation happens when species arise as a result of
geographic isolation
“different homeland”
Reproductive Isolation: results from barriers
to successful breeding between
population groups in the same area.
Pre-zygotic: occurs
before fertilization
active at different seasons
or times of day
Post-zygotic: occurs after
zygote dies
F1 hybrids have reduced
fertility or sterility
Example: mule from horse
and donkey
Sympatric Speciation: when two subpopulations
become reproductively isolated within the same
geographic area
200 years ago, ancestors of apple
maggot flies laid their eggs only
on hawthorns. Today, these flies
lay eggs on hawthorns and
domestic apples. Females choose
to lay eggs on fruit they grew up
in and males look for mates on
the fruit they grew up on.
Gene flow between parts of the
population that mate on different
types of fruit is reduced.
Led to new species
Rates of Speciation
Happens at a regular, gradual rate
(few million yrs)
Sudden, rapid change followed by little to no change
(few thousand years)