Population Genetics
and Speciation
A look at evolution from a genetic standpoint…
Population Genetics
 Population genetics is the study of evolution from a
genetic point of view.
 Microevolution is defined as a change in the collective
genetic material of a population.
 A population is a group of individuals of the same
species that routinely interbreed…they are important
because they are the smallest unit in which evolution
occurs.
It doesn’t only apply to grades…
 In population genetics, we study observable traits.
 Biologists often study variation in a population and
notice that when they measure a trait in a large sample,
the frequency of that trait exhibit a bell curve.
Bell Curve
Causes of Variation
1. Mutation – a random change in a gene that is passed
on to offspring.
2. Recombination is the reshuffling of genes in a diploid
individual. Recombination occurs during meiosis by
independent assortment and crossing over.
3. The random pairing of gametes occurs because each
organism produces large number of gametes, so…the
union of any particular egg or sperm is partly a matter
of chance.
Microevolution
• Changes occur in gene pools due to
mutation, natural selection, genetic drift,
etc.
• Gene pool changes cause more
VARIATION in individuals in the population
• This process is called MICROEVOLUTION
• Example: Bacteria becoming unaffected by
antibiotics (resistant)
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The Hardy-Weinberg Principle
• Used to describe a non-evolving
population.
• Shuffling of alleles by meiosis and
random fertilization have no effect on
the overall gene pool.
• Natural populations are NOT expected
to actually be in Hardy-Weinberg
equilibrium.
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The Hardy-Weinberg Principle
• Deviation from Hardy-Weinberg
equilibrium usually results in evolution
• Understanding a non-evolving
population, helps us to understand how
evolution occurs
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5 Assumptions of the H-W
Principle
1. Large population size
- small populations have fluctuations in allele
frequencies (e.g., fire, storm).
2. No migration
- immigrants can change the frequency of an allele
by bringing in new alleles to a population.
3. No net mutations
- if alleles change from one to another, this will
change the frequency of those alleles
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5 Assumptions of the H-W
Principle
3. Random mating
- if certain traits are more desirable, then
individuals with those traits will be selected and
this will not allow for random mixing of alleles.
4. No natural selection
- if some individuals survive and reproduce at a
higher rate than others, then their offspring will
carry those genes and the frequency will change
for the next generation.
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Traits Selected for Random Mating
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Disruptions of Genetic
Equilibrium(Breaking the HardyWeinberg! You rebel!)
1. Mutation-can produce totally new alleles for a trait.
Some are harmful, some are positive, some have no
effect.
2. Gene Flow- Genes can move in and out of a
population.
a.
b.
Immigration: Movement of individuals into a population.
Emigration: Movement of individuals out of a population.
Disruption Continued…
3. Genetic Drift- When allele frequencies in a population
change as a result of random chance or events.
-This can result in significant changes within a
population especially if the population is small!
Genetic Drift Example
Population Bottleneck
 Population Bottleneck: An event or period in which the size of a
breeding population is significantly reduced. This produces a
sudden plunge in genetic variability, and greatly increases genetic
drift as the population recovers.
 Examples: Humans may have had a population bottleneck about
70,000 years ago, when the breeding population decreased to as
few as 15,000 individuals. This catastrophe may have been
caused by the eruption of Mount Toba in Indonesia, leading to
global cooling.
 Northern Elephant Seals have greatly reduced genetic variability
as a result of a bottleneck that occurred in the 1890's, when
hunting by humans reduced their numbers to as few as 20 seals.
Founder Effect
 Founder Effect: A population that is descended from a relatively
small number of individuals who became separated from the
original population shows significant differences from the original
population. This is because its genetic inheritance depends upon a
small number of Founders, who were not representative of the
population from which they came, and did not possess the full
range of variability of the original population.
 Examples: The Afrikaner population of South Africa is descended
from a small group of Dutch settlers. Today, their descendents
have an unusually high frequency of a gene that causes
Huntington's Disease. This is a Founder Effect which originates
from the fact that the original settlers chanced to have a high level
of this allele.
Non-Random Mating
 Many species do not mate randomly, but they mate
based on proximity (how close a mate is to them.)
 This results in the mating of related individuals which
can amplify certain traits and also allow recessive
genes to be expressed.
Sexual Selection
 In many species, the males are brightly colored in order
to attract a mate.
 The bright colors is an adaptation for a male bird to
attract mates, but how could this adaptation be
detrimental to the species?
 Females can use these extreme traits to determine if
the male is fit or the makeup of his genes.
Natural Selection
 Three types: Stabilizing Selection, Disruptive
Selection, and Directional Selection.
 Stabilizing: Individuals with the average form of the
trait have the greatest fitness.
 Disruptive: Individuals with either extreme variation of
a trait have the greatest fitness.
 Directional: Individuals that display a more extreme
form of the trait have the greatest fitness.
Can you tell which is which?
Formation of a Species
 We know that existing species are change versions of
older species.
 This is called Speciation.
 Geographic Isolation can lead to Allopatric
Speciation.
 Reproductive Isolation can lead to Sympatric
Speciation.
 So what’s the diff? Let’s first define a species.
The Gene Pool
•Members of a species
can interbreed & produce
fertile offspring
•Species have a shared
gene pool
•Gene pool – all of the
alleles of all individuals in
a population
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The Gene Pool
•Different species do
NOT exchange genes
by interbreeding
•Different species that
interbreed often
produce sterile or less
viable offspring e.g.
Mule
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Geographic Isolation
 Sometimes there is a physical separation of members
in a population.
 Could be by a river, a valley, a mountain range.
 Natural selection will cause the two populations to
diverge and become new species.
 How and why? Let’s read about populations of fish on
page 327 in your book…
 THIS LEADS TO ALLOPATRIC SPECIATION.
Reproductive Isolation
 Sometimes organisms become genetically isolated (cannot
interbreed) without being geographically isolated.
 Reproduction isolation results from barriers to successful
breeding between population groups in the same area.
 Sometimes arises through disruptive selection.
 Prezygotic isolation—occurs before fertilization (different mating
seasons or calls)
 Postzygotic isolation—occurs after fertilization.(producing
sterile offspring)
 THIS LEADS TO SYMPATRIC SPECIATION.