Population Genetics and
Speciation
1.
2.
3.
4.
5.
Population
Genetics
Microevolution
Gene Pool
Allele
Frequency
Phenotype
Frequency
A.
B.
C.
D.
E.
Total genetic information
in a population
Portion of gene copies of
a given allele
Study of the frequency
and interaction of alleles
and genes in populations
Change in the collective
genetic material of a
population
Ratio of individuals with a
given phenotype to the
total population
A.
B.
C.
D.
E.
Population Genetics
Microevolution
Gene Pool
Allele Frequency
Phenotype Frequency
C
D
A
B
E
 Population
Genetics is the
study of evolution from a
genetic point of view (it is
the study of microevolution)
 Microevolution—a change
in the collective genetic
material of a population
• Population–members of the
same species that can
interbreed. It is the smallest unit
in which evolution occurs.
http://www.abc.net.au/science/news/enviro/EnviroRepublish_1417697.htm
 Populations
show natural
variety within
a species.
 Many
quantitative
traits (height
and weight etc.)
follow a bell
shaped curve.
http://www.bulbnrose.org/Heredity/Mather/poly1.jpg
 Environmental
factors—amount of food,
quality of food, etc.
 Genetic factors
• Mutations—random changes in genes
• Recombination—reshuffling of genes
• Random pairing of gametes
 Gene
pool —
total genetic
information
available in a
population
http://www.cartoonstock.com/lowres/rde3053l.jpg
 Allele
frequency—expressed as a
percent: it is determined by dividing the
number of a certain allele by the total
number of alleles of all types in the
population
 Phenotypic frequency—expressed as a
percent: it is the number of individual
with a particular phenotype divided by
the total number of individuals in the
population.
 Developed
by Wilhelm Weinberg
(German physician) and Godfrey Hardy
(British mathematician)
Godfrey Hardy
Wilhelm Weinberg
http://anthro.palomar.edu/synthetic/synth_2.htm
 States
that genetic
frequencies in a
population tend to
remain the same
from generation to
generation unless
acted on by outside
influences.
 It is based on a
“hypothetical
population” that is
not evolving.
http://www.cartoonstock.com/directory/h/human_evolution.asp
http://mariewinnnaturenews.blogspot.com/2007_11_25_archive.html
 In
Genetic
Equilibrium:
• No net mutations occur
• Population size remains
constant
• The population is
infinitely large
• Individuals mate
randomly
• Selection does not occur
This flock of mallards probably
violates some or all of the
conditions necessary for the
Hardy-Weinberg genetic
equilibrium
 It
is highly unlikely that all five of the
conditions in the Hardy-Weinberg Model
will happen in the real world.
 Therefore, Genetic Equilibrium is
impossible in nature.
 It is a theoretical state that allows us to
consider what forces could disrupt such
balance (equilibrium).
1.
2.
3.
4.
5.
6.
7.
8.
Immigration
Emigration
Gene Flow
Genetic Drift
Sexual
Selection
Stabilizing
Selection
Disruptive
Selection
Directional
Selection
A.
B.
C.
D.
E.
F.
G.
H.
Individuals move out
Individuals move in
Choice of mates based on
favorable traits
Genes move from one
population to another
Average trait is selected
One extreme trait is
selected
Two extreme traits are
selected
Allele frequencies
change randomly
1.
2.
3.
4.
5.
6.
7.
8.
Immigration
Emigration
Gene Flow
Genetic Drift
Sexual Selection
Stabilizing Selection
Disruptive Selection
Directional Selection
B
A
D
H
C
E
G
F
 Disruptions
to the Hardy-Weinberg
equilibrium can result in evolution.
 The five requirements for genetic
equilibrium can be disrupted by the
following outside forces:
•
•
•
•
•
Mutation
Gene Flow
Genetic Drift
Nonrandom Mating
Natural Selection
 Mutations
occur constantly at very low
rates under normal conditions.
 Exposure to mutagens (mutationcausing agents, i.e. radiation and
chemicals) can increase mutations rates.
 Mutations produce new alleles for a trait
 They can be harmful, harmless or
helpful
 Helpful mutations are a vital part of
evolution.
 Individuals
enter and leave populations
constantly. Their “genes” move with them.
This is called Gene Flow.
 Factors influencing gene flow include:
• Immigration—movement of individuals into a
population
• Emigration—movement of individuals out of a
population
• Migration and dispersal patterns can also
influence the movement of individuals into new
populations
• Birth and Death Rates also remove or add genes
from individuals to a population.
 In
nature, population sizes are
restricted rather than infinitely large.
 Genetic Drift can occur in small
populations of organisms
• Genetic Drift—the random change in allele
frequency in a population
• Significant changes can happen in small
populations if even a single organism either
fails to reproduce or reproduces too much.
http://en.wikipedia.org/wiki/File:Northern_Elephant_Seal,_San_Simeon2.jpg
If the frequency of an
allele reaches zero in a
population, then
(assuming you started
with two alleles), there is
only one left.
 All individuals will be
homozygous for that
trait---creating no
variations.
 This weakens a species.

• Ex: Northern Elephant Seal
Homozygous for every gene tested
 Genetic
Drift
can lead to a
bottleneck
effect in which
variations are
reduced
overtime.
http://biology.unm.edu/ccouncil/Biology_203/Summaries/PopGen.htm
 Organisms
do not mate
randomly in nature.
 Mate selection is
influence by:
• Geographic proximity—
choose mates nearby: can
result in kinship mating
• Assortative Mating—
choose mates with similar
traits: reduces variation
• Sexual Selection—choose
mates based on favorable
traits
http://upload.wikimedia.org/wikipedia/commons/5/51/Oregon_zoo_peacock_male.jpg
 Natural
Selection—organisms with
favorable traits are more likely to survive
and reproduce, passing on their
favorable genes to the next generation.
 It is an ongoing process in nature and an
important disruption to equilibrium.
 Three patterns of Natural Selection:
 Stabilizing
Selection:
individuals with
the average form
of a trait have the
highest fitness.
 Ex: Lizard body
size
http://upload.wikimedia.org/wikipedia/commons/c/c6/Selection_Chart.PNG
 Disruptive
Selection:
individuals with
either extreme
variation of a trait
have the highest
fitness.
 Ex: Shell Color of
Limpets
http://upload.wikimedia.org/wikipedia/commons/c/c6/Selection_Chart.PNG
 Directional
Selection:
individuals with
one extreme of a
trait have the
highest fitness
 Ex: Nose and
tongue lengths of
anteaters
http://upload.wikimedia.org/wikipedia/commons/c/c6/Selection_Chart.PNG
1.
2.
3.
4.
5.
6.
7.
Speciation
Geographic Isolation
Allopatric Speciation
Reproductive
Isolation
Sympatric Speciation
Gradualism
Punctuated
Equilibrium
A.
B.
C.
D.
E.
F.
G.
A slow change in a
species over millions of
years
Bursts of rapid change
Formation of a new
species
Physical separation of
populations
Inability to mate or
produce offspring
Speciation resulting from
geographic isolation
Speciation resulting from
reproductive isolation
1.
2.
3.
4.
5.
6.
7.
Speciation
Geographic Isolation
Allopatric Speciation
Reproductive Isolation
Sympatric Speciation
Gradualism
Punctuated Equilibrium
C
D
F
E
G
A
B
 Speciation—formation
of a new species
• Species: a single kind of organism whose
members are morphologically similar and
can interbreed to produce fully fertile offspring.
• Two types of speciation: Allopatric Speciation
and Sympatric Speciation
 Allopatric
Speciation: species arise as a
result of geographic isolation. (Allopatric =
different homelands)
 Geographic Isolation—physical separation
of members of a population
• Gene flow between the new subpopulations stops
and the two begin to diverge
• Eventually, they become incompatible for mating,
creating new species.
• Debate exists as to whether or not allopatric species
are different enough to be considered new species.
Examples of Geographic Isolation
http://cas.bellarmine.edu/tietjen/images/geographic_isolation.jpg
http://evolution.berkeley.edu/evosite/history/images/geog_isolation.gif
 Sympatric
Speciation —occurs when
two subpopulations become
reproductively isolated within the same
geographic area.
 Reproductive Isolation—the inability of
members of the same species to mate
• Can be caused by disruptive selection
• Two types: prezygotic isolation and
postzygotic isolation
 Prezygotic
(premating) isolation:
(different mating seasons, different
mating calls, etc.)
Behavioral Isolation
Habitat
Isolation
Other Prezygotic Isolation Mechanisms
include
Mechanical
Isolation
Temporal
Isolation
Gametic
Isolation
 Postzygotic
(postmating) isolation:
offspring do not fully develop, die, or are
infertile.
Hybrid is weak and likely to die
Hybrid is sterile
http://www.geo.arizona.edu/Antevs/nats104/SymptrcSpctnSml.jpg

Two Theories:
• Gradualism —slow change over millions of years
• Punctuated Equilibrium—short bursts of rapid
change
http://bioap.wikispaces.com/file/view/gradualism.gif/94608242/gradualism.gif
 Evidence
exists that suggests that both have
taken place over time.
http://silvertonconsulting.com/blog/wp-content/uploads/2010/06/c7-1-23-finches.jpg
http://www.doctortee.com/dsu/tiftickjian/cse-img/biology/evolution/horse-evolution-2.jpg
 Hardy
and Weinberg went on to develop an
equation that can be used to discover the
probable genotype frequencies in a population
and to track their changes from one generation
to another.
 The equation is:
p2+2pq+q2 = 1
p= frequency of the dominant allele
q = frequency of the recessive allele
See handout
 Punnett
Squares allow geneticists to
predict the probability of offspring
genotypes for particular traits based on
the known genotypes of their two parents
 The Hardy-Weinberg equation
essentially allowed geneticists to do the
same thing for entire populations.
 Before
Hardy and Weinberg, it was
thought that dominant alleles must, over
time, wipe out recessive alleles
(genophagy = “gene eating”)
 According to this wrong idea, dominant
alleles always increase in frequency from
generation to generation.
 Hardy and Weinberg demonstrated that
dominant alleles can just as easily
decrease in frequency.