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Purposeful Population
Genetics
Chapter 23
I.
Darwin’s Problem
He couldn’t explain the
variation! He didn’t know
Genetics!
Populations evolve (not individuals),
natural selection is the most important
mechanism of evolution, and gradualism
explains how small changes accumulate
over time to cause larger changes (like
new species)
II. Vocabulary
1. Species – group of individuals that can
interbreed and produce fertile offspring
2. Population – group of individuals of the
same species, living in the same place at
the same time
 Individuals near the population center are more
likey to interbreed in the same populations, and
are therefore more closely related
Vocabulary (cont)
3. Gene pool – all of the genes in a
population at any one time
 diploid organisms – have 2 copies of each
gene and can be homozygous or
heterozygous
 if ALL organisms in a population are
homozygous for the same allele the allele is
fixed in the gene pool
III. Hardy-Weinberg Theorem
Gene pool of a NON-EVOLVING population
If a population is not evolving, the frequencies of
alleles and genotypes stay constant over time
Hardy-Weinberg Equilibrium – occurs when
Mendelian genetics (crossing over/meiosis) do
not alter the genetic frequencies of a population
– this state does NOT normally exist in nature
for a long period of time
IV. How do figure out gene
frequency?
• Frequency of r = # of r
Total
• Frequency of R= # of R
Total
Example
• In wildflowers – Red color (R) is dominant
to white color (r). There are 500 plants in
a population. 20 plants are white (rr). 480
are Red. 320 are RR, 160 are Rr.
• What is the TOTAL number of genes in the
population?
• 1000 (500 x 2)
• Frequency of r = # of r = 20+20+160 =200 =0.2
Total
1000
1000
• Frequency of R= # of R=320+320+160=800=0.8
Total
1000
1000
Predicting Next Generation…
• To determine the frequencies of the next
generation, use the data above to predict.
Assume random mating:
• Chance of RR =
• .8 x .8 = .64
• Chance of Rr =
• 2 x .8 x .2 = .32 (x2 because Rr or rR)
• Chance of rr =
• .2 x .2 = .04
• Allele frequencies remain constant
•
Freq R = .8
Freq r = .2
• So after one generation, this population
has remained stable
Hardy Weinberg Equation
• Scientists use p to represent frequency of
one allele, and q to represent the
frequency of the other allele
• For problem above p = .8 q = .2
• p+q=1
• Genotype frequencies p2 + 2pq + q2
RR Rr rr
Practice
• One out of 10000 babies are born with
PKU. The disease is caused by a
recessive allele. What is the frequency of
carriers in the population (Aa)?
Solution
• q2 = .00001 q = .01
• p + q = 1 so p = .99
• Frequency of carriers (Aa) = 2pq = 2 x .01
x .99 = .0198 = 2%
Hardy Weinberg cont
• In order for a population to be in HardyWeinberg equilibrium (non-evolving), the
following conditions must occur:
• 1. Very large population size
• 2. No migration
• 3. No net mutations
• 4. Random mating
• 5. No natural selection
Hardy Weinberg cont
• Populations in nature are not expected to
be in equilibrium and because of this, the
population is evolving.
Microevolution
• generation to generation change in a
population’s frequency of alleles. Some
genotypes may be in equilibrium while
others are not. This population is still
evolving.
There are 4 causes of
microevolution:
• 1. Genetic drift – change in a
population’s allele frequency due to
chance
• Occurs when working with a SMALL
population
• The gene pool of a small population may
not be accurately represented in the next
generation
Genetic drift (cont)
• Examples
• a. Bottleneck effect: due to disasters such as
earthquakes, fires etc reducing the population size
dramatically
• -the new population may not represent the original gene
pool – some alleles may be eliminated totally.
• b. Founder effect: occurs when a few individuals from a
larger population colonize an isolated island, lake or
other new habitat
• -may cause a high frequency of inherited diseases in
some human populations
There are 4 causes of
microevolution:
• 2. Natural Selection – occurs when some
variations help organisms survive better in an
environment than others. This goes against
Hardy-Weinberg.
• -Traits that help in survival are more likely to be
passed on to the next generation because those
organisms are able to reproduce
• -This is the only evolutionary factor that adapts a
population to it’s environment
There are 4 causes of
microevolution:
• 3. Gene Flow – occurs when a population
gains or loses alleles due to migration
(either in or out)
• -can bring neighboring populations
together to one
There are 4 causes of
microevolution:
• 4. Mutation – change in an organisms DNA
• -may change one allele to another and therefore
alter the genotype frequency of a population
• -overall, mutations are rare events, but they are
still important because they can be a source of
new variations that may help organisms survive
changing conditions in their environment
Genetic Variation
• provides the raw material for Natural
Selection
• 1. Variation within populations:
• all populations have variation (easy to see
in humans, not always as easy in other
organisms)
• in order for natural selection to occur, the
variation MUST be heritable (in the genes)
Variation within populations
• *Quantitative Characters – vary along a
continuum (i.e. height) – usually a result of
polygenic inheritance – more than one gene
controls the trait
• *Discrete characters – either/or (i.e. red or white
flower) – usually controlled by only one gene
• If two or more distinct characters are seen in
high frequency within a population it is said to be
Polymorphic
Variation within populations
• *Measuring diversity
• gene diversity – measure of genetic
variation in whole genes (humans have
about 14% gene diversity)
• nucleotide diversity – done by comparing
nucleotide sequences of two individuals,
and then pooling data from many
comparisons (humans only about 0.1%
diverse in nucleotides)
2. Variation between populations
• -Geographic variation - differences in
gene pools between populations or
subgroups of populations (due to
differences in environments)
• *cline – graded change in a trait along a
geographic axis (i.e. avg body size of
many birds increases with increasing
latitude)
Generating Genetic Variety
• 1. Mutation – the ONLY source of new alleles in a
population
• *most occur in somatic cells and can’t be passed on
• *mutations only have an effect if they are in gametes
• *most mutations are harmless but some can have a
significant impact (usually negative)
• *On rare occasions – a mutation actually helps an
organism survive in a changing environment
• *Mutation can be significant in microorganisms due to
their short generation span
Generating Genetic Variety
• 2. Sexual Reproduction – accounts for
nearly all of the genetic differences of
sexually reproducing organisms
• *crossing over during meiosis
• *random fertilization of egg/sperm
• *independent assortment of alleles
Preserving Variation
• natural selection might eliminate variety but the
variation is preserved by the following methods:
• 1. Diploidy – most eukaryotes have two copies
of each allele (diploid). Because of this, variety
(due to recessive alleles) can be hidden in
heterozygotes
• *Heterozygote protection helps keep the allele
frequency variable and this can help if the
environment changes
Preserving Variation
• 2. Balanced Polymorphism – ability of
natural selection to maintain stable
frequencies of two or more phenotypic
forms in a population
• A. Heterozygote advantage – individuals
that are heterozygous survive better and
have a higher reproductive success than
homozygous individuals. (i.e. sickle cell
anemia)
Balanced Polymorphism
• B. Frequency-dependent selection –
survival and reproduction of one morph
declines if that phenotype becomes too
common in the population
(i.e.parasite/host)
Preserving Variation
• 3. Neutral variation – gives no selective
advantage for one individual over another
(i.e. different fingerprints)
Evolutionary Fitness
• contribution an individual makes to the
gene pool of the next generation relative to
the contributions of other individuals
Relative Fitness
• contribution of a genotype to the next
generation compared to alternative
genotypes
• *The most reproductively successful
variant has a relative fitness of 1. Other
genotypes are determined as a
percentage of the most successful
genotype
Effect of Selection on a Population
• 1. Directional selection – usually seen
during periods of environmental change or
migration
• *the frequency curve for variation is shifted
in one direction or another by favoring
relatively rare individuals
Effect of Selection on a Population
• 2. Diversifying Selection – occurs when
environmental conditions favor individuals
on both extremes of a phenotypic range
instead of intermediate phenotypes
Effect of Selection on a Population
• 3. Stabilizing selection – favors
intermediate phenotypes over the
extremes
• *reduces variation and maintains status
quo
Sexual Selection
• secondary sex characteristics cause
marked differences between males/
females called sexual dimorphism – this
can cause differences in size, plumage,
adornments etc
Sexual Selection
• A. Intrasexual selection – direct
competition among individuals of one sex
for mates of the opposite sex ( i.e. males
fighting for a female)
Sexual Selection
• B. Intersexual selection – mate choice
• *individuals of one sex are choosy in
selecting their mates from individuals of
the other sex
Why not “perfect” organisms after
natural selection?
• 1. Evolution is limited by historical
constraints – organisms are not built from
scratch but use existing structures to
adapt to new situations (i.e. back pain may
be due to skeleton being better adapted
for four legged existence)
Why not “perfect” organisms after
natural selection?
• 2. Adaptations are often compromises –
organisms must do many tasks. An
adaptation that helps in one area may
hinder another (seals live on rocks and
water-legs might be more helpful for rocks,
but fins in water outweigh the benefits of
legs)
Why not “perfect” organisms after
natural selection?
• 3. Not all evolution is adaptive – chance
affects the genetic structure of populations
(i.e. storms blowing insects/seed to new
areas)
• 4. Selection can only edit existing
variations – new alleles do not arise as an
organism needs them.