Download Chapter 23: Population Genetics

Chapter 23: Population Genetics
(Microevolution)

Microevolution is a change in allele
frequencies or genotype frequencies in a
population over time

Genetic equilibrium in populations: the
Hardy-Weinberg theorem

Microevolution is deviation from HardyWeinberg equilibrium

Genetic variation must exist for natural
selection to occur
.
•
Explain what terms in the HardyWeinberg equation give:
– allele frequencies (dominant allele,
recessive allele, etc.)
– each genotype frequency (homozygous
dominant, heterozygous, etc.)
– each phenotype frequency
.
Chapter 23: Population Genetics
(Microevolution)

Microevolution is a change in allele
frequencies or genotype frequencies in a
population over time

Genetic equilibrium in populations: the
Hardy-Weinberg theorem

Microevolution is deviation from HardyWeinberg equilibrium

Genetic variation must exist for natural
selection to occur
.
Microevolution is a change in allele frequencies or
genotype frequencies in a population over time

population – a localized group of individuals capable of
interbreeding and producing fertile offspring, and that are
more or less isolated from other such groups

gene pool – all alleles present in a population at a given
time

phenotype frequency – proportion of a population with a
given phenotype

genotype frequency – proportion of a population with a
given genotype

allele frequency – proportion of a specific allele in a
population
.
Microevolution is a change in allele frequencies or
genotype frequencies in a population over time
allele frequency – proportion of a specific allele in a
population


diploid individuals have two alleles for each gene

if you know genotype frequencies, it is easy to calculate allele frequencies

example:

population (1000) = genotypes AA (490) + Aa (420) + aa (90)

allele number (2000) = A (490x2 + 420) + a (420 + 90x2) = A (1400) + a (600)

freq[A] = 1400/2000 = 0.70

freq[a] = 600/2000 = 0.30

note that the sum of all allele frequencies is 1.0 (sum rule of probability)
.
Chapter 23: Population Genetics
(Microevolution)

Microevolution is a change in allele
frequencies or genotype frequencies in a
population over time

Genetic equilibrium in populations: the
Hardy-Weinberg theorem

Microevolution is deviation from HardyWeinberg equilibrium

Genetic variation must exist for natural
selection to occur
.
•
Explain what terms in the HardyWeinberg equation give:
– allele frequencies (dominant allele,
recessive allele, etc.)
– each genotype frequency (homozygous
dominant, heterozygous, etc.)
– each phenotype frequency
.
Make up and do some pop gen problems.
Suggestions: Start with a population of either 100 or 10,000 individuals. Have the
number of individuals with the recessive phenotype be the square of a
whole number (so that q2 is easy to solve). Then answer the frequency
questions below.
What is the frequency in the population of:
• the recessive phenotype?
• the dominant phenotype?
• the dominant allele?
• the recessive allele?
• homozygous recessive individuals?
• homozygous dominant individuals?
• heterozygous individuals?
.
The Hardy-Weinberg Theorem
the Hardy-Weinberg theorem describes the
frequencies of genotypes in a population based on
the frequency of occurrence of alleles in the
population that is in a state of genetic equilibrium
(that is, not evolving)

the usual case for calculations: if allele “A” is dominant to “a”, and
they are the only two alleles possible at the A-locus, then



p = freq[A] = the frequency of occurrence of the A-allele in the
population

q = freq[a] = the frequency of occurrence of the a-allele in the population
Then p + q = 1 (following the sum rule for probability)
.
The Hardy-Weinberg Theorem
Allele associations follow the product rule for
probability, so you multiply to predict the genotype
frequencies:


( p + q ) x ( p + q ) = p2 + 2 pq + q2

p2 = frequency of homozygous dominant genotypes

2 pq = frequency of heterozygous genotypes

q2 = frequency of homozygous recessive genotypes

note that ( p + q ) x ( p + q ) = 1 x 1 = 1, so
p2 + 2 pq + q2 = 1
.
Hardy-Weinberg Equilibrium
if the Hardy-Weinberg theorem
can be used to accurately predict
genotype frequencies from allele
frequencies for a population
then…


the population is in Hardy-Weinberg
equilibrium or genetic equilibrium

in such cases you can use data from one
generation to predict the allele, genotype,
and phenotype frequencies for the next
generation

such populations are not evolving, but are
static instead
.
Make up and do some pop gen problems.
Suggestions: Start with a population of either 100 or 10,000 individuals. Have the
number of individuals with the recessive phenotype be the square of a
whole number (so that q2 is easy to solve). Then answer the frequency
questions below.
What is the frequency in the population of:
• the recessive phenotype?
• the dominant phenotype?
• the dominant allele?
• the recessive allele?
• homozygous recessive individuals?
• homozygous dominant individuals?
• heterozygous individuals?
.
•
Describe the assumptions of the HardyWeinberg equilibrium model.
.
Hardy-Weinberg Equilibrium
the assumptions of this model are:


large population size (due to statistical
constraints, to minimize genetic drift)

no migration – no exchange of alleles with other
populations (no gene flow)

no mutations of the alleles under study occur

random mating of all genotypes

no natural selection
.
•
Describe the assumptions of the HardyWeinberg equilibrium model.
.
Chapter 23: Population Genetics
(Microevolution)

Microevolution is a change in allele
frequencies or genotype frequencies in a
population over time

Genetic equilibrium in populations: the
Hardy-Weinberg theorem

Microevolution is deviation from HardyWeinberg equilibrium

Genetic variation must exist for natural
selection to occur
.
•
Describe conditions that can keep
populations from establishing or
maintaining genetic equilibrium.
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium

if allele and/or genotype frequencies in a population change
over time, then it is by definition evolving (evolution means
changing over time), undergoing microevolution

things that can cause microevolution

small population size: genetic drift

migration – gene flow; individuals leave and/or join a population

mutations

nonrandom mating

natural selection
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
consequences of small population size: genetic drift

Consider taking a small sample of individuals from a larger population


If only two individuals were picked they almost certainly won’t reflect the
allele frequency in the larger population (in many cases, they can’t even
possibly do so).

The same holds true for 3, 4, or 5 individuals.

As the selected sample gets larger it becomes more likely that the sample
reflects the allele frequency in the larger population.
Mating to produce the next generation is effectively sampling the
population


It takes a very large sampling size (thousands) to have a good chance of the
sample essentially matching the allele frequencies and genotype
frequencies of the population.
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium

genetic drift is a change in gene
frequencies of populations because of
small population size
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium

genetic drift tends to decrease genetic
variation within a population

genetic drift tends to increase genetic
variation between populations
NOTE: genetic drift places a major factor in
evolution, especially when populations are
split, but does NOT involve natural
selection
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
consequences of small
population size:
genetic drift

two general mechanisms lead
to small population sizes


genetic bottlenecks are
created by dramatic reduction
in the population size –
endangered species face a
genetic bottleneck on a
species-wide scale, and suffer
lasting effects even if
population size later recovers
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
consequences of small
population size:
genetic drift

two general mechanisms lead
to small population sizes


founder effect – when a new
population is established,
typically only a few individuals
(founders) are involved in
colonizing the new area,
essentially an “isolation
bottleneck” for the new
population; this is common for
islands
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
migration – when individuals leave or
join a population


migrating individuals carry their alleles with
them (gene flow), usually resulting in changes
in allele frequencies

gene flow tends to decrease genetic variation
between populations
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
mutations increase variation in the
gene pool of a species


remember that mutations may be neutral,
harmful, or beneficial

even at the risk of harmful effects, mutations
are necessary to increase variation in the
population so that natural selection can
produce organisms more suited to their
environment
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
nonrandom mating


if individuals do not mate at random, then
Hardy-Weinberg equilibrium is not
achieved

the most common cases of nonrandom
mating involve inbreeding – mating
between individuals of similar genotypes,
either by choice or due to environmental
factors such as location
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium

inbreeding does not change allele frequencies, but increases
the frequency of homozygous genotypes

inbreeding depression is seen in some cases, where inbred
individuals have lower fitness that non-inbred individuals


fitness – relative ability of a genotype to contribute to future
generations

fertility declines and high juvenile mortality associated with
“unmasking” harmful recessive alleles can reduce fitness for inbred
individuals

hybrid vigor also leads to higher relative fitness for hybrids
self-fertilization is the most extreme case of inbreeding
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
assortive mating – a type of nonrandom mating
where mates are (sexually) selected based on
phenotypes – really is an aspect of natural selection


positive assortive mating – selection for the same
phenotype; works like inbreeding for the genes governing
that phenotype, and for loci closely linked to those genes

negative assortive mating – selection for the opposite
phenotype

less common than positive assortive mating

leads to a decrease in homozygous genotypes for the genes
governing the selected phenotype, and for loci closely linked to
those genes
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
natural selection changes allele frequencies in a
way that leads to adaptation to the environment


fitness is the ability of an organism to compete successfully in
passing its alleles on to the next generation (in a vessel that can
continue that process)

populations undergoing natural selection are evolving, with alleles
that contribute to better fitness increasing in frequency over
successive generations

natural selection only operates based on the current environment
– as conditions change, different alleles will be selected for
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium

sexual selection (mate choices based on
inherited characteristics) is an aspect of
natural selection
.
•
Describe conditions that can keep
populations from establishing or
maintaining genetic equilibrium.
.
•
Explain three main types of natural
selection.
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
there are three types of natural selection


stabilizing selection

directional selection

disruptive selection
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
stabilizing selection –
occurs in populations well
adapted to their
environments, selecting
against phenotypic
extremes


this is probably the type of
selection most commonly
faced by populations

example - human birth weight
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
directional selection – permits species to adapt
to environmental change by favoring selection of
one extreme over the other


example – peppered moth
.
Microevolution is a deviation from
Hardy-Weinberg equilibrium
disruptive selection – when more than one extreme phenotype is
favored over intermediate phenotypes


really a special case of direction selection, where there are trends in more
than one direction

can produce a genetic “split” in a population and thus serve as a
mechanism for speciation

example – pocket mice in the Tularosa Basin of New Mexico
Michael E.N Majerus, Nicholas I Mundy. Mammalian
melanism: natural selection in black and white. Trends in
Genetics Volume 19, Issue 11, November 2003, Pages 585-588
.
•
Explain three main types of natural
selection.
.
Chapter 23: Population Genetics
(Microevolution)

Microevolution is a change in allele
frequencies or genotype frequencies in a
population over time

Genetic equilibrium in populations: the
Hardy-Weinberg theorem

Microevolution is deviation from HardyWeinberg equilibrium

Genetic variation must exist for natural
selection to occur
.
•
Discuss the importance of genetic
variation for evolution, and the concept
of neutral variation.
•
Give a hypothetical example of how
genetic variation that was once neutral
may no longer be neutral.
.
Genetic variation must exist for
natural selection to occur

the ultimate source of genetic variation is mutations

once variation exists, it can be affected by
independent assortment and genetic recombination
during gamete formation

consider the cross AaBb x AaBb – 9 different genotypes arise

this involves only 2 alleles at 2 loci; if there were 6 alleles possible
at just 5 loci, over 4 million genotypes are possible

thus, given that there are thousands of genes in an organism, and
that many alleles are possible at most of these loci, it becomes
clear that in nature there is great genetic variability
.
Genetic variation must exist for
natural selection to occur
the demonstrated presence of two or more alleles at
a given locus is genetic polymorphism


biologists have produced tools for studying the genetic
polymorphism of populations at the molecular level (RFLP, DNA
sequencing, etc.)

these tools can be used to demonstrate and study polymorphism in
populations without necessarily knowing the specific genes involved
.
Genetic variation must exist for
natural selection to occur
genetic variation can be
maintained by heterozygote
advantage or hybrid vigor


when either the homozygous dominant
or recessive is more suited to an
environment than the heterozygote, the
homozygous genotype will be more
likely to be fixed in the population

…but when heterozygous genotypes
have advantage over either of the
homozygous genotypes, variation tends
to increase in the population

example - sickle cell anemia and malaria
resistance
.
Genetic variation must exist for
natural selection to occur
genetic polymorphisms can be
maintained due to frequencydependent selection


there are cases where the frequency of a given
genotype affects the degree to which it is or isn't
selected in the population

example - predator/prey relationships, where
individuals with a rare phenotype may be ignored
by a predator, but as they become more abundant
the selective advantage decreases because the
predator is more likely to notice them
.
Genetic variation must exist for
natural selection to occur
much of the genetic variation in a
population will produce no selective
advantage – called neutral variation


the role of neutral variation in evolution is debated
today

remember that what is neutral in one context may
not be neutral in another context, so as
environments change some previously neutral
variation may be acted on by natural selection
.
•
Discuss the importance of genetic
variation for evolution, and the concept
of neutral variation.
•
Give a hypothetical example of how
genetic variation that was once neutral
may no longer be neutral.
.