Microevolution
Individuals are selected; populations
evolve
Population Genetics
Genetic Context of Evolution
• Gene Pool: the various alleles at all the gene
loci in all individuals within a population
• Allele frequencies: the proportion (%) of each
allele at the same loci in the entire gene pool
of a population
– A change in allele frequencies of a population
from one generation to the next quantifies
evolution
Population Genetics
Ex. Snapdragon flower color
CR = Red, CW = White
Modeling a population of 100 snapdragons results
in 25CRCR, 50CRCW, and 25 CWCW
What are the allele frequencies for flower color in
this population?
Assuming random mating can sexual reproduction
alone change allele frequencies?
A punnett square describes the allelic frequencies
for this population
Population Genetics
•Sexual Reproduction alone cannot
change allele frequencies
•The dominant allele doesn’t always
increase from one generation to the
next (Allele dominance does not
cause an allele to become more
common in a gene pool)
Hardy-Weinburg Equilibrium
• A binomial expression used to calculate genotypic
and allele frequencies of a population
Genotypic frequency: p2 + 2pq + q2 = 1
Allele Frequency: p + q = 1
p2 = % homozygous dominant
q2 = % homozygous recessive
2pq = heterozygous
p = % of dominant allele
q = % of recessive allele
Example
In a snapdragon population’s gene pool 80% of
the alleles are CR . Assuming Hardy-Wienburg
equilibrium, calculate the frequency of the CW
allele, how many homozygous red,
Heterozygous pink, and homozygous white
flowers in this population.
Conditions
The Hardy-Weinburg Equilibrium will only remain if:
1)
2)
3)
4)
5)
There are no mutations
There is no migration
There is random mating
The population is large with no chance events
There is no selection
These conditions are rarely, if ever, met in natural
populations so why do we use the HardyWeinburg equation?
Departure from these conditions results in
evolution
1.
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•
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What changes the genetic equilibrium
(allele frequency) of a population?
Mutations: raw material for
variation (reason for diff. alleles
other than +)
Might not affect phenotype
directly; can be detrimental
some mutations neither “good”
or “bad” just different
phenotype.
Ex. : Daphnia mutation allows to
live 25oC to 30oC when normal
to 20oC
Have to be recombined to be
passed on
Rare in animals and plant
(1/100,000 genes per
generation), but much more
common in microorgs. (HIV gen.
2 days = drug resistance daily
2.
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Gene Flow: Migration of
individuals between populations
When individuals move between
populations, they bring the
alleles they carry in their genome
Can be constant.
– These two caribou
populations in the Yukon are
not totally isolated—they
sometimes share the same
area. Nonetheless, members
of either population are more
likely to breed with members
of their own population than
with members of the other
population.
Continued flow makes gene pools
more similar and eventually
decreases variability between
populations (therefore continued
gene flow can prevent speciation)
3.
Non-random Mating: when mates are chosen by their phenotype or
genotype
A. Assortative Mating: individuals mate with those that have the same
phenotype, behavior, or biochemical signal
•
Causes populations to subdivide into sub-species or phenotypic
classes
B. Sexual Selection: Mates are chosen by opposite gender. males compete
for reproductive rights of females or visa versa
•
Ex. Peacock tail or elk antlers
•
What’s being selected?
4. Genetic Drift: Change in allele frequencies of a
population due to chance (not phenotypes)
•
The smaller the population the greater the effect
•
•
Ex. Flipping of a coin 1000x should get closer to a 1:1 ratio
of heads to tails than flipping a coin only 10x
Totally random process: does not produce the same
results from population to population
Patterns of Genetic Drift
A. Bottleneck Effect:
the prevention of
the majority of
genotypes from
reproducing (only a
few will produce
offspring)
•
Caused by natural
disasters,
overharvesting,
habitat loss,
epidemics (random)
Patterns of Genetic Drift
B. Founder Effect: Rare alleles occur at a higher
frequency when compared to other larger
populations
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•
Isolated populations rely on the alleles of the
founding individuals (random)
Ex. Amish: 1 in 14 individuals have recessive
allele for dwarfism and polydactalism when US
population is 1 in 1000
5. Natural Selection: Adaptation of a
population to the abiotic and biotic factors in
an ecosystem
Requirements:
1) Variation and Inheritance are genetic (behaviors)
2) Differential adaptiveness: some variations affect
success of organisms to the environment
3) Differential Reproduction: better adapted, more likely
to reproduce
Types of Selection
• Selection can act on traits that are inherited by
polygenic inheritance (range of phenotypes)
– The different types affect what happens to this
distribution
A. Directional Selection: 1 extreme is favored for
environment, making the curve shift in that direction
•
Ex. Bacterial resistance to antibiotics, insects resistant to
pesticides, body size of modern horse got bigger as
environment changes from forest to grassland.
B. Stabilizing Selection: When intermediate is favored.
(average) = heterozygote
•
Ex. : Birth weight of human infants are more around the
average than before
C. Disruptive Selection: 2 or more extremes are favored
•
Ex. British land snails
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Habitat: low vegetation  Forest
In low vegetation the banded shells are favored
In the forest the dark shells are favored
 Can lead to speciation
Maintenance of Variation
• Variation is important to species for adapting when
ecosystems change
Promote Variation:
• Mutation = with recombination & fertilization
• Gene flow = small isolated populations (most
homozygotes) could be source of new alleles
• Disruptive Selection = polymorphisms
• Genetic Drift (?)
• Diploidy and heterozygous advantage = (stabilizing
selection) in diploid organisms the heterozygote is the
protector of recessive alleles (without they could be
weeded out) Slows down Evolution
• Ex. : Sickle cell disease
Decrease Variation:
• Gene flow = many populations with lots of
migration
• Non-random mating = Assortative and sexual
selection
• Genetic Drift = bottleneck and founder effects
• Directional selection (phenotypes)