Population Genetics:
How do Genes Move through Time
and Space?
How Do We Characterize
Variation?
• Variation can be smooth or
discontinuous.
– Two views of biology
• “Naturalists”
–Supported Darwin’s ideas.
• “Experimentalists”
–-Supported Mendelian Ideas.
Naturalists’ Viewpoint
• Saw most traits in a population exhibited a
continuum of forms.
• Believed the ability to survive and
reproduce might depend on having traits
that fall within some range of a spectrum.
• Believe that traits within populations
change or evolve as features of
environmental change.
Experimentalists’ Viewpoint
• Rejected naturalists’ view of evolution.
• Viewed variation as a sudden change due to
mutations.
• Maintained evolution progressed by leap
and bounds by sudden random mutations.
• Evolution could not be a gradual process
Stalemate Broken
• Herman Nilsson-Ehle
(1909)
– Using wheat
kernels proved that
traits that appear in
populations as a
continuous
spectrum of forms
with a genetic basis.
Stalemate Broken
• Nilsson-Ehle showed a
cross between
– true-breeding red- and
white-kerneled plants
– produced all light redkerneled plants
• Cross between light
red-kerneled plants
yielded 7 categories of
color.
Frequency Diagrams Illustrate
Variation
• Useful graphing tool
for illustrating
variation in population
• X-axis:range of
different forms that a
trait can exhibit
• Y-axis number of
individuals in
population that exhibit
each form of the trait
Frequency Diagram of Human
Height
• Graphing human
height creates a
bell-shaped curve.
• So many different
forms that the
categories blend.
Frequency Diagram of Wheat
Kernels
• Frequency diagrams of
Nilsson-Ehle F2
variation.
– Plants grown in
controlled laboratory
vs. those grown in the
wild
• Proved that genes can
be responsible for
seven different forms.
Continuous Variation is Determined
by Two or More Genes
• Polygenic
(quantitative) traits:
– Influenced by two or
more genes residing
at different loci on the
same or on different
chromosomes.
How Do Populations Differ?
• Brachydactyly
– Human trait in which the terminal bones of the fingers
and toes do not grow their normal length
Populations are Collections of
Alleles
• Populations:
– Group of interbreeding organisms of the same species
that exist together in both time and space.
• Gene pools:
– All of the alleles found in the population.
• Think of a beanbag
– Beans are analogous to alleles and the entire bag of
beans is the population’s gene pool.
Alleles Occur at Certain Frequencies
• Example: gene pool for sickle cell anemia
– Possible alleles humans could have: HBA or HBS
– If we let p = HBA and q = HBS, the sum should
equal 100 % of the alleles in the gene pool.
– This could be rewritten
p+q=1
Hardy Weinberg Principle
• Makes it possible to calculate allele frequencies (p, q)
based on phenotypes.
• Can calculate the sum of the genotypes:
p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant genotype
2pq = frequency of heterozygous genotype
q2 = frequency of homozygous recessive genotype
Hardy Weinberg Principle
• States allele frequencies for a population will
remain the same from generation to generation
as long as specific conditions are met.
• Populations in which p and q do not change are
said to be in a genetic equilibrium.
Hardy Weinberg Principle
• Required conditions for genetic equilibrium:
–
–
–
–
1. Populations are large.
2. Individuals mate randomly.
3. Populations do not gain or lose individuals.
4. Natural selection is not occurring in the
population.
– 5. Mutation is not occurring at a high enough rate to
influence genetic variation.
Hardy Weinberg Principle
• Power of this principle:
– Allows us
• To calculate what would happen if natural selection were
not occurring
• To compare what does happen in the real world
• Also allows us to calculate the proportions of
individuals in the population that have each of
the three possible genotypes.
Microevolution
• Definition: Change in allele frequencies in a
gene pool over time
• Factors that contribute to microevolution:
– Natural selection
– Genetic Drift
• Founder effect
• Bottleneck effect
– Mutation
– Gene flow
Natural Selection
• Example of natural
selection: industrial
melanism
– Rapid shift in the color
of peppered moth
populations during the
19th century in England
Natural Selection
• Color of moth due to
pair of alleles:
– Carbonaria = M
– Speckled = m
• Before industrialism:
– Speckled moths had
advantage because
their coloring served to
camouflage them
Natural Selection
• After industrialism:
– Lichens on the trees
died.
– Made speckled moths
visible.
– Darker moths were
more likely to survive.
– Resulted in change in
the allele frequency of
the population.
Natural Selection
• Heterozygote advantage:
– Tendency of red blood
cells to sickle makes
these cell resistant to
penetration by the
parasite that causes
malaria.
– Heterozygotes can
survive disease and have
immunity against malaria.
Types of Selection
• Directional selection
– Selection that acts on
one extreme of the
range of variation for a
particular
characteristic.
– Example: Frog tongue
length.
Types of Selection
• Stabilizing selection
– Selection that operates
against the extremes in
the distribution of a
particular trait in a
population.
– Example: human birth
weight
Types of Selection
• Disruptive selection
– Selection that favors
the extremes and
disfavors the middle
range of particular
traits in a population.
– Example: bird beak
size
Some Changes in Allelic Frequency
Are Random
• Genetic Drift:
– Random change in allelic frequencies as a
result of chance alone.
– Seen in small populations
– Two types:
• Founder effect
• Bottlenecks
– Often referred to as neutral selection
• Occurs independent of natural selection
Founder effect
• Eventual genetic
difference between an
isolated offshoot
population and the
original population
from which it came.
– Example:
Pennsylvania Amish,
settlers of Tristan da
Cunha
Bottlenecks
• A drastic decrease in
the size of a
population with a
resulting decrease in
the genetic variability
within a population.
– Usually due to a
catastrophe (drought,
hunting, flood etc.)
Mutation
• A permanent change in the genetic material
of a cell or organism.
– Can be inherited from generation to generation.
• Introduces new alleles into the population
• Effects can be lethal, neutral or
advantageous in a population.
Gene Flow
• A shift in the allelic
frequencies within a
population and
between populations
resulting from
migration.
– Either immigration or
emigration
– Example: DDT and
mosquitoes