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Population genetics
Evolution in more detail
Populations
• Group of individuals of the same species in a given area
• All of the genes in a population = their gene pool
– All alleles at all loci
Outline
• 1. Modern Evolutionary Synthesis
• 2. Individuals live or die, but
POPULATIONS evolve
• 3. Mechanisms of Evolution
Modern Evolutionary Synthesis
• In Darwin’s time, only one person had figured out how inheritance
works
• Mendel’s work was not widely known
• Darwin knew less about the mechanisms of genetics than you do!
• This gap caused difficulties for understanding Evolution by N.S.
• In particular, if you assume blending inheritance, N.S. seems not
to work.
?
!!!
Blending Inheritance Example
Large population of 20mph bunnies
Lesson:
• If inheritance happens by blending,
rare variants will get diluted over
time - useful variation cannot be
preserved for long.
Particulate (Mendelian)
inheritance
x
30mph bunny
20mph bunny
What happens if rare variant is due to a new allele?
Common allele:
b20
Rare allele:
b30 (assume it’s dominant for this example)
Lesson
• 30 mph bunnies can become more common
if they survive or reproduce better
(without losing speed through blending).
• Mendelian inheritance allows N.S. to work.
Now you’re talking!
Modern Evolutionary Synthesis
or “Neodarwinism”
Darwin gathered extensive evidence on geographical
patterns of biodiversity, on variation in domestic and
wild animals, and on artificial selection. But it’s a big
planet out there.
In the 50 years after the publication of the Origin of
Species, many paleontologists, biogeographers,
population geneticists and others made more
intensive studies. Those studies, together with the
rediscovery of Mendel’s papers on genetics, led to a
new, more complete understanding of evolution
Modern Evolutionary Synthesis
or “Neodarwinism,” summarized
Darwin, Wallace
+ Mendel
+ Wright, Fisher
+ Mayr, others
(Natural Selection)
(Genetics)
(Population Genetics)
(Biogeography, Paleontology)
= Modern Evolutionary Synthesis - a more complete
understanding of how evolution proceeds in the wild.
Macro- vs. Micro- Evolution
• Macroevolution - change that results in the
formation of new species, new taxonomic
groups, evolutionary trends, adaptive
radiation, and mass extinction.
• Microevolution: the generation-togeneration change in the frequency of
alleles in a population
– Evolution at its smallest scale
– The core definition of evolution
 Microevolutionary changes give rise to
macroevolution
Mechanisms of Evolution:
How do allele frequencies change?
‘I am convinced that Natural Selection
has been the most important, but not
the exclusive, means of modification’
- Charles Darwin, On the Origin of Species
Mechanisms of Evolution
• Why does the allele frequency in a
population change?
–
–
–
–
Genetic drift
Natural selection
Gene flow
Mutation
Mech. of Evol. (1) Genetic Drift
• Changes in a population’s allele frequencies due
to chance
– Is genetic drift a form of evolution?
• Drift can cause rapid change in small populations
• A problem when populations get too small
• Related issues with small populations
– Bottleneck effect
– Founder effect
Bottleneck effect
• Survivors after large reductions in populations do not
represent the gene pool as well as before the reduction
• Reduced genetic variability + drift lead to changes in allele
frequencies that are not necessarily adaptive
Fig. 23.5
Cheetah
Founder effect
• When a few individuals colonize a new area, they
represent the entire gene pool
• Reduced genetic variability + drift lead to
changes in allele frequencies that are not
necessarily adaptive
Polydactyly due to Ellis-van
Creveld syndrome
Mech. Of Evol. (2)
Natural Selection
• Natural selection: differential success in
survival and reproduction
• N.S. leads to populations becoming better
adapted to their environments (adaptation)
• N.S. works most powerfully in large populations
(effect of drift is small, and changes due to
N.S. are preserved)
Fitness
• No, not the body-builder kind
• Darwinian fitness: an individual’s
contribution to the gene pool of the next
generation compared to the contribution by
others
• This is what biologists really mean when they
talk about ‘survival of the fittest’
Modes of selection
• Three ways that natural selection
affects the frequency of heritable
traits
– Directional selection
– Diversifying (disruptive) selection
– Stabilizing selection
Modes of selection (2)
Fig. 23.12
It is Possible to
Measure Natural
Selection in the Wild.
Sexual selection
a form of Natural Selection
• Darwin’s term
• Why is there sexual
dimorphism?
If natural selection is so powerful,
how come we’re not perfect already?
• Evolution is limited by history or
ancestry
• Adaptations are often compromises
• Not all evolution is adaptive
• Selection can only edit existing
variation
Mechanisms of Evol. (3): Mutation
• Mutations are the origin
of all differences
between alleles
• But mutations are rare
• So mutations must still
spread by drift or
selection if they are going
to impact allele
frequencies in a
population
Sickle cell anemia is
the result of a single
point mutation
Mech. Of Evol. (4): Gene Flow
• Migration of
individuals can
change allele
frequencies in a
population
Mechanisms of Evolution:
Summary
• Drift and Natural Selection seem to
be the most powerful forces in
natural populations
• Drift is important in very small
populations
• Natural Selection is important in all
other populations, and is especially
powerful over long time periods.
Hardy-Weinberg theorem
• What would the gene pool (and thus the gene
frequency) of a NON-evolving population look
like?
• In 1908, Hardy and Weinberg independently
developed this idea
• The frequencies of alleles in gene pool remain
constant over generations unless they are acted
upon by things other than segregation in meiosis
and random fertilization
– In a state of Hardy-Weinberg equilibrium
Allele frequency in a gene pool
• Evolution is measured as changes in gene
frequency
Fig. 23.3a
Allele frequency in a non-evolving
gene pool
• Using the same example as before, what do
the offspring look like?
Fig. 23.3b
Hardy-Weinberg Equation
•
•
•
•
p2 + 2pq + q2 = 1
p = frequency of allele 1
q = frequency of allele 2
Using this equation, you
can calculate frequencies
of alleles in a non-evolving
gene pool
Hardy-Weinberg
assumptions
•
•
•
•
•
Very large population size
No migration
No mutations
Random mating
No natural selection
• How realistic are these conditions?
Assumptions of HWE
•
•
•
•
•
•
1. Large population size
2. No migration
3. No mutation
4. Random mating
5. No selection
Deviation from HW assumptions usually means
allele frequencies are changing between
generations - Evolution is happening
• HWE is more interesting when absent!