Transcript Population Genetics.

The Modern Synthesis
• Populations are the units of evolution
• Natural selection plays an important role in
evolution, but is not the only factor
• Speciation is at the boundary between
microevolution and macroevolution
The Modern Synthesis
• Integrates ideas from many different fields:
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Comparative morphology & molecular biology
Taxonomy – relationships of taxa
Paleontology – study of fossils
Biogeography – distribution of species
Population genetics – Hardy-Weinberg Theorem
Darwin
Mendel
Population
genetics
• Mendelian genetics
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explains inheritance
patterns between parents
& their progeny
Evolution works first in
populations
(microevolution)
Then at the taxonomic
level (macroevolution)
• Mendel’s work rediscovered in early 1900s
• Seemed to be at odds with Darwin’s theory
* Darwin felt that natural selection must operate
on continuous (polygenic) traits
* Mendel showed the inheritance of discrete traits
• New theories dealing with populations,
described next, reconciled the two views
The genetics of populations
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Population = localized, interbreeding group of
individuals of one species
Population gene pool = all the alleles of all the
individuals in the population
Consider one locus,
* If you could count all alleles in all individuals,
* e.g. in a population of yellow- and green-seeded peas
There are YY, Yy and yy individuals
* Of all the alleles, a certain fraction are Y, say p is that
fraction
* Then the rest of the alleles are y; that fraction is q
Hardy-Weinberg Theorm
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In non-evolving populations with Mendelian transmission
of traits
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Frequencies of alleles & genotypes in an interbreeding
population remain the same for any number of
generations
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Certain assumptions (see last part of this lecture), the most
important of which is:
* All alleles in population have equal chance of uniting with all other
alleles (in other individuals) during sexual reproduction
Hardy Weinberg formulas
(for one locus)
p + q = 1.00
(by definition)
p2 + 2pq + q2 = 1.00 (this because of
Mendelian inheritance, expressed using laws of
probability – multiplication & addtion)
Where,
p = frequency of 1 allele
q = frequency of alternate allele
both expressed as decimal
fractions of a total of 1.00
and,
p2 = frequency of YY
2pq = frequency of Yy
q2 = frequency of yy
The Hardy-Weinberg equilibrium of allele frequencies in non-evolving
populations
This equilibrium will hold true no matter what the frequencies of the
alleles in the parent population. Try it with p = .24 and q = .76, for
example, in a population of 1000 peas.
Why is this theorem
important?
• Extends Mendelian genetics of individuals
to population scale (where evolution works)
• Shows that if Mendelian genetic processes
are working, variation is maintained at the
population level
Assumptions of HardyWeinberg equilibrium
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Very large population size (no genetic drift)
Random mating
No migration (no gene flow in or out)
No mutations (change in form of an allele –
the ultimate source of genetic change)
No natural selection
Microevolution
• Generation-to-generation change in allele
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frequencies in populations
The Hardy-Weinberg theory provides the
baseline
Microevolution occurs even if only a single
locus in a population changes
Causes of microevolution
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Genetic drift *
Natural selection *
Gene flow
Mutation
* The 2 most important factors
• All are departures from the conditions required
for the Hardy-Weinberg equilibrium
Genetic Drift
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Changes in gene frequencies due to chance events in
small populations
Hardy Weinberg assumes reproduction works
probabilistically on gene frequencies,
(p + q = 1)
Reproduction in small populations may not work
this way
Two similar situations lead to genetic drift
* Bottleneck effect
* Founder effect
Genetic drift example
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Wildflower population with a stable size of only 10 plants
Some alleles could easily be eliminated
Bottleneck Effect
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Large population drastically reduced by a disaster
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By chance, some survivor’s alleles may be over- or underrepresented, or some alleles may be eliminated
Endangered species
• Bottleneck incidents cause loss of
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some alleles from the gene pool
This reduces individual variation
and adaptability
Example: cheetah
* Genetic variation in wild
populations is extremely low
* Similar to highly inbred
lab mice!
Founder effect
• New population starts with a few genetically
unrepresentative of a larger source population.
* Extreme: single pregnant female or single seed
* More often larger sample, but small
• Genetic drift continues until the population is
large enough to minimize sampling errors
Natural selection
• Review: overpopulation, unequal reproduction,
survival of the fittest, microevolution
• Only factor that generally adapts a population to
its environment
• The other three factors may effect populations in
positive, negative, or neutral ways
Natural
selection
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Examples:
* Herbivory higher for white flowered plants than red
flowered – red-flowered alleles (R) increase
* Pollinators attracted by white flowers rather than red
flowers – white flower alleles (r) increase.
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Natural selection accumulates and maintains
favorable genotypes
Gene flow
• Genetic exchange due to migration of alleles
* Fertile individuals
* Gametes or spores
• Example:
* Wildflower population has white flowered plants only
* Pollen (with r alleles only) could be carried to another
nearby population that lacks the allele.
• Gene flow tends to reduce differences between
populations
Mutation
• Change in DNA
* Rare and random
* More likely to be harmful than beneficial
• Only mutations in cell lines that produce
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gametes can be passed along to offspring
One mutation does not effect a large population
in a single generation
Very important to evolution over the long term
* The only source of new alleles
* Other causes of microevolution redistribute
mutations
Phenotypic Variation
• Combination of inheritable and non•
heritable traits
Phenotype is the
cumulative product of:
* Inherited genotype
* Environmental influences
• Only the genetic component
can be selected
Same genes, different seasons
Genotypic variation
• Expressed in these ways:
* Quantitative (continuous – multilocus?)
• ex. plant height
* Discrete (single locus?)
• ex. flower color
• Measured by:
* Gene diversity - % heterozygosity
• Human – 14%
* DNA base diversity
• Human – 0.1 %
Geographic variation
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Between or within
populations
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Natural selection
working in response
to differences in
environment
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Genetic drift
Geographical distribution of
variation in Yarrow plants
Variation in
isolated
populations
• Discretely separated
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populations exhibit
discrete differences
Example:
karyotypes of mice
House mice on Madiera
What keeps mutations?
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Diploidy – masks recessive alleles
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Hardy-Weinberg Equilibrium says that, without
natural selection, gene frequencies remain the
same
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A balance of recessive alleles can be kept even
without Hardy-Weinberg
* Heterozygote advantage
* Frequency-dependent selection
Heterozygote advantage
• Sickle-cell allele
* Homozygous
recessives
unhealthy
* Heterozygotes
protected from
malaria
Sickle-cell allele and malaria
Frequency-dependent
selection
• Common
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morphs of
snails more
likely to die
from parasites
Rare morph
less likely
Infection of snails by parasitic worms
Neutral variation
• Have negligible impact on reproductive success
* Not selected by natural selection
* But their gene frequencies can change
• Hard to assess
• Some neutral alleles will increase and others will
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decrease by the chance effects of genetic drift
May provide basis for future evolution
How natural selection acts on
allele frequency
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Directional
Diversifying
Stabilizing
Frequency of individuals showing a range of phenotype
Directional
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Phenotype moves
toward one end of the
range
Ex. Beak size in
Galapago ground finch
* During dry years big
beaks advantageous and
increase in frequency
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Stabilizing selection is
similar
Diversifying
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Selects for two ends of
a range
Can result in balanced
polymorphism
Ex. Beak type in blackbellied seedcrackers
* Two types of seeds –
hard and soft
* Intermediate billed birds
inefficient at feeding on
either type
Macroevolution
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The evolution of species and larger taxa
Evolutionary theory must also explain macroevolution
Speciation is the keystone process in the origination of
diversity of higher taxa
Galapagos tortoise
Figure 1.17
Galapagos
finches
“Species”
• Latin meaning “kind” or “appearance”
• Traditionally distinguished by morphological
differences
• Today distinguished in addition by differences
in body function, biochemistry, behavior, and
genetic makeup
“Biological species”
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Concept emphasizes
reproductive isolation
Similarity between species
Diversity within species
How are biological species
isolated?
• Prezygotic barriers – impede mating
* habitat isolation, behavioral isolation, temporal
isolation, mechanical isolation, and gametic
isolation
• Postzygotic barriers – prevent development
* reduced hybrid viability, reduced hybrid
fertility, and hybrid breakdown
Alternative species concepts
• Ecological species defined in terms of its
ecological niche
• Pluralistic species defined by combination of
reproductive isolation and ecological niche
• Morphological species defined by structure
• Genealogical species defined as a set of
organisms with a common and unique genetic
history as shown by molecular patterns
Speciation
• Allopatric speciation geographic separation
restricts gene flow
• Sympatric speciation biological factors
reduce gene flow
Fig. 24.6
Allopatric speciation
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Geological processes that isolate populations
* Mountain ranges, glaciers, land bridges, or splintering of lakes
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Colonization of new, geographically remote areas
How significant the
barrier must be
depends on the
A. harrisi South Rim A. leucurus North Rim
species
Increases in small
and isolated populations
2 species of antelope squirrel
near Grand Canyon
Adaptive
Radiation
• The evolution of
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many diverselyadapted species from
a common ancestor
Seen in some island
chains (Hawaii,
Galapagos)
Sympatric speciation
• Reproductive barriers must evolve between
sympatric populations
• In plants, sympatric speciation often results
from polyploidy
• In animals, sympatric speciation may result
from gene-based shifts in habitat or mate
preference
Transformation of one
species into another
Creation of one or more new species
from a “parent” species
Promotes biological diversity by
increasing the number of species
Tempo of speciation
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Gradualism
* Traditional view
* Not supported by
fossil evidence
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Punctuated
equilibrium
* Rapid appearance
* Slow to no
change later
Evolution of
complex
structures
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Continued
modification of
older structures
Often fossil
evidence of
sequence not
complete
Limpet
Slit-shell
Nautilus
Murex, snail
Squid
Range of eye complexity in mollusks
Mass extinctions – Periodic events in which large
numbers of taxa go extinct simultaneously
Permian Extinction – 90%
of marine invertebrate
species go extinct
Cretaceous Extinction – less
dramatic, but killed off
remaining dinosaurs
Asteroid hypothesis
Cambrian Explosion (543-510 MYA)
Mass extinctions and adaptive radiation have affected the
organisms we find on earth today.
Evolution does not have goals
Evolution does not have direction
or goals
Humans as the
pinnacle of
evolution?
Remember, evolution is the
touchstone of biology
• a criterion for determining the quality or
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genuineness of a thing
a fundamental or quintessential part or feature