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Today:
•Wrapping up Darwin
•Population Genetics
• The Black Death…
MOLECULAR
HOMOLOGY:
Some HOMOLOGIES are MISLEADING…
as they result from CONVERGENT EVOLUTION.
EVOLUTION.
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BIOGEOGRAPHY,
BIOGEOGRAPHY or the geographic distribution of
species, provides many clues about their evolution.
Islands are especially good for studying
biogeography as they contain many
isolated, endemic species.
The FOSSIL RECORD provides the last major
line of evidence in support of Darwin’s ideas.
The Darwinian
view of life
predicts the
existence of
evolutionary
transitions.
Bailosaurus
Thus today, Darwin’s views are supported by
multiple, independent lines of evidence.
Evolutionary patterns of Homology that match
patterns in space (Biogeography
Biogeography) and time (the
Fossil Record).
Record
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But originally, Darwin’s mode of natural
selection was problematic.
Paradox: Chance Variations in a population
reproducing through precise transmission of
characteristics.
POPULATION GENETICS resolves the paradox.
It emphasizes the genetic variation within populations and
acknowledges the importance of QUANTITATIVE
CHARACTERS.
Discoveries in paleontology, biogeography, population
genetics, combine in the MODERN SYNTHESIS
•populations as the units of evolution
•natural selection as most important mechanism
•gradualism to explain large changes evolving as a series
of small changes over time.
Population: localized group of individuals
belonging to the same species.
•Every species distributed over range, but
composed of multiple populations.
•Populations may be more
or less isolated.
•The total aggregate of genes in a population at
any one time is its GENE POOL (all alleles at all
loci)
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In a nonevolving population, we can relate the
allele and genotype frequencies using the HardyWeinberg Theorem:
The frequencies of alleles and genotypes in a
population’s gene pool remain constant over
generations unless acted upon by agents other
than Mendelian segregation and recombination
of alleles.
What are the other “agents”?
Within a given POPULATION, we can calculate
the allele frequency and the genotype frequency.
Applying Hardy-Weinberg, the next generation will be…
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This population is said to be in HardyHardy-Weinberg
EQUILIBRIUM.
EQUILIBRIUM. Its allele frequencies could
remain constant forever!
It can be described by the Hardy-Weinberg Equation:
p2
+
Frequency
of RR
Genotype
2pq +
Frequency
of Rr plus
rR
q2
=
1
Frequency
of rr
genotype
Note that if p + q = 1, then p = 1 – q and q = 1 - p
Using HardyHardy-Weinberg: An Example
Phenylketonuria (PKU):
„
„
disease caused by a
recessive allele.
frequency in the U.S. is
1:10,000 (0.0001)
If the frequency of PKU is 0.0001, then
q2 = 0.0001,
0.0001, and q = 0.01
and
If p = 1 - q, then p = 0.99
We can then determine the frequency of carriers
(heterozygotes)
2pq = 2 x 0.99 x 0.01 = 0.0198 (~2%)
But only if the Hardy-Weinberg assumptions are true…
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The Hardy-Weinberg Assumptions:
1. Very large population size
2. No migration (Gene Flow)
3. No net mutations
4. Random mating
5. No natural selection.
How often do you expect this to occur??
Microevolution
If the HardyHardy-Weinberg conditions aren’t met, then
there may be generationgeneration-toto-generation
change in a population’s frequencies of
alleles
= MICROEVOLUTION!
Microevolution:
• generation-to-generation
change in a population’s
frequencies of alleles
• Below the species level!
Four Primary Causes of Microevolution:
1. Genetic Drift
2. Natural Selection
3. Gene Flow
4. Mutation
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1. Genetic Drift: a change in a population’s
allele frequencies due to CHANCE.
Genetic Drift is most effective in small
populations!
Small Populations May Experience…
The Bottleneck EffectDisasters randomly reduce the population size. The
surviving population may not be representative of the
original population’s gene pool (a version of genetic drift)
Small Populations May Also Experience…
The Founder EffectEffect- a few individuals from a
larger population colonize an isolated new habitat.
Genetic drift occurring in this new colony is known
as the founder effect (a version of genetic drift)
Polydactyly -- extra fingers or sometimes
toes -- is one symptom of Ellis-van
Creveld syndrome. The syndrome is
commonly found among the Old Order
Amish of Pennsylvania, a population
that experiences the "founder effect."
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3. Gene Flow:
Populations may gain
or lose alleles due to
the migration of fertile
individuals or gametes
between populations.
Gene Flow acts in opposition to Genetic
Drift!
4. Mutation:
•
A change in an organism’s DNA can change
the gene pool of a population
•
Important as the original source of Genetic
Variation!
2.Natural
2.Natural Selection:
Natural selection sorts through variations,
increasing the frequencies of certain genotypes
and “fitting” organisms to their environment.
It is the entire organism that is “sorted”, as
natural selection acts on phenotype.
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Types of Selection: Examples??
Diversifying Selection: An Example
The female Papilio butterflies exist in several morphs, some of which
resemble two other species which are noxious. Intermediate butterflies
do not gain the advantage of mimicry and thus are more likely to be
preyed upon.
Looking at GENETIC VARIATION, the
Foundation of Natural Selection:
•Variation occurs both within and between
populations
•Not all variations are heritable! (Phenotype)
Seasonal variations of Arachnia levana, the map butterfly. Individuals that emerge in spring are orange and
brown; individuals that emerge in late summer are black and white.
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„
„
Variation may consist of QUANITATIVE or
DISCRETE Characters
Quantitative variation usually indicates polygenic
inheritance
If two + discrete characters are present in a
population, it is POLYMORPHIC.
Genetic Diversity may be measured most
directly as GENE DIVERSITY or
NUCLEOTIDE DIVERSITY
Gene Diversity: the percent of loci
that are heterozygous. Fruit flies
typically have ~14% gene diversity.
Nucleotide Diversity: percent difference of the
nucleotide sequence. Fruit flies typically have ~1%
sequence diversity. (humans = ~0.1%)
GEOGRAPHIC VARIATION may also be
observed BOTH within and between
populations.
Cline
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Geographic
Variation may
also occur in
DISCRETE
CHARACTERS
Variation is generated by MUTATION
and SEXUAL RECOMBINATION
•Many single point mutations have little or no effect
•Those that do have serious effects of the protein are
usually harmful
•Thus random change rarely improves the genome,
especially in a stable environment
•However, organisms that reproduce rapidly, or
sloppily (i.e. HIV) can achieve massive variation
through mutation.
Variation is generated by MUTATION
and SEXUAL RECOMBINATION
•Sexual reproduction alters the combination of
alleles an individual receives
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Preserving Variation:
1. Diploidy- “hides” genetic variation from selection
2. Balanced Polymorphism- natural selection favoring
2+ phenotypes
1. Heterozygote advantage- Sickle Cell Disease
2. Frequency-dependent selection- survival and
reproduction of any one morph declines if that
form becomes too common.
3. Neutral Variation- no known selective
advantage “shields” from natural selection.
Frequency-dependent selection-survival and
reproduction of any one morph declines if that
form becomes too common.
Sexual selection
Darwin: "...the advantage which certain
individuals have over others of the same sex
and species solely in respect of reproduction“
or
Selection that arises from
differential mating
success
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Sexual Selection frequently involves a TRADEOFF between survival and attracting mates.
Sexual selection arises because one sex is a
limiting resource for another.
Two Types of Sexual Selection:
1. INTRASEXUAL selection
• Well documented
• Arises (typically) from competition
among males for access to females or
mating sites
• Often results in larger body sizes or
weaponry that may be advantageous in
battle
Two Types of Sexual Selection:
2. INTERSEXUAL SELECTION (Mate Choice)
Females are usually
more selective in
mate choice than
males (why?)
More Problematic!
A male and female Bowerbird. Males build ornate nests to compete for mates.
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Female preference can be a complex trait!
Preference may serve to increase her
fitness (examples?)
Fitness may not be affected by her
preference (if not, why should she have a
preference?)
Three Hypotheses to Explain Intersexual
Selection:
1.
2.
3.
Direct BenefitsBenefits- increased food, protection, etc
Indirect BenefitsBenefits- good genes?
Sensory BiasBias- artifact of other selective force
If females are generally the "choosy" ones,
how might you explain some of the
exceptions (like sea horses) where males
are the ones who are choosy?
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Reminders: Natural Selection is limited!
„
„
„
„
Evolution limited by historical
constraints
Adaptations are often
compromises
Not all evolution is adaptive
(Chance!)
Selection can only sort existing
variations
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