Download Chapter 23: The Evolution of Populations

Chapter 23: The Evolution of
Populations
Evolution
Scientific theory
Supported
by all current evidence
New information added

ex. Natural selection no longer only mechanism
Microevolution
Change in population’s genetics over
generations
Mendel’s research made sense of natural
selection
Population genetics


how populations change genetically over
time
Combines Darwin & Mendel

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Population- localized interbreeding individuals
that produce fertile offspring
Gene pool- all alleles for all individuals in the
population
Modern synthesis

Theory of evolution integrating statistics,
genetics, biogeography, paleontology,
botany, etc.
Major factors for evolutionary
change

Natural selection

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Differential success based on suitable environmental
trait
Acts on individuals evolutionary impact seen
in populations
Genetic drift

Chance fluctuations in allele frequencies

Bottleneck effect

Founder effect

Survivors after a catastrophic event pass on only possible
alleles
Isolated individuals establish a new gene pool
Gene flow

Population gains/loses alleles through
immigration/emigration
Heritable variations which may lead
to evolution

Mutation
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Point mutations (ex. Sickle cell allele, HIV resistance)
Alteration in gene number or sequence
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Translocation
duplication
Sexual recombination

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Gene shuffling by meiosis & fertilization
Variation may aid in disease resistance/co-evolution
with another species
Closer look at variation

Phenotypic polymorphism
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Genetic polymorphism

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Discrete characters= “either or traits” ex. Blue or
brown eyes
Quantitative characters= traits on a continuum ex.
height
Geographic variations

Environment can cause one allele to be
beneficial in one locale but a different allele to
be beneficial in another locale
Preservation of genetic variation

Unfavorable variations are not eliminated
completely due to:
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Diploidy
Balancing selection
Heterozygote advantage
Frequency dependant selection
Neutral variation
Sexual selection
Intrasexual selection
 Intersexual selection

A closer look at Natural selection

Fitness

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Contribution an individual makes to the gene pool
relative to others
Fitness acts on phenotypes (not genotypes)
with the environment & reproductive success
playing key roles
Types:

Directional
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Disruptive


Shift in phenotypes toward one extreme ex. Giraffe neck
length
Intermediate has low fitness ex. Barn swallow tail length
Stabilizing

Intermediate phenotype has high fitness ex. Birth weight
Natural selection can not create
perfect organisms

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Evolution can not scrap ancestral anatomy
Adaptations are often compromises
Chance & natural selection interact
Selection can only edit existing variations
Hardy-Weinberg Theorem


Determines when evolution is not
occurring; alleles are constant in the gene
pool
Hardy-Weinberg equilibrium

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Alleles retain the same frequency across
generations
p=frequency of one allele for a locus in the
gene pool
q=frequency of second allele for a locus in
the gene pool
Equilibrium equation

p2 +2pq=q2 = 1
Conditions for H-W equilibrium
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large population
No gene flow
No mutations
Random mating
No natural selection
Natural populations are rarely in true
H-W equilibrium but rate of evolution
may be so slow it appears to be close
to equilibrium
Ex. PKU allele