Download The Evolution of Populations

The Evolution of
Populations
Chapter 23
Topics

I. Hardy Weinberg Theorem
 Introduction
 The
theorem
 Computing allelic frequencies
 Microevolution
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II. Genetic Variation
 Variation
within a population
 Variation between populations
 Mutations/sexual recombination
 Balanced polymorphism
 No perfection
“Populations evolve, individuals
do not”
Variation in a Natural Population
Introduction
Gene Pool all the alleles of all the
individuals of a population
 Genetic Structure the allelic
composition of the population
 Fixed Alleles same allele for everyone,
no variation in a population
 Population Genetics how populations
change over time
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Modern Synthesis Theory
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Darwin found a mechanism of change
Mendel a model particulate hypothesis
Modern Synthesis Theory brought all these
together evolution, ecology, population genetics,&
taxonomy
It emphasizes:
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the importance of populations as the units of evolution,
the central role of natural selection as the most important
mechanism of evolution, and
the idea of gradualism to explain how large changes can
evolve as an accumulation of small changes over long periods
of time
Hardy & Weinberg two mathematicians developed
theorems separately at the same time
Hardy Weinberg Theorem
 The
frequencies of alleles in a
population’s gene pool remain
constant over the generations
(unless acted upon by agents other
than sexual recombination)
Describes a non-evolving population
 Makes 5 assumptions
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Assumptions
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No mutations
No emigration or immigration
Large breeding population
Random mating
All alleles are equally viable (no
selection
Computing Allelic Frequencies
Population geneticists use p to represent
the frequency of one allele and q to
represent the frequency of the other
allele
 p frequency of the dominant allele
 q frequency of the recessive allele
 The combined frequencies must add to
100%; therefore p + q = 1
 If p + q = 1, then p = 1 - q and q = 1 - p.

In situations in which there are three or
more alleles AA , Aa and aa
 p= frequency of A
 q = frequency of a
 pq = frequency of Aa
p +q = 1
 Square the equation p2 +2pq +q2 = 1
 Ex 1000 /360 recessive Dominant?
Heterozygote?
 Always find q
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q2 = 360/1000 =.36,q=.6
p + q = 1 , p = 1- q =.4
p2 = .16 = 160
2pq = 2(.4)(.6) =.48 =480
Microevolution
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The Hardy-Weinberg theory provides a
baseline against which we can compare the
allele and genotype frequencies of an evolving
population
Microevolutiongeneration-to-generation
change in a population’s frequencies of alleles
Causes of Microevolution
 Mutation
 Gene Flow
 Genetic Drift
 Founder Effect
 Bottleneck Effect
 Non random mating
 Natural selection
Mutation
A change in an organism’s DNA
 A new mutation that is transmitted in
gametes can immediately change the
gene pool of a population by substituting
the mutated allele for the older allele.
 Is vital to evolution because it is the
only force that generates new alleles
 Is the original source of genetic
variation that serves as the raw
material for natural selection
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Gene Flow
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A genetic exchange due to migration of fertile
individuals or gametes between populations
Population may lose or gain alleles
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For example, a wildflower population consisted entirely of
white flowers, its pollen (r alleles only) could be carried into
our target population
This would increase the frequency of r alleles in the target
population in the next generation
Gene flow tends to reduce differences between
populations
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If extensive enough, gene flow can amalgamate neighboring
populations into a single population with a common genetic
structure.
The migration of people throughout the world is transferring
alleles between populations that were once isolated,
increasing gene flow.
Genetic Drift
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When changes in gene frequencies from one
generation to another occur because of
chance events that occur when populations
are finite in size
Natural disasters  sharp reductions due to
randomness nothing to do with genes
Genetic drift at small population sizes often
occurs as a result of
 The
bottleneck effect
 The founder effect
Genetic Drift
wildflower population stable,
only ten plants some alleles can be completely
eliminated
The Bottleneck Effect
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When the numbers of individuals in a larger
population are drastically reduced by a
disaster (fire, flood)
Survivors pass through a restrictive
bottleneck gene pool no longer reflective of
original population
Reduces genetic variation and adaptability
 Ex:
Cheetah population, elephant seals in CA
The Bottleneck Effect: an analogy
Figure 23.5x Cheetahs, the bottleneck effect
The Founder Effect
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A new population is started by only a few
individuals that do not represent the gene
pool of the larger source population
A population could be started by single
pregnant female or single seed with only a
tiny fraction of the genetic variation of the
source population
Have been demonstrated in human populations
that started from a small group of colonists
Non Random Mating
Selection preferences
 Inbreeding get more homozygous and
less heterozygous
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 Ex:
self pollinating plants
 Explain more
Natural Selection
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A violation of the conditions necessary for the
Hardy-Weinberg equilibrium
HW all individuals in a population have equal ability
to survive and produce viable, fertile offspring
Natural Selection in a population with variable
individuals, will lead some individuals to leave more
offspring than others
Selection in some alleles being passed along to the
next generation in numbers disproportionate to their
frequencies in the present generation (wildflower
example)
Natural selection accumulates and maintains
favorable genotypes in a population.
Modes of Natural Selection
Directional
 Disruptive/Diversifying
 Stabilizing
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Directional
Directional selection for beak size in a Galápagos population
of the medium ground finch
Diversifying
Diversifying selection in a finch population
Small beak
Large beak
Stabilizing
Modes of selection
Which mode of selection might lead toward
speciation?
Sexual Selection
Natural selection for mating success
 Sexual dimorphismmales and females
look very different birds
 Intrasexual selection direct
competition of one sex for mates of the
opposite sex
 Intersexual selection females are
choosy about their mates, choice
depends on the showiness of the male
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Genetic Variation
Each individual is unique can observe
phenotypic variations
 Not all phenotypic variation is heritable
 Types of Variation
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 Variation
within a population
 Variation between populations
 Mutations/sexual recombination
 Balanced polymorphism
 No perfection
Variation Within a Population
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Emerged in
Spring
Morphs different
forms
Polymorphism2 or
more distinct morphs
These butterflies are
genetically identical
at the loci for
coloration, but they
emerge at
different seasons
Emerged in
Summer
Variation Between Populations
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Geographical variation in the form of graded
change in a trait along a geographic axis is
called a cline
Plant at a higher altitude are shorter
Mutations/Sexual Recombination
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New alleles originate only by mutation
Mutations are changes in the nucleotide
sequence of DNA
Mutations of individual genes are rare and
random
Mutations in somatic cells are lost when the
individual dies
Only mutations in cell lines that produce
gametes can be passed along to offspring
Diploidy hides a lot of variation
Sexual selection and the evolution of male appearance
SEXUAL DIMORPHISM
Male peacock
Balanced Polymorphism
Heterozygote Advantage sickle cell,
increases diversity, keeps both alleles in
the population
 Neutral Variation  not helpful for
success, variation doesn’t mean much
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 Ex
fingerprints no evolutionary advantage
Heterozygote advantage
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Heterozygotes at a
particular locus have
greater survivorship and
reproductive success
than homozygotes
In these cases, multiple
alleles will be maintained
at that locus by natural
selection.
No Perfection
Evolution is imperfect
 Every species constrained to historical
descent
 Evolution is an editing process, not a
creation process
 Not all evolution is adaptive bottle
neck and founder effects
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The two-fold disadvantage of sex