the evolution of populations chapter 23. topics i. hardy weinberg theorem introduction the theorem...
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The Evolution of Populations
Chapter 23
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Topics I. Hardy Weinberg Theorem
Introduction The theorem Computing allelic frequencies Microevolution
II. Genetic Variation Variation within a population Variation between populations Mutations/sexual recombination Balanced polymorphism No perfection
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“Populations evolve, individuals do not”
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Variation in a Natural Population
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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 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: 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
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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
No mutations No emigration or immigration Large breeding population Random mating All alleles are equally viable (no
selection
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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.
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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=.6p + q = 1 , p = 1- q =.4p2 = .16 = 1602pq = 2(.4)(.6) =.48 =480
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Microevolution The Hardy-Weinberg theory provides a
baseline against which we can compare the allele and genotype frequencies of an evolving population
Microevolutiongeneration-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
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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 A genetic exchange due to migration of fertile
individuals or gametes between populations Population may lose or gain alleles
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 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.
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Genetic Drift 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
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Genetic Drift wildflower population stable, only ten plants some alleles can be completely eliminated
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The Bottleneck Effect 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
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The Bottleneck Effect: an analogy
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Figure 23.5x Cheetahs, the bottleneck effect
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The Founder Effect 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
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Non Random Mating
Selection preferences Inbreeding get more homozygous
and less heterozygousEx: self pollinating plantsExplain more
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Natural Selection 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.
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Modes of Natural Selection
Directional Disruptive/Diversifying Stabilizing
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Directional
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Directional selection for beak size in a Galápagos population of the medium ground finch
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Diversifying
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Diversifying selection in a finch population
Large beakSmall beak
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Stabilizing
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Modes of selection
Which mode of selection might lead toward speciation?
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Sexual Selection Natural selection for mating success Sexual dimorphismmales 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
Variation within a populationVariation between populationsMutations/sexual recombinationBalanced polymorphismNo perfection
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Variation Within a Population Morphs different
forms Polymorphism2 or
more distinct morphs
These butterflies aregenetically identical at the loci forcoloration, but they emerge atdifferent seasons Emerged in
Summer
Emerged in Spring
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Variation Between Populations 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
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Mutations/Sexual Recombination 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
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Sexual selection and the evolution of male appearance
SEXUAL DIMORPHISM
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Male peacock
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Balanced Polymorphism Heterozygote Advantage sickle
cell, increases diversity, keeps both alleles in the population
Neutral Variation not helpful for success, variation doesn’t mean muchEx fingerprints no evolutionary
advantage
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Heterozygote advantage
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.
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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