population genetics reconciling darwin & mendel. darwin darwin’s main idea (evolution), was...
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Population GeneticsReconciling Darwin & Mendel
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Darwin• Darwin’s main idea (evolution), was
accepted • But not the mechanism (natural selection)
– Scientists did not understand Darwin’s mechanism because there was no understanding of genetics
• Even once scientists grasped Mendel, genetics was viewed as an either/or – didn’t understand many traits are polygenic
• So how do you get the variation on which selection works?
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Ideas About Evolution
• Orthogenesis – 1920’s – saw evolution as a predictable
progression to more & more elite forms of life
• Population Genetics – 1930’s – reconciled Darwin & Mendel
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Genetics of Populations
• Population – a localized group of individuals belonging to
same species– The definition of a species not always clear
• Gene pool = The total genes in a population
• Evolution on the smallest scale occurs when the relative frequency of alleles in a population changes over a succession of generations = microevolution
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Genetics of a Non-evolving Population
• The gene pool is in stasis• This is described by Hardy-Weinberg
Theorem:• The frequencies of alleles in a population’s
gene pool remain constant over the generations unless acted on by agents other than sexual recombination
• i.e. shuffling the deck has no effect on the overall genetic make-up of the population
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The Hardy-Weinberg Theorem
• Example• In pink flowers (A), is dominant over
white flowers (a)• 2 alleles for at this locus• Sample 500 plants:• 20 white flowers (aa)• 480 pink [320 (AA); 160 (Aa)]• Therefore there are 1000 genes for
flower color in the population•
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Example (Continued)
• The dominant allele accounts for 800 of these: – [(320 x 2) + (160 x 1)]
• Therefore:– the frequency of A in the population =
80%– the frequency of a = 20%
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Predicting Change• How will genetic recombination
during sexual reproduction affect the frequencies in the next generation?
• If mating is random:
– the probability of picking 2 AA = (0.8 x 0.8) = .64
– the probability of picking 2 aa = (0.2 x 0.2) = .04
– and of heterozygotes = 2(0.8 x 0.2) = 0.32 • There are 2 heterozygote combinations: aA &
Aa• sperm or egg
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Hardy-Weinberg Equilibrium
• This shows that the alleles are present in the gene pool in the same frequencies as they were in the previous generation
A: [0.64 + (0.32 2)] = 0.8 a: [0.04 + (0.32 2)] = 0.2
• The gene pool is at equilibrium• This is called Hardy-Weinberg
equilibrium
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The Hardy-Weinberg Equation
• This example is the simplest case: – 2 alleles, one is dominant
• For this case: – if p = frequency of one allele – q the frequency of the other
• Then: p + q = 1probability of AA = p2
probability of aa = q2
probability of Aa = 2pq
• Therefore: p2 + 2pq + q2 = 1
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Uses of Hardy-Weinberg
• Thus you can calculate the frequency of a gene in a population if you know the frequency of the genotypes
• This is important in genetic disease counseling
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Relevance to Evolution
• A population at genetic equilibrium does not evolve
• Hardy-Weinberg tells us what to expect in non-evolving populations
• Therefore it is a baseline for comparing actual populations where gene pools may be changing.
• Can determine if the population is evolving
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Genetic Equilibrium• Hardy-Weinberg equilibrium is
maintained only if the population meets all 5 of the following criteria:– Very large population size– Isolation from other populations
• migration can effect the gene pool– No net mutations– Random matings– No natural selection
• no difference in reproductive success)
• Describes an ideal that never exists in nature
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Altering Genetic Equilibrium
• For evolution to take place something must upset the genetic equilibrium of the population:
• Factors that change genetic equilibrium are:– Genetic drift– Migration (Gene flow)– Non-randon mating (Isolation) – Mutation– Natural selection
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Genetic Drift• Changes in gene frequency of a very
small population due to chance• Controlled by the laws of probability &
chance• Bottleneck effect
– Chance sampling error due to small population
• Founder’s effect – a few individuals colonize a remote spot – causes drift
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Illustrating Genetic Drift
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The Bottleneck Effect
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Gene Flow (Migration)
• Movement of organisms into or out of a population
• Takes their genes out of the gene pool
• Most populations are not completely closed– gain & lose alleles
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Non-random Mating
• More apt to mate with close neighbors
• Promotes inbreeding• Assortive mating
– seek mate like self (i.e. size)
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Isolation
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Mutation• A change in a
gene• An alteration of
DNA• The original
source of variation
• Raw material on which natural selection works
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Natural Selection
• If one type produces more offspring than another, upsets the balance of equilibrium
• There are three types of natural selection:– Stabilizing Selection– Disruptive Selection– Directional Selection