Chapter 23~ Microevolution- small changes in the genetics of populations.
Biology 331 Genetics Population Genetics (Microevolution)
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Transcript of Biology 331 Genetics Population Genetics (Microevolution)
![Page 1: Biology 331 Genetics Population Genetics (Microevolution)](https://reader035.fdocuments.net/reader035/viewer/2022062217/56649e6a5503460f94b688e6/html5/thumbnails/1.jpg)
Biology 331 Genetics
Population Genetics (Microevolution)
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Introduction to Evolution:
Population Genetics (Microevolution): Evolution occurring at and below the species level
Macroevolution:Evolution occurring at and above the species level
Outgrowth of agricultural revolution in the 20's-40's
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Modern importance of population genetics:
Agriculture
Other genetic engineering
Forensics
Medicine
Conservation
"Pure" Science speciation, systematics, evolution of behaviors etc.
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Variation
Qualitative
Quantitative
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Evolution:
What is it? Means Change
Biological/Organic Evolution Change in an organism over time
Change in allele frequency over time
Not = Natural Selection
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Natural Selection: How does it work? More offspring are produced than can survive (Species could reproduce at an exponential rate) Most populations have a stable size Therefore: There is a struggle for existence
Members of a population vary in their characteristics(short, tall, fast, slow)
Much of this variation is heritable Therefore: Struggle for existence is not random. It depends on individual characteristics
(which are heritable)
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Natural Selection ContinuedThose which are best adapted to the environment survive and reproduce (Differential Reproduction) Over time this process brings about changes in populations with favorable changes accumulating. Examples:
Cheetah's Speed, Cow's Milk etc.
Fitness: The ability of an organism to leave offspring in a given environment
Genetics: Darwin lacked a method. Mechanism provided by the monk Gregor Mendel. 1932-1953 modern synthesis
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Pepper Moth
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Items of note: Selection on individuals, but individuals do not evolve, populations do Natural selection acts on phenotypes but evolution is change in gene frequency Natural selection does not "think ahead". Selects organisms adapted to past environments. But, some traits may be favorable in new environments
human bipedalism
Natural Selection acts only on existing traits, variation is crucial Natural selection results in organisms better adapted for an environment...NOT optimally designed
human bipedalism
Natural selection normally acts on individuals not groups/species.
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Hardy-Weinberg:
An introduction
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Hardy-Weinberg Theorem:
Allele frequencies stay constant if there is no selection and it's other assumptions are met
Thus if we have 25% green eye genes, 25% blue eye genes, and 50% brown eye genes it will stay that way.
Heterozygosity will also stay the same
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Two allele equation:
p2 + 2pq + q2 = 1 p= frequency of allele A
q = Frequency of allele a
p + q = 1.
So p2 = AA, q2 = aa, and pq = Aa
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Sophisticated Punnet square:
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Genotype frequency
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Assumptions:
Random mating
Very large Population size
Diploid
Sexual
Non-overlapping generations
No migration
No mutation
No selection.
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So what good is it?
Provides an evolutionary baseline
Calculate deviations from the H.W. Ideal
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Hardy-Weinberg and Selection:
Problem #1 Assume a population has two co-dominant alleles for a gene (B, B')
Assume there are 1000 individuals, 250BB, 500BB', and 250B'B'
So: Freq. B = 500+500/2000 = .5; B'= 500+500/1000 = .5
Assuming H.W. BB = p2 = .25; BB'= 2pq = .5; B'B'= q2 = .25 (No Change)
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Add Selection:
Fitness = Survival (for this example) BB = 1; BB'= 0.9, B'B'= 0.8 BB = 250; BB'= 0.9(500) = 450; B'B'= 0.8(250) =200 Frequency BB = 250/900 = .278; BB'= 450/900 = 0.5; B'B'= 200/900 = .222 Frequency B = .278+1/2(.5) = .528, B' = .472 Deviation From H.W.!
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Types of selection
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Frequency dependant selection
Fitness of an allele depends upon its frequency
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Mutation and Hardy Weinberg:
Assume p has a frequency of 1 What is the frequency of q ?
Now allow a mutation to occur from p to q
Instant evolution!
But is this a "strong" evolutionary effect? Highest rate of mutation recorded is 0.0007/mutant cells/cell division
Result....no real effect over one generation
Over time?
Mutation alone is typically a weak evolutionary force
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Mutation over time
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So why does in matter?
Raw material for evolution
Creates new genes
Mutation selection balance
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Migration:
Transfer of alleles from one gene pool to another
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One island model:
Assume you have genotypes A1A1, A1A2, A2A2
frequencies p2, 2pq, and q2
A1 is fixed on the continent; A2 is fixed on the island
N on the island is much smaller than on the continent
Migration (m) from the continent to the island is more important than vice versa (Why?)
m=20% of the island population/generation A1A1 = 0.2 after migration (was 0)
A2A2 = 0.8 after migration (was 1.0)
Not H.W. equilibrium Both allele frequencies and genotype frequencies changed
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Islands
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Long term effect??
The general effect of migration is homogenization This effect is proportional to m, and the difference between Pc and PI
Migration selection balance
Migration as mutation
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Gene flow and natural selection
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Genetic Drift:
Random variation in allele frequencies due to sampling error
Yields evolution but not necessarily adaptation
Drift more important in small populations
Coin flipping/beanbag examples
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Drift
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Absorbing States:
The random fixation of alleles
The frequencies of alleles vary through time
Eventually alleles go to either fixation or loss
Assumes no Migration, mutation, selection etc.
Probability of loss or fixation proportional to initial frequency
"C" allele example
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Loss and Fixation
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Drift
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What determines probability of loss?
Probability of loss or fixation proportional to initial frequency
So why does population size matter?
"C" allele example
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Speed
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Bottle Necks:
"Random" reduction in population size (Disasters)
Only a fraction of the alleles in the initial population survive
• "Instant" Evolution (sampling error) • Small population size after the bottleneck enhances drift • Repeated bottlenecks have huge effect!
European Jews, Lynx, Whales S. African Cheetahs and Northern Elephant seals almost
"Clones"
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Bottlenecks
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Bottlenecks
• "Instant" Evolution (sampling error) • Small population size after the bottleneck enhances
drift • Repeated bottlenecks have huge effect!
European Jews, Lynx, Whales S. African Cheetahs and Northern Elephant seals almost
"Clones"
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Founder Effect:
• Genetic drift in a new colony
• May be only one gravid female
• Sampling error can result in "instant" evolution
• Very much like a bottleneck
• Extreme sampling error possible
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Founder Effect
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Picture Wing Drosophila
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Examples
• Tristan da Cunha (Classic Example) Founded by a small number of colonists (15) Retinitis Pigmentosa (one founder was a carrier)
• Amish in PA Founded by 200 people 1-2 founders have Ellis-van Creveld syndrom Frequency 0.07 in Amish, 0.001 in the population as
a whole
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Village of Salinas:In the remote mountains of the Dominican Republic:
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• One village founder Altagracia Carrasco
• Several children with at least 4 women
• Large contribution to a small population
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• Mutant for 5-alpha reductase-2 gene
• Low catalytic activity
• He was a heterozygote
• Enzyme responsible for conversion of testosterone to DHT
• Required for full masculinization of external genitalia
• Results in XY “females”
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What happens at puberty?
• Guevedoces (penis at twelve)
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Effective Population Size:• Theoretical "ideal" population having the same
magnitude of drift as the "Real"(tm) population • Census size:
All the individuals in a population Assume No selection, No migration, No mutation, Non
overlapping generations, Diploid, Sexual
• No population obeys the rules so we need a "fudge factor"
• Effective population size almost always smaller than the census size
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Example:
• Assume 500 individuals
• 250 breeding age
• Only 5 "dominant" males breed
• EPS = 130
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Drift and selection:
• Can allow selection to act
• "C" allele again!
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Nonrandom Mating:
ANY deviation from totally random mating
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Inbreeding:
• Mating between genetic relatives
• Need to calculate the probability that an allele is Identical By Decent (ibd)
• f = probability 2 gametes are ibd beyond random mating expectaions
• Does not require inbreeding in a social sense
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F values
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Effect of selfing with time:
• Increase in number of homozygotes Why?
• Selfing homozygotes yield homozygotes
• Selfing heterozygotes yield 50% heterozygotes and 25% of each homozygote
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Increase in homozygosity
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Inbreeding continued
• Does not affect gene frequencies
• Does affect genotype frequencies Excess homozygotes
• Potential affect on evolutionary process?
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Effects of inbreeding:
• Loss of heterozygosity What effect might this have?
• Inbreeding depression Due in part to deleterious recessives Effect of inbreeding depression varies among
lineages Resistance to inbreeding depression Inbreeding more likely to be detected in stressed
organisms Inbreeding affects often show up later in life
• Maternal effects
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Effect of low values of "f"
• f = 0.0005 (cousin mating)
• % of affected individuals from first cousin mating
• 18-24% albinism
• 27-53% tay-sachs
• 20-26% xeroderma pigmentosa
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Positive Assortative mating:
• Like breeds with like
• Acts like inbreeding but only for selected alleles Increases homozygosity
• Can increase variance in a trait Short w/ short, tall w/ tall More variation for selection to act on
• Alternative to inbreeding to fix "type"
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Negative assortative mating:
• Avoidance of like types
• To an extent this is the opposite of inbreeding
• Does not affect all genes equally
• Excess heterozygotes
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Avoidance of inbreeding:
• Behavior Dispersal (how far is far enough?) Not coming into season before dispersal
• Social Mores
• Mate Choice: Self incompatibility MHC rejection
• Why do we want variation at MHC?
Spontaneous abortion/mate choice