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Synthetic Theory Synthetic Theory of Evolution:of Evolution:MicroevolutionMicroevolution
Population Genetics
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POPULATIONA group of organisms of the same species living together in a given region and are capable of interbreeding.
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Individuals do NOT evolve – populations evolve…
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POPULATION GENETICS
The study of changes in the genetic makeup of populations
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GENE POOL
The total of all the alleles present in a population.
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FORCES THAT CAUSE MICROEVOLUTION
1. Recombination
2. Mutation
3. Genetic Drift
4. Non-Random Mating
5. Natural Selection
6. Gene Flow
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RECOMBINATIONRECOMBINATION
REARRANGING GENES
1
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SOURCESrecombination of existing
genes and mutationsMeiosis creates gametesCrossing-over recombines
genes during Prophase I of meiosis
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Crossing Over
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MUTATIONSMUTATIONS
CHANGES IN GENETIC MATERIAL
2
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MUTATIONSresult in entirely new allelescan be inherited if contained
in sex cellsFrequency in sex cells:
1 / 10-100,000
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Human cells have approx. 32,000 genes
Most sex cells contain at least one mutation of some sort
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Mutations
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Mutation rates in nature are low BUT it is the ultimate source of variation
Most mutations are neutral in their effect
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CAUSES:radiation, alcohol, lead, lithium,
mercury, virusesTeratogens: androgens,
tetracycline, vitamin ASpontaneous mutation: specific
cause is not known
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GENETIC DRIFTGENETIC DRIFT
CHANCE EVENTS CAUSE CHANGES
3
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GENETIC DRIFTRandom changes in gene
frequencies of small populations as a result of chance events
Net effect > rapid evolution
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EXAMPLEThe DunkersGermany to PA Had a higher percentage of A
blood type as a result of genetic drift
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FOUNDER EFFECT A small amount of people have many surviving descendants after a number of generations
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RESULTHigh frequencies of
specific genetic traits inherited from the few common ancestors who first had them
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EXAMPLE 1:One woman moved to
Venezuela, had an unusually large # of descendents who inherited the Huntingdon’s disease allele > extremely high frequency in that area
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EXAMPLE 2:Amish of Lancaster have
high incidence of microcephaly > all are descendents of a single Amish couple nine generations ago.
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EXAMPLE 3:South and Central
American Indians all have type O blood > founders migrated into the region from the north
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BOTTLENECK EFFECT
When most individuals die as a result of a crisis and the few survivors experience reproductive success > large populations
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RESULTDramatic reduction in
genetic diversity of a species because most variation is lost at the time of the bottleneck
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NON-RANDOM NON-RANDOM MATINGMATING
ORGANISMS CHOSE THEIR MATES
4
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HUMAN MATINGHumans select mates non-randomly because of cultural values and social rules.
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RANDOM MATINGGene pool will remain in equilibrium – the frequencies of alleles will NOT change
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NON-RANDOM MATING
Types:Positive assortativeNegative assortative
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POSITIVE ASSORTATIVE
common in humans = individuals mate with people like themselves
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EFFECTProgressive increase in the
number of homozygotes (AA, aa)
Decrease in heterozygotes (Aa) in a population
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NEGATIVE ASSORTATIVE
Least common pattern in humans > people mate with people who are different from themselves
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EFFECTprogressive increase in
frequency of heterozygotes (Aa)
Decrease in frequence of homozygotes (AA, aa) in a population
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POSITIVE ASSORTATIVE
Used to develop purebred varieties of animals
Increase in recessive diseases: hip dysplasia, epilepsy in dogs
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EXAMPLEAmish select mates from
within their own communities > high frequency of Ellis-van Creveld syndrome (dwarfism, extra fingers)
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Polydactyly
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Polydactyly
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INBREEDINGConsanguineous matingRisk for birth defects in
offspring of first cousins is ONLY 1.7-2.8% above normal but 6.8-11.2% higher for offspring of siblings.
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NATURAL NATURAL SELECTIONSELECTION
SURVIVAL OF THE FITTEST
5
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Natural selection is the most important mechanism of evolution.
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DIRECTIONAL SELECTION
Frequency of alleles in gene pool shifts towards the advantageous allele
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DIRECTIONAL SELECTION
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EXAMPLESSlow: albinism, juvenile
diabetesExtreme: AIDS
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HIV / BUBONIC PLAGUE
Connection:Homozygotes for CCR5-delta32
gene are immune to AIDS. This gene also provides immunity to the bubonic plague. Heterozygotes are partially immune.
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STABILIZING SELECTION
Also called - balanced polymorphism
Selection for the heterozygote (Aa) > no shift in gene pool frequencies towards either one of the alleles
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STABILIZING SELECTION
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EXAMPLEMalaria / Sickle Cell Anemia in Africa
aa: have sickle cell, but are immune to malaria
Aa: have partial sickle cell and moderately good resistance to malaria
AA: no sickle cell, can get malaria
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DISRUPTIVE SELECTION
Favors both homozygote extremes (AA, aa), selects against the heterozygote (Aa)
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DISRUPTIVE SELECTION
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When nature selects against all genotypes EXTINCTION of the population results
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GENE FLOWGENE FLOW
ORGANISMS MIGRATE
6
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GENE FLOWGenes are transferred from
one population to another as a result of migrationImmigration- enter populationEmigration- leave population
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EXAMPLEUS soldiers had children
with Vietnamese women during the war > altered gene pool frequencies of the Vietnamese population