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