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

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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

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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

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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

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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