Synthetic Theory of Evolution: Microevolution Population Genetics

POPULATION A 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

RECOMBINATION REARRANGING GENES 1

SOURCES recombination of existing genes and mutations Meiosis creates gametes Crossing-over recombines genes during Prophase I of meiosis

Crossing Over

MUTATIONS CHANGES IN GENETIC MATERIAL 2

MUTATIONS result in entirely new alleles can be inherited if contained in sex cells Frequency in sex cells: 1 / ,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, viruses Teratogens: androgens, tetracycline, vitamin A Spontaneous mutation: specific cause is not known

GENETIC DRIFT CHANCE EVENTS CAUSE CHANGES 3

GENETIC DRIFT Random changes in gene frequencies of small populations as a result of chance events Net effect > rapid evolution

EXAMPLE The Dunkers Germany 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

RESULT High 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

RESULT Dramatic reduction in genetic diversity of a species because most variation is lost at the time of the bottleneck

NON-RANDOM MATING ORGANISMS CHOSE THEIR MATES 4

HUMAN MATING Humans select mates non-randomly because of cultural values and social rules.

RANDOM MATING Gene pool will remain in equilibrium – the frequencies of alleles will NOT change

NON-RANDOM MATING Types: Positive assortative Negative assortative

POSITIVE ASSORTATIVE common in humans = individuals mate with people like themselves

EFFECT Progressive 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

EFFECT progressive 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

EXAMPLE Amish select mates from within their own communities > high frequency of Ellis-van Creveld syndrome (dwarfism, extra fingers)

Polydactyly

INBREEDING Consanguineous mating Risk for birth defects in offspring of first cousins is ONLY % above normal but % higher for offspring of siblings.

NATURAL SELECTION 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

EXAMPLES Slow: albinism, juvenile diabetes Extreme: 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

EXAMPLE Malaria / 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 FLOW ORGANISMS MIGRATE 6

GENE FLOW Genes are transferred from one population to another as a result of migration Immigration- enter population Emigration- leave population

EXAMPLE US soldiers had children with Vietnamese women during the war > altered gene pool frequencies of the Vietnamese population