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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
1
RECOMBINATION
REARRANGING GENES
SOURCES
recombination of existing
genes and mutations
Meiosis creates gametes
Crossing-over recombines
genes during Prophase I of
meiosis
Crossing Over
2
MUTATIONS
CHANGES IN GENETIC MATERIAL
MUTATIONS
result in entirely new alleles
can be inherited if contained
in sex cells
Frequency 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, viruses
Teratogens: androgens,
tetracycline, vitamin A
Spontaneous mutation: specific
cause is not known
3
GENETIC DRIFT
CHANCE EVENTS CAUSE CHANGES
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
4 NON-RANDOM
MATING
ORGANISMS CHOSE THEIR MATES
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
Polydactyly
INBREEDING
Consanguineous mating
Risk 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.
5
NATURAL
SELECTION
SURVIVAL OF THE FITTEST
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
6
GENE FLOW
ORGANISMS MIGRATE
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