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Population Genetics & Evolution
Population Genetics
• The study of evolution
from a genetic point
of view
Causes of Variation
• Mutations
• Recombination
• Random fusion of
Allelic Frequency
• Determined by dividing the number of a certain allele by the number
of alleles of all types in the population.
• Consider a population of ten individuals and a given locus with two
possible alleles, A and a. Suppose that the genotypes of the
individuals are as follows:
– AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA
• Then the allele frequencies of allele A and allele a are:
• so there is a 70% chance of the population getting that allele
• and there is a 30% chance of the population getting this allele
Gene Pool
• The total genetic
information available
in a population.
Hardy-Weinberg Principle
• Godfrey Hardy(18621937) = British
• Wilhelm Weinberg(1877 –
1947) = German
The H-W Principle
• Allele frequencies in a population tend to
remain the same from generation to
generation unless acted on by outside
• Genetic Equilibrium
• Formula
p2 + 2pq + q2 = 1
What do the letters mean?
p = dominant allele
q = recessive allele
p2 =homozygous dominant
q2 = homozygous recessive
2pq = heterozygous
• p+q=1
p2 + 2pq + q2 = 1
H-W Based on Assumptions
• No net mutations occur
• Individuals don’t enter or leave the
• The population is large
• Individuals mate randomly
• Selection does not occur
Disruptions to Genetic Equilibrium
• Mutations occur
constantly at a slow
• Some mutations will
be bad and some will
be good for the
Disruptions to Genetic Equilibrium
• Individuals and their
genes frequently
move in and out of
• Sometimes males will
leave when they
mature to form their
own group
Disruptions to Genetic Equilibrium
• Genetic Drift – The
changes of allele
frequencies in small
populations due to
• Small populations can
have genetic issues
Disruptions to Genetic Equilibrium
• Nonrandom mating
• Most organisms
choose their mates
• Females often pick
“more fit” males to
breed with.
Disruptions to Genetic Equilibrium
• Natural Selection is
an ongoing process in
• Traits are being
selected for and
against all the time.
Species Evolution
• A species is a
population of
organisms that can
interbreed but cannot
breed with other
Forming a New Species
• Geographic Isolation when the habitat of a
species is divided
over a long period of
time, two new species
can arise.
Forming a New Species
• Reproductive
Isolation Barriers to successful
breeding between
population groups in
the same area
Reproductive Isolation
Prezygotic - before
• Chromosomal
• Seasonal mating
Reproductive Isolation
Postzygotic = after
Creation of hybrids that
will be born infertile.
Rates of Speciation
• Gradualism – idea
that species originate
through a gradual
buildup of new
• Punctuated
Equilibrium = idea
that speciation occurs
quickly in rapid
bursts, with long
periods of stability in