Population Genetics
 Populations – is a group of organisms of the same species occupying a particular geographic region.
 Deme – a locally interbreeding group within a geographic population.
o May also be called a subpopulation or a Mendelian population
o They will be our topic of interest for genetics
 Measure of Genetic Variation
o Gene pool is all the genetic information present in the population at any given moment.
o If we could count every allele at every locus in every organism that would be fantastic, but since that is
not possible we instead look at allele frequencies.
o Allele frequencies are assumptions made by a simple equation
o First we assume that all members there are only 2 alleles at given locus (A, a), and from those we can
generate 3 different genotypes
o In a population consisting of ‘N’ dipoid individuals:
 let ‘X’ be the number of individuals that are homozygous for the A allele (AA)
 let ‘Z’ be the number that are heterozygous (Aa), and
 let ‘Y’ be the number that are homozygous for ‘a’ (aa).
 Note that X + Z + Y = N, and that the total number of alleles is 2N
 So for the ‘A’ alleles in the population is 2X + Y
 the ‘a’ alleles in the population is 2Z + Y
o
o So we can put this into a formula, where ‘p’ is the frequency of the ‘A’ allele and ‘q’ is the frequency of
the ‘a’ allele,
p = 2X + Y
q = 2Z + Y
2N
2N
o
E.g. Calculate the allele frequencies in 2 populations, each consisting of 200 diploid individuals.
Population 1 has mostly homozygotes (90 AA, 40Aa, and 70aa), whereas population 2 has mostly
heterozygotes (45AA, 130 Aa, and 25aa). Find the frequencies of ‘p’ and ‘q’ in both populations
o
Notice first that for each population p + q = 1, and the frequencies range from 0 – 1. So we can figure
out ‘p’ from ‘q’ and vice versa.
Another thing to notice is that the 2 population have the same frequencies, but they are distributed
differently among the populations. It is these distribution patterns, together with the allele frequencies
that describe the genetic structure of the population.
What would be more accurate is genotype frequencies
o
o
Hardy-Weinberg Rule
 Evolution is change in the genetic composition in a population over time. A population that is not changing, that
have the same genotypic and allele frequencies from generation to generation are said to be at equilibrium.
 We can measure to see if a population is at equilibrium by using the Hardy-Weinberg Rule.
 The rule specifies the conditions a population must meet to be at equilibrium and the genotype frequencies that
will be found in such a population.
o To meet the conditions:
 A population must be very large
 They must be sexually reproducing
 Diploid
 All individuals must survive and reproduce equally
 Mating must be at random
o If these condition hold, then a population will remain constant from generation to generation.
o The genotypic frequencies are related as follows:
p2 (AA) + 2pq (Aa) + q2 (aa) = 1
o
It is easy to show how alleles remain constant from generation to generation. Shown below:
Changing the Genetic Structures of Populations
 Evolutionary agents are forces that change the genetic structure of populations. They violate the HardyWeinberg equilibrium.
 Mutation
o They are either neutral or harmful to the organism unless the environment changes and then those
mutations may be of advantage.
o They both create and help maintain genetic variation within a population
 Gene Flow
o Migration followed by breeding in the new location between 2 subpopulation.
o
 Genetic Drift
o Chance alterations which change allele frequencies in small populations

Bottlenecks
 Only a small number survive from a population and are then repopulate based on a
small group of individuals (decreased variation)

Founder Effects
 A small group expands into a new region
 Small amount of variation
 Nonrandom Mating
o Produce generations where the homozygous generations are overrepresented
o Self-fertilization is an example
 Natural Selection
o Selects for certain traits, favoring some alleles over others.