Population Genetics
Defining a Population
Population: Localized group of individuals
that belong to the same species and are
capable of interbreeding to produce
fertile offspring.
Smallest ecological unit that can evolve.
Natural selection works directly on the phenotypes of
individuals.
But it is the allele frequencies of populations that change.
Once Mendelian genetics was reconciled with Darwin’s idea of wide
variation, the field of population genetics was born.
Population genetics: the study of how populations change genetically over
time.
This shift in thinking was also part the development of the modern
synthesis of evolutionary theory. It includes Darwinian natural selection,
Mendelian genetics, population genetics, paleontology, systematics.
biogeography, and several other related fields.
Gene pool: total aggregate of genes in a population at any one time.
-- all alleles
-- all loci
-- all individuals
As long as there are two or more alleles for any given loci, then each of
those alleles will have a frequency.
It is the changes of those frequencies over successive generations that is
the definition of evolution.
Hardy-Weinberg Theorem
Mathematician and physician who
developed a model to work as a null
hypothesis for determining if evolution has
occurred in a population.
Provided the 5 H-W conditions are met and assuming Mendelian
inheritance, allele frequencies between generations will not occur and,
therefore, there will be no evolution.
Five Conditions for Hardy-Weinberg Equilibrium
1. No genetic drift (population must be large)
2. No gene flow (no immigration or emigration)
3. No sexual selection (mating must be random)
4. No natural selection (all phenotypes are created equal)
5. No mutation (no alleles can randomly change)
Hardy Weinberg EquationsAssume a gene at a particular loci has 2 alleles: dominant and recessive.
The dominant is represented by the letter p.
The recessive is represented by the letter q.
All alleles in the population must equal 100%.
As a frequency, 100% = 1
Therefore: p + q = 1homozygous dominant is pp = p*p = p2
homozygous recessive is qq = q*q = q2
heterozygous is pq or qp = 2pq
All individual genotypes in the population must equal 100% or 1
Therefore: q2 + 2pq + q2 = 1
Practice: class as a population, eye color as a simple trait
Once you can calculate allele frequencies, you can track them over
generations and determine if evolution has occurred.
Once you have determined that evolution has occurred, you can
hypothesize reasons.
Likely there is a selection pressure that is providing higher fitness to a
particular phenotype.
But don’t forget to consider other explanations: genetic drift or gene flow.
Preservation of VariationDiploidy with complex interactions between many genes.
Recessive alleles can “hide” and, thus, is it very hard for even an unfavorable
recessive allele to reach a frequency of 0.
Most traits are polygenic and produce a very wide range of combinations
and possibilities.Up to 15 genes:
Dominant OCA2 or HERC2 mean more
melanin = brown eyes
Polymorphisms of those genes can result
in subtle variations of melanin
Recessive results in blue which is less to
no melanin
Also influenced by cellular density of the
stroma; not related to pigment
Genes that determine structure determine
scattering effect which plays a role
in green, blue, and hazel
Balancing Selection
Frequency dependent selection
Natural selection favors the preservation of multiple phenotypes.
Fitness of a phenotype is directly dependent on and inversely proportional
to its frequency in the population.
Thus, as the frequency goes down, the fitness goes up.
As the frequency goes up, the fitness goes down.
Wood tiger moths: morphs that are more abundant have lower fitness.
As predation acts over time, the morph that is abundant can vary so the
fitness of each morph will vary.
Balancing Selection
Heterozygote advantage
A codominant or incomplete dominant pattern that results in an intermediate
morph with the highest fitness despite the recessive phenotype being lethal.
Most common in individuals of African decent.
Experiencing bouts of sickle cell is more
survivable than malaria. Thus, the heterozyote has
the highest fitness and the recessive will never
reach a frequency of 0.
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