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.