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Lecture Twenty One : Population genetics and natural selection

4 important questions on Lecture Twenty One : Population genetics and natural selection

How do you calculate the genotype frequencies using Hardy-Weinberg equilibrium ?

These are equations we can use to calculate the genetic frequencies in a population at a given time. We can only use Hardy-Weinberg when we know are dealing with “normal” alleles (ie. no linkage or codominant alleles).

We can look at alleles separately:

Two alleles in a population, a dominant and a recessive, then there frequencies add up to one.
p+q=1

Or within their inherited genotypes:

p 2+2pq +q2=1
p2=homozygous dominant
pq= heterozygous
q2=homozygous recessive

What are the random genetic drift the occurs rapidly in small populations ?

Allele frequencies are not set in stone, they can change rapidly. This is particularly the case in small populations due to the small gene pool.

Changes can occur via:

Non-random mating
Natural Selection
Gene Flow or Migration
Mutations
Random Genetic Drift
Bottleneck
Founder Effect

What is Random Genetic Drift ?

A random change in allele frequencies due to sampling and chance. As you go from generation to generation, some alleles are bred out as a result of chance.

How do populations evolve over time in response to selection ?

Selection pressures acting on a population may change its distribution.

Directional : Selection pressure against short necks of giraffes
Stabilising : Selection against short plants because they aren’t getting sun. Tall plants get wind damage.
Disruptive : light and dark coloured oysters

Frequency Dependent Selection : This is where it is favoured to have an uncommon allele
Cline : This is where gradual geograhical change causes a gradual change in genetic/phenotypic composition