Lecture 5 - Introduction to Evolution in Finite Populations
From last time.
(redraw a cline, w/ p on the y, lat on the x)
Migration and Selection
What about selection
We can imagine two patches,
, the A1 allele is favored in
one and the A2 allele in the other, with a rate of migration
migration from patch
and vice versa.
Genotypic fitnesses in patch
; fitnesses in patch
Population sizes are equal in the two
Set the model up as random mating followed by selection followed by migration
(imagine selection acts mostly on juvenile survivorship, as for many fish).
After selection but before migration, let the frequency of the A1 allele in population
*, which follows from the general viability model for population
So, the frequency of the A1 allele in the next generation in population
' = (1-m
* + m
The equilibrium condition is (you guessed it!) marginal overdominance, i.e., the average
fitness of the heterozygote is highest, even if it isn't the most fit in either population.
equilibrium frequency will also depend on the migration rates.
Note that natural selection can also produce a
in allele frequency.
the gene for alcohol dehydrogenase (Adh) exhibits a latitudinal cline in several species
, on several continents.
There are two major allozyme alleles, "fast" and
One allele appears to be favored in warmer climates, the other in cooler
Other examples of latitudinal clines include growth rate in Atlantic Silversides (a fish),
ovariole number in several species of
(on several continents)
How would you determine if a cline was maintained by restricted migration (e.g.,
stepping stone) or by natural selection?
Evolution in a finite population: Introduction to Random Genetic Drift
A familiar example of genetic drift.
There ~25,000 genes in the human genome, and
since we are diploid, that means we have two copies of each.
On average, there is a
polymorphic nucleotide site about every 1000 bp (1 kb).
Let's further say the average
gene is 1 kb long.
So by this crude reasoning, we are all heterozygous at every locus,