Genetic Drift and Neutral Evolution

Exam 2 Lecture 8

UIC BioS 101 Nyberg

GENETIC DRIFT & NEUTRAL THEORY of EVOLUTION

There is much genetic variation within almost all species. The amount of genetic variation is too much to be maintained by selection.

Speaker Notes:

For populations the only way to keep multiple alleles in a population is to have heterozygotes superior to homozygotes.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Reading Assignment

The sections relevant to this lecture are 25.3 Genetic Drift,
Your lab on Population Genetics &
Box 27.1 (p. 551) on the Molecular Clock.
Phylogeny which is currently a major activity of biologists is based on the fact that neutral allelic substitutions accumulate with time.

Speaker Notes:

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Natural Selection
view of Evolution

Mutations arise by chance (meaning the mutations are not directed to match environmental needs).
Favorable (= higher fitness) mutations increase in frequency via selection (changed fitness often associated with changed environment).
Deleterious (= lower fitness) mutations are reduced in frequency (many resistance mutations are deleterious unless toxic agent present).

Speaker Notes:

The idea of natural selection does not directly address the dynamics of genetic variation. Those dynamics were worked out early in the 20th century.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Amounts of Genetic Variation

When molecular techniques allowed scientists to measure genetic variation in proteins they found 1) many polymorphic loci, and 2) including some with many alleles.
The amount of genetic variation was much greater than scientists expected from the population genetic models that existed.
Selection models were ‘tweaked’, but ….

Speaker Notes:

Dick Lewontin at University of Chicago was pioneer in measuring genetic variation.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

New Idea

The neutral theory of evolution developed by Motoo Kimura (1st publication 1968).
The neutral theory departed from all existing models by using N, the population size, as the most important population parameter.
What is the neutral theory of evolution?

Speaker Notes:

In finite populations alleles can reach a frequency of 100%. In the infinite population models frequencies get really close to 1 but never reach it.

Sampling Variation

Real populations have a finite number of members, N.
Whenever you have a finite number of individuals you have sampling variation.
Sampling leads to changes in frequency due to chance.
Allele frequencies are not expected to stay the same in a real population.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Speaker Notes:

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Genetic drift

Let the frequency of in next generation = p’
p will not = p’ if the population is finite, because of chance (sampling).
The proportion of deviation from one generation to next should
- Go up sometimes and down sometimes
- Be inversely proportional to N, pop size
- END when one allele reaches 100% of population, a state called fixation.

Speaker Notes:

Frequencies of 0 and 1 are called absorbing states because you can’t go back once you reach those special states.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Allele frequency in finite populations

- One way to see that the infinite population size (=Hardy-Weinberg model) is not compatible with finite populations is to calculate expected frequencies and realize that the calculated value is impossible in real population. If 1 of 10 snails have the recessive phenotype, the estimated recessive allele frequency is 0.316, but 0.30 & 0.35 are the closest possible actual frequencies in population with only 20 (=2N) genes in ten diploid snails.

Speaker Notes:

The square root of the phenotypic frequency will NOT generally equally a possible actually frequency.

GENETIC DRIFT in populations differing by 2 orders of magnitude
Note fixation of both alleles

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Speaker Notes:

Experimental Study of Drift

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Speaker Notes:

Exam 2 Lecture 8

UIC BioS 101 Nyberg

The Neutral Theory

1) There are no fitness differences between almost all of the molecular variation that is detected in populations.
Neutral is the word chosen to describe the lack of fitness differences (functionally equivalent alleles).
2) Amount of genetic variation in a population is determined by a balance between an increase due to mutation, rate =μ, and a decrease due to finite population size (=genetic drift).

Speaker Notes:

Frequencies of 0 and 1 are special. When either of those states are reached there is no more polymorphism.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Fixation (of alleles)

For allele frequencies the values of 0 and 1 are especially important, because a previously polymorphic population that had both A and a alleles now has only one kind, i.e. has no variation.
Alleles that reach 100% are said to be fixed.
Once lost, the only way genetic variation can be regained is through mutation.

Speaker Notes:

0 and 1 are almost called ‘absorbing states’.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Rate of loss of genetic variation
in one generation

If there are N individuals in a population, there are 2N genes. Only if every gene were different could there be as many as 2N kinds.
Sampling from a infinite gamete pool with 2N types, means there is a probability of 1/2N that the second allele drawn is identical to the first one drawn.
F, a measure of genetic variation loss, = 1/2N

Speaker Notes:

F is called the inbreeding coefficient or the probability of being identical-by-descent, IBD.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Genetic Variability Loss
over time

Proportion of variability retained from one generation to the next is: (1 - 1/2N) = 100% minus that which is expected to be lost.
After t generations remaining variability = (1 - 1/2N)t which goes to zero as t increases.
If N = 50 then 2N = 100 so in one generation (1 - 1/2N) = 0.99 = 99% retained
(0.99)40 = 0.67, so in 40 generations 1/3rd of initial variability is lost in population of 50 individuals (each generation).

Speaker Notes:

Try some different population sizes on your calculator.

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UIC BioS 101 Nyberg

Losses and gains of polymorphism

Each polymorphic population has a possibility of becoming monomorphic (= completely homozygous) in the next generation.
The rate that variability is lost is inversely proportional to population size.
Once monomorphic (=only one allele) the only return to polymorphism is via mutation.

Speaker Notes:

What is polymorphism? A monomorphic population has an allele at 100% frequency. We say such a population is fixed (for one allele).

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Bottlenecks

When population is now large, but was very much smaller in the past we say it has gone thru a “genetic bottleneck”.
The population will have less genetic variation than expected from its current population size, if, in the past, its population size was much smaller.

Speaker Notes:

The neck of a bottle is much smaller than the ‘barrel’.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Probability of eventual fixation
of neutral alleles

Each neutral allele has an equal chance of being the one that will eventually become “fixed” via drift, i.e. reach 100% frequency.
If the frequency of a is 0.15, then the probability it will eventually become the only allele in the population is 0.15.
If the initial frequency of a is 0.7, then the probability it will eventually become fixed is 0.7.

Speaker Notes:

Each allele has a equal chance of being eventually fixed.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

GENETIC DRIFT recapitulation

All populations have genetic drift.
The average change in allele frequency (drift) from the value of the last generation is greater the smaller the population size.
Thus drift is inversely proportional to population size.

Speaker Notes:

Inversely proportional means as population size increases the amount of drift decreases.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Neutral Theory recapitulation

Most evolution (change in allele frequency) is the result of:
- Biological populations have a size, N.
- Functionally equivalent alleles are expected to rise or fall by chance in finite populations.
- Genetic drift results in a loss of genetic variation
- Mutation is the only ‘mechanism’ to increase genetic variation.

Speaker Notes:

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Dynamics of Genetic Variation

Natural selection and drift are the main forces leading to allele frequency change.
- Both are expected to lead to genetic uniformity.
Mutation generates new alleles, variety.
Separate populations is vary independently, except if migration between the populations is occurring. Migration makes the populations less differentiated.

Speaker Notes:

Neutral theory does NOT say there is no selection. It says selection on favorable and unfavorable alleles is not very important in understanding the variation that one can detect.

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Geographic structure & migration

Species are composed of populations that are at least partially isolated from one another.
Most of the geographical genetic structure that is seen in populations is the consequence of genetic drift.
Migration between populations reduces the differences that tend to build up thru drift.

Speaker Notes:

Exam 2 Lecture 8

UIC BioS 101 Nyberg

Vocabulary

Genetic variation
Mutation
Neutral theory
Selectively equivalent
Drift
Fixation
Loss of variability

N
1/2N
Monomorphic
Genetic bottleneck
Directional selection
Stabilizing selection
inversely proportional

Speaker Notes: