BIO 370 Lecture Notes - Lecture 17: Allele Frequency, Neutral Mutation, Selection Coefficient
The Interplay of Drift, Mutation, and Natural Selection
What did Wright estimate as the probability of a neutral allele becoming fixed in a population?
Probability of fixation = frequency.
So a new neutral mutation always has a very low probability of fixation.
For neutral mutation, frequency of that mutation is very low at the beginning within a
population, low probability of fixation.
J.B.S. Haldane looked at a simple model in which a new, slightly beneficial allele with a fitness
of 1 + s arises through mutation and competes with the wild type allele that has a fitness of 1.
Haldane found that the fixation probability is approximately 2s; therefore, a new beneficial
mutation that confers a 0.1 % fitness advantage has only about a 0.2 % chance of being fixed
regardless of population size.
X seletio oeffiiet = he thee’s ot a huge adatage, ad does’t atte aout
population size = huge change from when considering genetic drift effects.
In a population of 100, for example, drift causes substantial fluctuations in allele frequencies,
ut a e allele ill egi at a feue of i . Relatiel speakig, it does’t hae that
far to go to become fixed due to drift alone, so a low positive s has relatively low effect.
Drift affects population more than selection coefficient will.
In a population of 1,000,000, a new allele will have a very low chance of being fixed initially, but
this disadvantage goes away as it rises in frequency.
Drift is not very likely to cause change in large population. If has a slight advantage selection,
I Haldae’s odel, these effets ael out, ad the poailit of fiatio is oe o less
independent of population size for alleles with a weak selection advantage.
For larger population, it only matters during part of that change in frequency.
While the populatio size is ot pat of Haldae’s epessio fo the fiatio poailit of the
initial mutation (probability = 2s), if we look at an allele present at some particular frequency –
say 1% or 10% or 50% – the population size matters as well.
Broadly, the interplay between selection and drift depends on the strength of the selection and
the population size.
Thee’s itepla / selection and drift, depends on strength of selection and population size.
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Effect of selection changes the rate at which probability of fixation occurs.
Pop size does’t hae to e e ig fo seletio to oeoe dift.
Having infinitely large population, selection will finally have an effect. You might eventually
come to selection and drift as well.
What happens when selection is very strong (e.g., s = 0.5)?
Low going to fixation – drift
High selection going to fixation – selection
Looking at this graph = comparing selection to drift.
The Neutral Theory of Molecular Evolution
In the study of molecular evolution, biologists look at evolutionary change not at the level of
the phenotype, but rather at the molecular level.
A lot of changes are potentially neutral, esp 3rd codon positions.
Studies in the mid-1960s showed that molecular variation is far more common in populations
than previously thought. At the time, most explanations for the presence and/or maintenance
of genetic variation in a population required strong natural selection.
Although most heritable phenotypic differences result in fitness differences and thus are
subject to natural selection, the same might not be true of molecular differences.
According to the neutral theory, most genetic variation within a population is neutral and thus
not subject to natural selection.
Therefore, when a DNA sequence does change over time, some process other than selection is
usually responsible. The neutral theory argues that the critical process is genetic drift.
Let’s look at sustitutios. A sustitutio ous he a e allele aises utatio ad is
subsequently fixed in the population. The substitution rate, usually measured in substitutions
per generation, is defined as the rate at which new alleles become fixed in the population.
Substitution = going to frequency of 1 (fixed) in population.
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