Neutral and Selectionist Theories

What is the neutral theory?

That most mutations at the molecular level (especially in non-coding regions or silent sites in coding regions) are neutral (they do not affect fitness)

Who proposed neutral theory?

Motoo Kimura

How did neutral theory come about?

In the 1950s-60s scientists observed more genetic variations in populations than expected under models driven purely by natural selection. This raised questions about how organisms could tolerate so many mutations without significant genetic load (the burden of harmful mutations)

How do neutral mutations accumulate over time?

Due to genetic drift (random changes in allele frequencies), not selection

What comparison does the theory use?

• Selection Coefficient (s) vs. Effective Population Size (Ne):

  • The theory uses the comparison:
    s <> 1/(2Ne)

    • If s < 1/(2Ne), drift dominates → mutation is effectively neutral.

    • If s > 1/(2Ne), selection can act on the mutation.

    • S represents selective forces

    • 1/2Ne represents the power of genetic drift

What does the neutral theory offer?

A theory as to why genomes can tolerate many mutations and offers the means to test the hypothesis of selection

What is there a fine balance between?

Selection and drift - most mutations are neutral meaning that they have no benefit and no disadvantage

What do the neutralist-selectionist debate agree on?

• That most mutations are delerious and are removed

• That some mutations are favourable and are fixed

What does the neutral theory suggest?

• Advantageous (adaptive) mutations are very rare

• Most of the amino acid changes and polymorphisms are neutral and created by genetic drift

• The concept of the molecular clock

What does the selectionist theory suggest?

• Advantageous mutations are more common

• Molecular evolution is dominated by selection

• No molecular clock