Friday, June 03, 2011

Mutations: The Good, the Bad, and the Neutral (Part 4)

We have so far learned from Dobzhansky that:

1.       Mutations are common in nature and are the source of all diversity
2.       Mutations can be really bad, to the point of being lethal, but there is a gradient from bad to good; bad mutations can hide as recessives within a population
3.       Environmental change (or a change in the genetic background) can turn a ‘bad’ mutation good, and a ‘good’ mutation bad.  Mutational value is contextual.

Lesson 4 – A single mutation can have a multitude of effects

Fruit fly sperm cells
Dobzhansky, prior to writing Genetics and the Origin of Species in 1937, studied the sperm of twelve different mutant strains of fruit flies.  (Yes, this is something people actually do.  No, I have never had the desire to do this myself).  He found structural differences from wild-type fruit fly sperm in ten of the twelve mutants.  Yet no one had even suspected that these mutations should in any way affect the sperm!  These were mutations for things like eye colour.  Dobzhansky concluded from this that a single mutation can have manifold effects.  He further studied a mutation that caused fruit flies to have white eyes, and discovered that the testicular membrane, the shape of the sperm, length of life, and viability were also affected.  At the time it was thought that manifold effects were rare, but Dobzhansky concluded that

‘most, and possibly all, genes have manifold effects…In general, there is no conclusive evidence to show that genes have a circumscribed province including only one class of characters or physiological reactions.’

Today we call multiple effects of a single gene pleiotropy, and it is very common.  If a gene produces multiple effects, and a mutation alters the effect of that gene, we can therefore expect mutations to have multiple effects.

As an example of how this works, think about hormones like adrenaline.  Adrenaline affects heart rate, blood vessel constriction, fight-or-flight response, and a host of other things.  Any mutation that affects the synthesis, regulation or recognition of adrenaline would then also affect all of those things.

For evolution, pleiotropy can have interesting consequences.  Imagine that a mutation produces two effects (A and B), which really correspond to two different phenotypes.  Remember that selection does not act on mutations, it acts on phenotypes.   Now, if A and B are both positive, selection will favour the mutation; if they are both negative, selection will disfavour them.  But what if A is positive and B is negative?  This is called antagonistic pleiotropy, and the result is that selection tries to favour A and remove B, when they both have the same source.  Depending on the relative magnitude of the good effect to the bad effect, a mutation that produces ‘bad’ effects could spread throughout a population.

Come back tomorrow for the final instalment on Dobzhansky’s view of mutations.  

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