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Does heterogeneity in opsin genes demand selection pressure

C.J.L.Wolf C.J.L.Wolf at ncl.ac.uk
Fri May 4 17:18:20 EST 2001


Many thanks for your reply and the links to the illuminating simulations.

On 4 May 2001, Bob Friedman wrote:

> "C.J.L. Wolf" <C.J.L.Wolf at ncl.ac.uk> wrote in message
> > My question is whether, as is often stated, we must invoke positive
> > selection pressure for the female monkeys that have enhanced colour vision
> > in order to account for the maintainance of several opsin isoforms in the
> > population. If the hardy-weinberg equilibrium applies to genes on the
> > X-chromosome, presumably the proportion of individuals in the population
> > that carry each alternative opsin gene should not change from generation
> > to generation so there's no need to search for a reason for the
> > continuation of established heterogeneity in the population. But I'm sure
> > I remember vaguely some maths in a pop-science book that said that a
> > million years from now we'd all end up with the same surname, as the
> > statistics of surname inheritance tend to select names out at random. As
> > an embryo is more likely to carry an X chromosome from its mother than its
> > father, presumably similar stats should apply?
> The effects of genetic drift are also important.  See:
> http://www.evotutor.org/GenDrift/GD2A.html
> The example you made about surnames -- does it also model mutations?

The model didn't - I believe it was in one of Steve Jones' books. There is
evidence that the 3 alleles in marmosets are ancient (i.e. the genes
diverged many millions of years ago, so are presumably not the result of
recent mutations creating new variants to replace ones that are quickly
eliminated by genetic drift.)

> Did you try a journal search of population genetics studies of the above?
> If there are no studies then you can't presume whether the gene frequencies
> are in H-W equil.

To be in H-W equil. I understand we must assume:
1. No Genetic Drift (Infinite Population Size)

I've no idea how many marmosets there are of each species, but I assume we
could assume it was large (millions)- although marmosets live in groups
perhaps interbreeding's common enough that population = total number of
individuals in an area. (i.e. in a big population formed by many
subpopulations that mix fairly freely, can we assume that N should
be the total number of individuals in all subpopulations - very aware of
my vague use of terms such as 'fairly freely').

2. No Migration (No Gene Flow)

I don't know if there are any wild populations with alternative alleles -
I haven't heard them reported.

3. No Mutation

Presumably false - opsins tend to be labile as changes in AA sequence at
only a few sites may lead to big changes in spectral absorbency. Would
this be at odds with the fact that the 3 alleles appear to have diverged
tens of millions of years ago?

4. No Selection (No Differential Selection)

My essay question. In males that carry just one pigment, there are good
reasons to assume that there's no advantage to carrying any of the 3
alternative alleles. Females may be at an advantage if they carry both
alleles and have improved colour vision, and I can think of reasons why it
may also put them at a disadvantage.

5. Random Mating (No Differential Reproduction)

As above. Females can't know which allele a given male carries / vv.

In other words, other than point 4, based on several assumptions it seems
likely that the HW equilibrium holds. Is there any other way to test it
empirically? (I tried a search for population AND genetics AND opsin* and
didn't find anything that seemed relevant).

The argument is stated...

There are several different alleles present in the wild population

This implies selection pressure for individuals that carry several
alleles in order to maintain heterogeneity in the population
(reference to another paper that says... "therefore there must be
selection pressure to maintain the alternative alleles...").

The most likely explanation is that colour vision is advantageous because 
it allows us to see fruit better.

... It's the 2nd bit I don't follow. I can think of numerous scenarios by
which I think the alleles could be established and maintained in the
absence of selection pressure. So colour vision could just have happened
by chance and there's no need to think up explanations of why it's useful
to us on this evidence, at least. I brought up the X-chromosome issue, as
I wasn't sure if the HW equilibrium applied to genes inherited in this
fashion, and if not, then it would eliminate my dilemma. I can't see
straight out why genes shouldn't be in H-W equilibrium if on the
X-chromosome, though.
> > Kit Wolf

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