Re: Remine & Haldane on sci.bio.evolution


Friar Broccoli wrote:
As I understand this, the problem of deleterious mutations was not
what Haldane was concerned with. (Please correct me, if you know
I am wrong.)

You are right. But the problem of deleterious mutations has become
tied up with the Haldane problem as a result of later work. The
problem
Haldane was concerned with is sometimes called 'substitutional load'.
The problem of removing recurrent harmful mutations is called
'mutational
load'. These two 'loads' (and some others) all take a cut out of
natural
selection's total 'budget', and both can possibly lead to a limit on
the
amount of evolution that natural selection can provide.

He was concerned with the rate at which an existing
beneficial mutation could be fixed in a population under high
selective
environmental pressure, (ie really bad stuff happening in the
evironment
like say a change in pH in a marine ecosystem)

Not true. The question of how fast the frequency of an existing allele

changes was dealt with adequately by Fisher and others. Fisher also
showed, under the assumption that there aren't too many loci under
selection and that the selection coefficients are fairly small, that
the
rates of change at the different loci don't interfere with each other.

And it doesn't much matter in Fisher's math whether the beneficial
allele is beneficial because the environment has changed or whether
it is beneficial because it is just a wonderful idea which has never
been
tried before.

Haldane addressed a slightly different question. He investigated what
would happen if you really pushed against Fisher's assumptions.
Haldane
tried to look at what would happen if you had hundreds or thousands of
beneficial alleles all moving toward fixation at once. It is no longer
provable that the alleles don't interfere with each other. There might
even be a limit on total throughput.

Thus, one of the many problems his analysis faced, was the fact that
not all (and probably very little) evolutionary change occurs under
such adverse conditions. Most change seems to happen under very
favourable conditions, like after all your competition has just been wiped
out by a catastrophy and there are open ecological niches all over the place.

In short he analysed a very unusual situation (change under high
selective pressure), and then generalised it to the entire process

Are you saying this because you have studied Haldane's paper, or
because
you just don't like ReMine and want to blow him off?

Adverse conditions, as such, are not an essential part of the problem
Haldane was looking at. But there are real difficulties modeling a
situation
in which lots of beneficial alleles are becoming fixed in a relatively
short
period of time. The fitness of the average member of the population is
increasing in this scenario. The standard models say that if your
fitness
is higher, then you have more children. But everyone's fitness is
higher,
so everyone is having more children. So why doesn't the population
size
explode? If you want to address Haldane's question using the usual
kinds of models, then you are FORCED to make the assumption that the
environment is getting harsher. That is what holds the population size
down.

So don't see what Haldane did as only applicable to a particular kind
of
harsh-environment evolutionary scenario. That the environment gets
harsher
can be seen as the inevitable (assuming roughly constant population
sizes)
consequence of the adaptive evolution of the population, not the cause.

.

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