Epistasis and Linkage - was: Ernst Mayr: Where Are We (1976)


"John Edser" <edser@xxxxxxxxxx> wrote in message news:d7q4jm$13ss$1@xxxxxxxxxxxxxxxxxxxxxx
>
>
> "Perplexed in Peoria" jimmenegay@xxxxxxxxxxxxx
> > JM:-
> > ... it is an "empirically true fact" that epistatic gene fitnesses
> > (as you seem to use this term) are not heritable.

[snip Edser comments to which I have no useful response]

> > JM:-
> > There do exist models of two epistatic loci dependently selected at
> the
> > organism level.

[snip more Edser comments to which I have no useful response]

>
> > JM:-
> > The key issue in such models is the degree of linkage
> > (segregation independence) between the two loci. It turns out that if
> > the loci are unlinked (on different chromosomes, say), then
> > **under the assumption of random mating**
> > the process of recombination undoes any linkage disequilibrium created
> > by selection.
>
> JE:-
> Linkage and fecundity rates (the rate of reproduction of sterile
> immatures) determine the probability that two alleles at two different
> loci may be inherited together within just a sterile immature body. When
> a female tick, which can only reproduce itself once, mass produces about
> 10,000 sterile immatures and then dies, many recombination events are
> represented in these immatures. What matters is which of these 10,000
> are raised to fertile adulthood and _exactly_ how is each gene is
> actually selected in nature. In this species only about 2.something
> sterile immatures are mostly raised to fertile adulthood, i.e. just a
> tiny fraction of the fecundity rate. The mass of infertiles that die
> along with their gene combinations are only subject to a process of sub
> selection and not selection because they are only dependently and not
> independently selectable. This means that high fecundity rates can be
> advantageous because they allow the less costly act of sub selection of
> the better epistatic gene combinations. Sub selection is more efficient
> than selection because a selection of them would require immatures to be
> raised to fertile adulthood so they acquire an independent fitness.

You raise an interesting point here. I think that your analysis is wrong
but I would appreciate comment from someone who understands the issues
better.

The population of fertile fecund female ticks (and their mates) is
presumably far from linkage equilibrium because selection for
epistatic gene fitnesses has moved the population away from linkage
equilibrium. This population then gives birth to a much larger population
of infertile immature ticks. This immature population is presumably
much closer to linkage equilibrium due to the processes of recombination
and fertilization. Then nature takes its course and selection takes its
toll (even though you choose to call it "sub selection" for some reason),
and we end up with something much like our original population -
small again, but back far from linkage equilibrium. This second
generation population might, by stretching the terminology, be said to
have "inherited" epistatic information from its parents.

However, ISTM (and this is the point I would appreciate comment upon)
that this kind of "inheritance" is not really inheritance at all. The
"epistatic information" was created by selection acting on the population
of infertile ticks. It was not "inherited" in the sense needed in order
for NS to act cumulatively. (As "proof" of this, notice that even ticks
whose parents did not have the "epistatic information" may have it after
recombination, fertilization, and selection.) So, even with John's
eccentric choice of sexual maturity as the starting point in the life-cycle,
the epistatic information is mostly not heritable - and therefore we are not
doing violence to the truth by ignoring it in our models of evolution.

Comments?


.

Relevant Pages

  • Re: Epistasis and Linkage - was: Ernst Mayr: Where Are We (1976)
    ... >> Linkage and fecundity rates (the rate of reproduction of sterile ... >> immatures) determine the probability that two alleles at two different ... >> selection and not selection because they are only dependently and not ... > have "inherited" epistatic information from its parents. ...
    (sci.bio.evolution)
  • Re: Understanding and Testing TDF (Total Darwinian FItness)
    ... > which only about 1.something survive to fertile adulthood then intense ... You seem to be saying that highly fecund species have intense ... selective pressure on immatures, ... > intense level of survival sub selection but then pass the one definitive ...
    (sci.bio.evolution)
  • Re: Reproductive excess model
    ... >>> independent of the reproductive excess. ... >> It is impossible to separate the fitness of alleles from ... > and Haldane) treats all selection as "survival selection". ... > acts by killing off unfit immatures. ...
    (sci.bio.evolution)