Re: Felsenstein and reproductive excess


"William Morse" <wdmorse@xxxxxxxxxxxx> wrote in message news:d8lqof$qkp$1@xxxxxxxxxxxxxxxxxxxxxx
[snip much]
> ... there is
> probably no such thing as a universal definition of "reproductive
> excess", at least if we want to use that definition to determine the cost
> of substitution ...

I think I agree. I will say this for Walter's infuriating coyness.
It is forcing me to think about these issues more deeply than I otherwise
would.

A natural and obvious definition of "the reproductive excess of a population"
can be formulated in the case of a semelparous organism with non-overlapping
generations if all selection is survival selection. In such a model, the
deaths of the post-reproductive parents are not Haldane's "selective deaths",
so no useful substitution gets accomplished by these deaths. The only
"useful" deaths are the deaths of the unfortunate immatures which don't
make it to the next population census. Those deaths ARE selective. They
accomplish substitution. And clearly, nature can only "afford" to take
those deaths out of the "reproductive excess". Wirt's computer model tells
the same story.

But now let us complicate this model by adding fertility (or is it
"fecundity"?) selection. Some types of individuals are better at reproducing
than others. Now, not all of the work of selection is accomplished by
"selective deaths". Some of it is accomplished by "selective failure to
produce as many offspring as your peers". So what is our definition of
"reproductive excess of the population" for this situation? If we continue
to use the old definition, then it is no longer true that all of the work
of selection comes out of the "excess". Or, if we want to save the "rule"
that substitution is limited by the "excess", we can choose to define the
"reproductive excess" of the population as the excess of its most fertile
type. Those "selective failures to reproduce as many offspring" now act
exactly like "selective deaths". And, again, they have to come out of the
"excess".

But then my head starts to hurt when I try to extend this further to
iteroparous organisms with overlapping generations.

Which is the right course to take? My current thinking is that the best
course is to discard the whole concept of "reproductive excess" in any
but the simple first case. It was a useful concept in that model, but
it is not useful in more complicated models. However, I would be willing
to revise this thinking if Walter were to present us with a definition of
"reproductive excess" which is applicable in a wide variety of models.


.

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