Re: A simple model exhibiting Haldane's limit


"William Morse" <wdmorse@xxxxxxxxxxxx> wrote in message news:e95ppk$qea$1@xxxxxxxxxxxxxxxxxxxxxx
"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in
news:e939uh$2men$1@xxxxxxxxxxxxxxxxxxx:

I will also observe that reducing generation time does not directly
increase fitness (and would definitely not do so for a species with a
fixed number of offspring, like my slime mold cell. In fact,
shortening the generation time is a mistake in a situation in which
adaptation is not keeping up with a deterioration in the environment
and populations are decreasing. Even for my slime mold, decreasing the
generation time is a good idea only for those cells that are already
so fit that they almost never die while growing to reproductive size.


Well, in that case would decreasing the generation time help when the
population is rapidly increasing?

Yes. And if the selective expansion phase is followed by a non-selective
population collapse (with the survivors being randomly selected without
reference to differences in generation time), then you can repeat the
expansion. And again, and again. In such a selective dynamic there
can be long term selective pressure in favor of shorter generations.

(Of course, the selective dynamic favoring longer generations is just
the opposite - a series of selective population declines with some
non-selective population expansions to permit repetition. How to
get a non-selective population expansion that doesn't automatically
favor short generations? Well, colonization of a new environment by
a small founding population without much genetic variation for generation
time would do the trick.)

The situation I am thinking of is after
an extinction event. The remaining organisms might in this case be engaged
in a Haldane limit rate race. It has been observed that the species in a
family that survive an extinction event tend to be those with the smallest
individuals. I have attributed this to a tendency for species with small
individuals to have larger populations, and so be more likely to survive
the crash phase of an extinction event. But perhaps it is in fact due to
species with small individuals being more easily able to reduce their
generation time, and thus being able to outcompete species with large
individuals in a Haldane limit rate race to invade new niches during the
rebound phase following an extinction event.

I'm not sure that bringing Haldane's limit into this discussion helps.
And in any case, a species will only be able to evolve to a reduced
generation time long after the extinction event. So better evolvability
doesn't explain differential species survival through the extinction
event. And, in any case, I don't see why it should be easier to reduce
generation time from 4 years (say) to 3 than it is to reduce generation
time from 8 years to 6. Why does being small help here?

I like your first explanation for why small species survive. Or, if
you want an explanation for why small, short-generation-time species
preferentially radiate after the extinction, it may be simply that they
can most rapidly build up their populations and exploit the new
opportunities. Evolution of life history properties may not come into
it at all - at least not in the immediate aftermath of the extinction.


.

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