Re: population sizes for colonising a planet

On Fri, 04 Nov 2005 19:46:34 GMT, Thomas Lindgren
<***********@*****.***> seems to have said:

>Kristopher <eoslives@xxxxxxxxxxxx> writes:
>
>> Thomas Lindgren wrote:
>>
>> > Mary K. Kuhner writes:
>> >
>> >> This part doesn't work for me at all, because (a) bacterial
>> >> genomes, unlike ours, are remarkably junk-free, and (b)
>> >> any part of a bacterial genome that isn't useful to the
>> >> critter tends to change or be outright deleted extremely
>> >> quickly.
>> >
>> > A related issue concerning colonies (which struck me while
>> > reading PSYCHOHISTORIC CRISIS): even assuming a nice enough
>> > setting, how quickly can one expect the human population in
>> > a new colony to speciate?
>>
>> How long does it normally take in mammals? Mammals with long
>> lives and long generations and relatively low birth rates?
>
>I went and had a quick look in the literature (at the intro level).
>
>First, speciation occurs in three phases:
>
>1. Isolation: a population splits into two for some reason (let's say
> geography here, which is called allopatric speciation)
>
>2. Divergence: the populations genetically diverge.
>
>3. Confluence: the populations meet up again. Either they blend back
> together (no speciation) or they remain separate.
>
>One test for speciation is whether the offspring has "markedly lower"
>fitness than the parents. (At the extreme end, infertility or sterile
>offspring.) If so, the parents are from different species. (Well, I'm
>oversimplifying a bit, but you can see the reasoning.)
>
>Also, as a point of trivia it seems eukaryotes (that's us) tend to
>stop exchanging genes with each other if their genomes diverge by more
>than 2%, while bacteria and archaea, the hussies, stop only at 16%.
>
>Finally, speciation as a rule of thumb seems to be driven mostly by
>natural and sexual selection, not so much by genetic drift. Also,
>small populations diverge quicker.
>
>OK, now for some numbers. These were found from an article by
>G.L. Bush et al, PNAS vol 74, no 9, pp.3942-3946. Sep 1977, also
>conveniently located on the web. (http://www.pnas.org/cgi/content/abstract/74/9/3942)
>
>If I have interpreted the tables in the article correctly, they get
>from the fossil record:
>
>* the primate family as a whole speciated 2.6 times per million years
>
>* a primate species lasts a mean of 500,000 years (carnivores and
> lagomorphs 1.2 million year).
>
>* The highest rate of speciations was among mammals, esp. horses and
> primates. (Horses speciate 2.8/Myr, compared to carnivores 1.1/Myr.)
>
>* There was a lower rate of speciation among the lower
> vertebrates. E.g., lizards: speciate 1 per 3 Myr, mean species
> duration 26 Myr; frogs: speciate 1 per 5 Myr, mean species duration
> 16 Myr.
>
>By examining a phylogenetic tree with a time scale, I guess one could
>estimate what the values would be along our branch if we, say, placed a
>small population of humans on an otherwise identical counter-earth.
>
>Here is one estimation of when other species split off "our" branch:
>
>(http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3934395&dopt=Citation)
>
>mouse 92.6 +- 11.7 Myr
>gibbon 13.3 +- 1.5 Myr
>orangutan 10.9 +- 1.2
>gorilla 3.7 +- 0.6
>chimpanzee 2.7 +- 0.6
>
>Though there are quite a few intermediates after chimpanzee. This
>article (http://www.actionbioscience.org/evolution/johanson.html)
>mentions a predecessor, Homo Ergaster, appearing at 1 Myr, followed by
>H.Sap. arising about 400,000 years ago. That means about 600,000
>years between speciations, or less. I don't know how many
>intermediates there were, either.
>
>Moving up a bit, in this survey:
>
> http://www.geocities.com/SoHo/Atrium/1381/hominids2.html
>
>Homo Antecessor split to H. Heidelbergiensis to H. Neanderthaliensis
>in somewhat less than one million years. This one
>
> http://www.channel4.com/history/microsites/N/neanderthal/facts/tree_evolution.html
>
>shows Heidelbergiensis and Neanderthaliensis appearing with a 300 000
>year spacing. Aaand, here's a final reference, which gives Neanderthaliensis
>about 200,000 years before going extinct:
>
> http://en.wikipedia.org/wiki/Human_evolution
>
>Those with shiny new books on human evolution may wish to correct this
>picture, of course. Anyway, 300,000 years between speciations seems a
>reasonable amateur's estimate for a nice habitat like counter-earth.
>

I was puzzled because you had three homo species that I am unfamiliar
with but you didn't have homo habilis.

I did find other sources with h. heidelbergensis, but some of them
said that it was more properly "archaic homo sapiens." H. ergaster and
h. antecessor just don't appear in material I'm familiar with. Which
means I get two less homo species total than you do, and you haven't
listed any of the australopithecus species -- whose exact number seems
to change every few months as current species are lumped together or
split apart with new data -- or any of the other anthropoid ape
species which I have to admit I'm not familiar with.

I think I'm saying that it's pretty wildly speculative to try to
develop a schedule at which speciation will take place. And that
counting species is probably the wrong way to do it. If I wanted to
quantify it, I'd look at two close anthropoid species -- oh, say, two
gorilla species, or chimps and bonobos -- and count the numbers of
different genes and say that was a representative number of by how
much close anthropoid species differ. And i would know I was
bullshitting. Then I would go find some source for cladistics and I
would look up about how fast mutation in anthropoid genes seems to
take place. Then I would stick my thumb carefully in my cheek and try
to logic out what that would mean for how long it would take isolated
populations to drift far enough to speciate.

And then I would say "what does my story need?"

If my story needed faster speciation, I'd posit that my colonizing
group had a bunch of rare variations to start with, and that there was
a bottleneck at some point, during which some especially rare
combinations survived, and some other events which I can't think of at
the moment, to speed things up.

Lucy Kemnitzer, still
chapter 15 is up:
http://www.baymoon.com/~ritaxis/donor/donorweb/donorindex
also, see Frank's hurricane relief mail:
http://www.livejournal.com/users/franksmail/
.

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