Re: Standards of Evidence

r norman wrote:
> On Sun, 20 Nov 2005 13:03:17 +1000, John Wilkins <john@xxxxxxxxxxxxx>
> wrote:
>
>
>>Matt Silberstein wrote:
>>
>>>On Fri, 18 Nov 2005 09:37:51 +1000, in talk.origins , John Wilkins
>>><john@xxxxxxxxxxxxx> in <dlj463$26i$5@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>>...
>>
>>
>>>>>>I would challenge anyone to show a modern theorist who thinks that
>>>>>>macroevolution *doesn't* involve processes that aren't part of microevolution
>>>>>>- unless somebody wants to assert that tectonic movement and solar variation,
>>>>>>etc, are microevolutionary processes.
>>>>>
>>>>>
>>>>>Excuse me, but doesn't microevolution involve processes that are not
>>>>>part of microevolution then? I mean, unless you want to assert that
>>>>>weather and landslides are microevolutionary processes.
>>>>>
>>>>
>>>>I probably would treat them as external variables for a microevolutionary process.
>>>
>>>
>>>Different than you would treat them as external variables for
>>>macroevolutionary processes? I strongly doubt that any biologists are
>>>studying (or, more to the point, making contributions to) tectonic
>>>movement or bolide collision.
>>
>>No, you miss my point (that is, I express it badly, as always).
>
>
> Try writing an equation for it !! ;-)
> No you don't express it badly. The situation is complex and with
> quite subtle distinctions. It takes several passes through it to make
> it clear.

Careful, or you'll be seen aiding and abetting the philosophical enemy. They
drum you out of the Academy for that...
>
>
>>In microevolutionary biology (that is, in ordinary population genetics) one
>>treats the external environment as constant - selection occurs, for instance,
>>when there is a coefficient that remains constant enough that the background
>>fitness landscape is stable. So for the purposes of microevolution, these
>>things aren't *processes* but constants.
>>
>>Macroevolution results, I believe, from these background processes not being
>>constant. That is, a description of the case will necessarily have to attend
>>to the rates of change of these factors.
>>
>>>Now I am not denying that macroevolution is either a different subject
>>>of study nor that we can learn distinctly different things, but I see
>>>lots of sloppy and/or misleading statements about this. Lets take one
>>>issue, how diversity is affected by environment size. There is
>>>evidence that the larger the environment, the more diversity we see.
>>>It may be that the species diversity follows the same pattern as
>>>intrapopulation diversity. Or it may be that one speciation occurs we
>>>find a different pattern. That is clearly something we learn by
>>>looking and something we learn by looking both at micro and macro
>>>evolution. But the detailed processes are the same. That plate
>>>movement, for example, connects two areas changing the size is of
>>>interest, but it is not a biological process. Neither is a landslide
>>>increasing the size of a lake. (And lets ignore that the landslide may
>>>be due to an earthquake and so caused by plate movement.)
>>>
>>>
>>
>>Speciation usually results from population structure within the species and
>>the differing ecological regimes each deme finds itself in. Sure, the detailed
>>processes are the same, but the reason why species occur at all has to do with
>>the patchiness of the differing conditions of the demes that end up new
>>species endure.
>>
>>If novel environments or niches occur for reasons that have nothing to do with
>>the genetics of the population, then the subsequent speciation event is due to
>>something other than the genetics of the demes. Whereas if a selection
>>coefficient or ordinary drift changes a population, the relevant variables are
>>all about the population structure and genetics.
>>
>>And something happens when speciation occurs that is more than the sum of
>>internal properties of the metapopulation. In a way this is trivial because it
>>is a relational set of properties - the geography of the two populations,
>>founder and ancestral. But there are also material aspects to speciation due
>>to rather more than the traditional popgen.
>>
>>I am reading Sergey Gavrilets' _Fitness Landscapes and the Origin of Species_.
>>He dissolves a lot of these antinomies, for example between selection and
>>drift. He does this by pointing out that in a sufficiently high dimensionality
>>fitness landscape (which is constituted by the genomic structure of the
>>species as well as the external properties of abiotic and biotic environments)
>>there will be "fitness ridges" throughout most of the landscape of roughly
>>equal fitness, with a high degree of connectivity. Hence, although the
>>organisms of a population are maintained by selection to a certain fitness,
>>the populations can undertake a Markov chain drift through the genome
>>space/fitness landscape.
>>
>>In other words, selection is *not* the polar opposite of drift (apparently the
>>original idea is Maynard Smith's and goes back to 1962). Speciation is drift
>>through fitness space along these nearly neutral networks. But the fact that
>>they drift is not entirely a matter of population dynamics but of the
>>structure of the adaptive landscape imposed on them from without, and
>>speciation often occurs because that landscape changes independently of the
>>population.
>
>
> Since I am mathematically trained, I think of selection vs. drift
> quite differently. The fact is that the transmission of specific
> alleles from parent to offspring is inherently a random process. And
> the survival of a parent and the specific number of offspring a parent
> has and the number of offspring that survive to maturity are all
> random processes. It is just that the probabilities are not uniform;
> parents with one particular set of alleles will have a higher
> probability of reproducing successfully than another with a different
> set of alleles. You can conceptually separate this situation into two
> mathematical terms; a random process with equal probabilities and a
> deterministic process with unequal offspring. Whether or not the
> separation corresponds with the way the "real world" works is a
> different story. It is a very useful separation to make, though.
> Just don't expect it to correspond all that well with
> microprocesses, the mechanisms at work behind the scenes. They may
> well confound the two terms irretrievably.

Selection has always been stochastic. It happens merely to bias the normal
distribution, that is all. But a drunkard's walk through genome space has
previously been thought to be the antithesis of selection - what Gavrilets
points out is that if the fitness "ridge" is roughly equivalent, the
drunkard's walk *requires* selection. That's new.
>
> There are many physical processes that show a similar mix of random
> plus deterministic, or unequal probabilities within a random process.
> Charged particles moving in an electric field (but not in a vacuum)
> are subject to both random (Brownian) motion and a deterministic
> motion due to electrical forces. This happens to electrons in a metal
> or semiconductor or proteins during electrophoresis. Confusingly, the
> deterministic term is called "drift", just the opposite of the way
> biologists use the term.

Interestingly, the term "drift" is deterministic in linguistics, too. Causes
all kinds of trouble when linguists talk about the evolution of languages :-)
>
> So drift is indeed not the polar opposite of selection. Together they
> describe the dynamics of the gene pool.
>
> But there is more going on, as you so well described. There is the
> environment which can well vary. The results of evolution depend very
> strongly on whether the environment is stable for long stretches of
> time, whether it changes gradually or abruptly from one state to
> another, whether it changes in a regular pattern or erratically from
> time to time, whether the changes occur only over many generations or
> within one generation. And that is just the temporal variation.
> Then you add in the same considerations for spatial variation and you
> have completely new evolutionary patterns. Are all these really part
> of the population genetics or are they part of a larger story? That
> is the issue.

There's an operational question (what is it tractable to deal with?) and a
theoretical question (what happens at *this* level that doesn't happen at
*that* level?). I think Matt was saying that the answer to the second question
is "nothing much". I was saying that the answer to the first question is
"quite a lot". How those two relate will have to await Larry to pronounce upon :-)

--
John S. Wilkins, Postdoctoral Research Fellow, Biohumanities Project
University of Queensland - Blog: evolvethought.blogspot.com
"Darwin's theory has no more to do with philosophy than any other
hypothesis in natural science." Tractatus 4.1122

.

Relevant Pages

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    (talk.origins)
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