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Re: courses




On Sunday, December 8, 2002, at 02:45 AM, Philidor wrote:

You might want to clarify '...character acquisition is not random.' If it
is not random, then it has direction, no?

I was talking about characters that would be used in a morphological cladistic analysis (which presumably would have already undergone selective pressure), not random mutations. No direction, it's just that characters - as pieces of morphology - must be functional, which reduces the possible characters any organism can evolve (and pathways too). This reduces the improbability of matching sequences, the improbability cladistics relies on to work.


What I had in mind in my comments, aside from their direct import about the
meaning of random, is the idea that if characters arise at random to be
selected for as a kind of weeding out process (or just held as neutral),
then by definition similar species with different ancestries will have some
similar mutations, and that could mislead you about a connection between the
species. (There are physical constraints to mutation.)

True, and exactly my point too.

There's also the fact that something you see as a significant character
linking two species together may be in actuality a mutation that showed up
for only a short time, even in a single individual.
The likelihood may be small that chance will produce exactly matching
mutations, but given enough time, any speed of mutation will produce a
match. The 'improbable', the fluke, happens.

The chance-like acquisition of characters makes cladistics work, but it will of course occasionally mislead. Cladistic analysis can produce wrong results, obviously. But I think that this has more to do with biomechanical constraints on character development than occasional flukes.


Unless you allow for some
functional significance or other linkage between characters in an
individual...

Well it'd be nice to have a synthetic approach. I'm sure it can't be _that_ hard to figure it out - but I'll need some beer. :-)



John Conway, Palaeoartist

"Do I contradict myself? Very well, then I contradict myself, I am large, I contain multitudes." - Walt Whitman

Systematic ramblings: http://homepage.mac.com/john_conway/
Palaeoart: http://homepage.mac.com/john_conway/_palaeoart.html
Skeletals: http://homepage.mac.com/john_conway/_skeletals/