[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: A bit of cladobabble



Original Message by Philidor
Saturday, 28 December 2002 15:00

> When you say 'much more' you could be referring to characters by simple raw
> count or to characters significant in distinguishing related taxa.

Simple raw count. Sadly this is the only thing one can reasonably do in 
morphology. In molecular biology, things like "maximum likelihood" is 
possible, but how much more probable is it -- in percent! -- that teeth are 
lost than that a tail is lost?

> And yet, we're reading (with interest) about a dispute where certain
> characters are stated to be more significant than others diagnostically.

Actually not. Each side is trying to bring up greater numbers of characters 
which the other has supposedly overlooked.

> It's this contrast between total number of characters and asserted
> significant characters that creates the apparent contradiction.  Characters
> which are not significant create the homoplasy,

Homoplasy can happen everywhere. To use the example above, teeth have been 
lost more often than tails, but the latter has happened repeatedly, too.

> but how do we know they are
> not significant?

We don't. Therefore "we" don't try to seperate significant and insignificant 
characters a priori, instead we let the analysis decide on the basis of raw 
numbers.
        Of course trees in which supposedly significant characters evolve very 
often 
are subjectively considered suspect by many. But then their goal is (ideally) 
to show that a tree with more characters and more taxa produces a less 
suspect result, not simply to keep saying "I don't buy it".

> This is not an example working simply from the
> tree-with-the-fewest-steps model.

Of course it is. :-)