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Date: Thu, 19 Oct 2000 17:11:18 -0700 (PDT)
From: Nathan Wilson <velosa@cinenet.net>
To: Kevin de Queiroz <Dequeiroz.Kevin@NMNH.SI.EDU>
Cc: PhyloCode@ouvaxa.cats.ohiou.edu
Subject: Re: Hybrid specifiers
Thanks for your reply. It's good to know that my input is valued.
Kevin de Queiroz wrote:
> One thing that puzzles me is Nathan's suggestion "to allow for multiple
> most recent common ancestors in node-based clades." First, I'm unclear
> on which ancestors he has in mind in the example--Y and (Z) or Y, (Z),
> and Z. Second (and more importantly), regardless of which ancestors he
> has in mind, I'm unclear on what he means by allowing multiple
> ancestors. Would the name ("Alpha," to use Gerry's example) be applied
> (separately) to clades stemming from two or more different ancestors
> (e.g., Y and (Z))? Or would it be applied to a group composed the
> clades stemming from those different ancestors? Neither of these
> alternatives seems desirable to me. The first results in ambiguity, the
> second is potentially confusing in applying the name not to a clade but
> to a set of clades.
The ancestors I have in mind are Y and (Z).
When I talk about allowing multiple ancestors I am in a sense changing
what is meant by the term clade. To my way of thinking, the phylogentic
'tree' is not actually a tree at all. Rather it is a directed acyclic
graph or 'DAG'. The difference between a tree and a DAG is that the nodes
within the graph can have mulitple parents. In our discussion so far
these would be the inter-clade hybrids. In actuality the history of life
is locally a strongly connected DAG (within 'species'), but globally it
approximates a tree where the nodes are species.
As I understand the concept, a clade is the set of all descendents of a
particular hypothetical species. For this definition to be well defined
you need a solid definition of species. My personal belief is that
there is no such definition.
However, what is possible is to create a precise definition of interesting
subgraphs within the DAG of life that approximate clades to exactly the
degree to which the so called 'species' actually exist in the DAG. The
really cool thing is that this precise definition is very close to the
node-base clade definition given in the phylocode. The only thing that
is needed is to allow for the potential of multiple parents.
Unfortunately, I have to take off for a long weekend right now, so I can't
quite complete this thought, but hopefully this will clarify things a bit
for you.
Regards,
-Nathan