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Date: Wed, 29 Aug 2001 13:03:36 -0300 (ADT)
From: "Alastair G. B. Simpson" <simpson@hades.biochem.dal.ca>
To: "Jonathan R. Wagner" <jonathan.r.wagner@mail.utexas.edu>
Cc: PhyloCode mailing list <phylocode@ouvaxa.cats.ohiou.edu>
Subject: Re: Apomorphy-based definitions
Jonathan Wagner wrote:
Actually, I would regard species as being self-bounded, and those
boundaries as (necessarily) "fuzzy." This is entirely analogous to
determining the most recent common ancestral organism of two individuals:
the most recent common female ancestor of myself and my sister (at the
organismal level) is our mother, despite the grey area surrounding the
point
in space and time where she became an individual distinct from her mother,
and despite the grey area surrounding the point in space and time where we
became distinct from her. Likewise, no one seems to be bothered with the
notion that some cells in my mother's body might share a more recent
common
(cellular) ancestor with my sister than with me (although this seems
unlikely to me). That is a question which is best addressed at the
cellular
level, and not the organismal level.
To return to your point, I find that looking at clades in terms of
species (as individuals) clears up the point rather nicely. Would it be
possible for you to clarify how ambiguities in the recognition of species at
and around speciation events affect the issue at hand (which, I believe,
involved difficulties in recognizing ancestors)?
Alastair writes in reply:
To me a crucial difference between treating people as individuals (despite
being composed of lots of related cells) and treating species as
individuals (despite being composed of lots of related organisms) is that,
in the former case, the succession from one individual to the next is
marked by an extreme bottleneck (and, as sexual organisms, a co-incident
'hybridisation event'). This gives a very obvious place to mark the
arising of a new individual that few reasonable people would dispute.
In the latter case there is no a priori expectation that a species lineage
changing anagenetically over time will be punctuated
by such profound marking events: i.e. most of the time it will be much
much harder to decide precisely when a new individual (ie. new
'species') has emerged.
So, If you tie the origin of a clade to the origin of a species (as you
necessarily do when you treat a species as the 'individual', however
identified, from which the clade arose), it would seem to me inescapable
that any difficulty in determining the point of emergence of the species
(as an individual) would equate to a difficulty in determing the point of origin
of the clade.
At this point I start to flounder a bit because I dont know how people
propose to actually identify species (within a 'species are lineage
segments' or related model): I cant offhand think of a way of doing it
without leaving wide uncertainty zones between species ('fuzz'), unless
you just treat them as clades which are 'truncated' by the emergence of
descendant species (wouldn't 'paraphyletic taxa' buffs love that!). Does
anyone who has actually thought in depth about species definitions want to
comment?
Alastair Simpson