Cladistics and "Eclecticism"

Thomas DiBenedetto tdib at OCEANCONSERVANCY.ORG
Tue Feb 5 09:26:50 CST 2002

-----Original Message-----
From: P.Hovenkamp
Two questions, well, three actually:
1. If a group of people (say, a mixed group of 100 men, women and children)
embarked on a spaceship and were to settle on Mars - would that effectively
split Homo sapiens into two species (assuming contact would be scarce and
limited to raw materials, not frozen sperm etc.) the moment the spaceship
took off?
2. If a group of people (say, a mixed group of 100 men, women and children)
embarked on a plane which later crashed into a high mountain in the Andes
(alas, no survivors) - would the situation to us survivors on the ground
(and in other planes) be different in any way from the situation we would
have had in the first case?
3. Considering the fact that (2) is a fairly common occurrence, are we
still in the same species as we were this time last year?
 I addressed this issue in a post a few weeks ago. Would you mind greatly if
I save a little time and simply reprint my response? I think it addresses
the concerns you have - if not I'll try again.....

from 1/16/02-
If two individuals in a species simply die, then clearly the species is not
affected in any way that has any real consequence for our conception of what
the species is. So I understand of course the argument that if these two
individuals, instead of dying, go off to the other side of a mountain and
start a new isolated breeding population, that then evolves into a new
"species", the "parent" grouping is similarly unaffected. So it is easy, and
seemingly logical to conclude that "species" are not necessarily
monophyletic - for here we have a species that budded off a new species, and
yet persists effectively unchanged (i.e it remains the same "species").
But, I would argue, the entity that we judge to be unchanged is a "species"
in all of the non-systematic meanings of the word. From a systematic point
of view, we demarcate groups (species) relative to other groups. The
criterion we use to delineate groups are the relationships that we infer to
exist between the indiviuals in the groups, and between the groups
themsleves.  Although we necessarily use characters as evidence of these
relationships, it is the relationships that are the ultimate criterion.
Before going over the mountain, the two individuals in our example had a
certain type of relationship with other members of the group, and these
relationships caused us to recognize that they all constitute a single
species. After the journey over the mountain, the groupings on either side
have this same relationship within their group - there are now two groups,
two species. Is either one identical _in terms of relationships_ to the
parent group that existed yesterday? I would say - clearly not. In fact they
both are made up of individuals that are descendants of the individuals in
the parent - and this descendant relationship is exactly the same in both of
the two groups. In addition, the parent species had a historical
relationship with some fourth group not previously mentioned - that other
group being the one that shared a most recent common ancestor with the
parent. But this type of historical relatioship is what the two new species
share with each other, not with that fourth group. Clearly there is a novel
set of relationships throughout this situation.
I am not saying that systematists must reject non-systematic meanings of
"species" - but perhps we need to do that when we are actually doing
systematics. In systematics, "species" are taxa, and they are defined in
reference to the relationships within and between the individuals and groups
of individuals we percieve. Natural species-level taxa are monophyletic in
the same way that higher taxa are - it is only if we forget about the
criterion of relationship do we begin to concieve of "species" remaining
unchanged while some of their descendants form new species - i.e. of species
being paraphyletic. When relationships change, relationship-based taxa
change, even if an ecologist can't tell the difference.

Tom DiBenedetto

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