Cladistics and "Eclecticism"
Hovenkamp at NHN.LEIDENUNIV.NL
Tue Feb 5 15:42:15 CST 2002
At 09:26 AM 2/5/02 -0500, Tom DiBenedetto wrote:
>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
>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.....
Thanks - I had overlooked this post as it was part of a discussion about
monophyly of species - a red herring that I try to ignore...
Anyway, as far as I can make out from this post, your answers are Yes, No, No.
>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.
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