defining species phylogenetically

Subject: defining species phylogenetically
From: (Kim Sterelny)
Date: Thu, 21 Jan 1999 12:02:14 +1300
On 19th Jan, Hugo Phillips wrote (regarding the roughly 9700 bird
species in the world):

"Moreover, this is using the traditional Biological Species Concept. If
you adopt an alternative way of defining species, the Phylogenetic
Species Concept, the total might be closer to 30,000."

Can someone out there provide a simple explanation of the Phylogenetic
Species Concept (or suggest a suitable reference)?
I can't find it in any of my books.


       The basic idea of phylogenetic species concepts is to identify a
species with a segment of a phylogenetic tree between two speciation
events, or between speciation and extinction. Two organisms are members of
the same species if they are part of the same lineage, and no speciation
event has separated them. So organisms alive at very different times can be
part of the same species; Mayr considered the biological species concept to
apply only to organisms alive at the same time, which is one of its
problems. Moreover, on this view, two conspecific organisms could be
morphologically very different from one another, so long as the lineage of
which they are part has not split in the course of its evolutionary
transformation. Phylogenetic species definitions therefore owe us an
account of speciation, and a method for counting lineages. Granted
satisfactory solutions to these problems, phylogenetic accounts have clear
attractions. They may apply to both sexual and asexual species. If a
suitable way of individuating lineages can be developed, a phylogenetic
account might be noncommittal on the causes of "cohesion", of why, most of
the time, two mebers of the same species are morpholgically and
behaviourally similar. The BSC is committted to the idea that the limits of
gene flow explain within species similarity, but there is good evidence
that in different lineages, gene flow, stabilising selection and
phylogenetic inertia have different weights in establishing cohesion.
Limits to gene flow seem less important in plant species, for example. The
idea is that the phylogenetic species concept is founded in the objective
branching pattern of nature rather than in a specific theory of
evolutionary process.

There is a HUGE literature on species concepts (to which I have contributed
a share!) but there is a good overview paper on species concepts in
ornithology at the beginning of volume 4 of Hoya's Birds of the World.

Hope this helps

Kim Sterelny
Victoria University of Wellington
PO Box 600, Wellington
New Zealand

phone: 64/(0)4/4721-000
Fax: 64/(0)4/495-5130

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