[Taxacom] An improved definition of cladogenesis

[Bob Mesibov]

Hi, Hubert.

You wrote:

"Systematics/phylogeny reconstruction is a historical science, interpreting past events, not possibilities. If two lineages are split, they are split, regardless of the possibility they will reunite in the future or whether under artificial circumstances members of both lineages might/... produce (viable, fertile) offspring."

But the issue under discussion here is not how to tell whether two lineages have split (happens all the time in Hennig's 'tokogeny'), but how to decide if the split is permanent enough to be cladogenesis (Hennig's 'phylogeny'). And this is the point that O'Hara discusses so clearly in the 1993 paper (I said 1994 earlier, sorry) I gave the URL for. He argues that none of the leading notions of speciation are free of the 'prospective' view you want to avoid:

1. Wiley's evolutionary species: "A species is a single lineage of ancestral descendant populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate." Fails if the split in descendant populations is only temporary.

2. Phylogenetic species: "A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent". Fails because the diagnostic gap could be closed in future. O'Hara: "Wheeler and Nixon (1990), advocates of the phylogenetic species concept, have recognized this difficulty, and concede that “temporary isolation” of individuals within a phylogenetic species does not change the species status of those individuals (1990: 78). But this leaves us with the same dilemma that confronted the notion of historical fate in the evolutionary species concept: how do we tell whether the isolation is temporary or not?"

3. Biological species: "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups". O'Hara: "Since no population is instantaneously panmictic, our judgement as to what individuals belong to a particular population or reproductive community will always depend to some extent upon our expectation of the future behavior of those individuals and their descendants."

O'Hara: "All three of these species concepts, then—the evolutionary, phylogenetic, and biological—depend upon prospective narration: upon notions of fate, temporariness, and permanence. As a consequence, it is logically impossible for them to be applied with certainty in the present, because they all depend upon the future."
-- 
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery, and
School of Zoology, University of Tasmania
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 64371195; 61 3 64371195
Website: http://www.qvmag.tas.gov.au/mesibov.html