Rectifying Evolution and Genealogy

Robert Mesibov mesibov at SOUTHCOM.COM.AU
Sun Jul 14 17:05:23 CDT 2002

Richard Pyle wrote:

"When we see evidence of a gap
between two evolutionarily "sister" populations, how can we predict whether
that gap represents the genesis of a perpetual chasim, versus whether that
gap will eventually close itself up, becoming a "hole"?"

I may be miscounting, but this could be the third time this year I've cited
R.J. O'Hara, Syst. Biol. 42:231-246 (1993) in a Taxacom post. O'Hara
discusses this fascinating issue at length, and shows how the evolutionary,
phylogenetic and biological species concepts all depend on predicting that
certain gaps will never, ever close. However, says O'Hara:

"I do not regard future dependence as a flaw of these species concepts; it
is simply an ineliminable characteristic of them, and I consider Wiley's
explicit recognition of future dependence, under the name of "historical
fate," to be particularly insightful" (p. 242)

What did Wiley say? "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." (p. 18, E.O. Wiley, Syst. Zool. 27: 17-26 (1978))

I suspect that 'closing the mesh' happens to many lineages when a vicariant
event reverses itself. A barrier appears, populations get separated and
start differentiating; the barrier disappears, the differentiated
populations come into secondary contact. Classic Mayrian scenario, but I
suspect that you don't always get reinforcement of new lineage identities.
Vicariant events can separate lots of species at one time, and some of them
might resume sharing genes and amalgamating lineages. Speculating wildly, I
can even imagine that it isn't the duration of the "failed vicariance
event" that's crucial in whether or not a mesh gets closed, it's the mating

Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 64371195; 61 3 64371195

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