Lineages (was real species)

Bob Mesibov mesibov at SOUTHCOM.COM.AU
Mon Apr 19 21:08:09 CDT 2004

Don Colless wrote:

"Bob Mesibov wrote:
That's why Wiley's 'evolutionary species' concept is so useful: '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'. Since we can never be sure
what the historical fate of a particular lineage will be (it hasn't happened
yet), describing that lineage as a species, i.e. an evolutionary species, is
always a judgment call. It's an hypothesis, and explicitly so.

I find a problem here with the term "lineage".
De Queiroz defined one fairly precisely, but I don't think that is what is intended here.
Could someone enlighten me?"

Bob: I quoted Wiley from his 1978 paper on evolutionary species in Systematic Zoology.
If I'm not mistaken, this antedates De Queiroz' work by about 10 years. Nevertheless,
I think both would accept the broad definition of a lineage as 'an historical sequence
of ancestors and descendants', a phrase I pinched from Mike Crisp's glossary of
cladistic terms. However, there would be a fair bit of qualification needed for
different uses of the term 'lineage'.

An operational definition could be harder to come by. The op def of a 'species' as
'something which an experienced systematist calls a species' isn't a good model,
because geneticists and phylogeneticists might recognise different lineage
relationships in the same set of creatures!

In any event, as I suggested above, both 'species' and 'lineages' are hypotheses,
in my humble opinion. All I know are individuals and their breeding networks.
All higher-level and longer-term relationships are inferred from morphological,
genetic, biogeographic, etc data. No matter how fervently I believe in my inferred
'species', I should be prepared to be dead wrong. Exactly that happens to
rock-solid morphological species when genetic analysis reveals cryptic speciation.

It's interesting that many biologists see such revelations as the discovery of
'true' species among 'false' ones. The underlying reality is a vast, intricate history
of individual organisms giving rise to other individual organisms through time.
We simplify the picture by picking out particular threads of relationship and dealing
with them as units ('species' for the taxonomist, 'monophyletic taxa' for the
systematist). It's very easy to forget that the underlying reality is not, in fact,
simple and is, in fact, unknowable. We may be good guessers, and we're certainly
good arguers, but the subject matter consists of hypotheses. Some of these hypotheses
are very useful, and I personally won't stop describing millipede species because
I don't believe they have a reality separate from the individual specimens I'm
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery
Home address: PO Box 101, Penguin, Tasmania, Australia 7316
Home phone: (03) 6437 1195

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