[Taxacom] Geodorcus sororum

Stephen Thorpe stephen_thorpe at yahoo.co.nz
Sat Jul 3 20:52:50 CDT 2010

Hi Ken,

Yes, Geodorcus sororum is an interesting case, and one for which the relevant taxonomist changed her mind completely between 1961 and 2007 (hopefully as the result of greater accumulated wisdom!)
There isn't male dimorphism as such in stag beetles, but there is male allometry, which may be equivalent for your argument. Another species of Geodorcus (G. helmsi) has a wide latitudinal range, and the northermost males are VERY different to the southernmost males, BUT the extremes are linked by morphological/geographical intermediates in this case. I expect that the same process is responsible for the G. sororum/G. capito distinction, but here there are no intermediates. Yes, it clearly is just a result of the isolation of a population on a tiny islet, but however the distinction came about we can still ponder the question as to possible resultant reproductive barriers between the two populations/species. If, as Holloway does, you postulate reproductive isolation, then you must call them distinct species.
I would use subspecies for a somewhat different scenario whereby two populations have small but consistent differences, but not of sufficient magnitude to suggest reproductive isolation. So, they would be distinct "morphological species", but the same (biological) species...
The main point here is that it doesn't matter how reproductive isolation came about, only that it is postulated to exist ...


PS: Having said all that, Beverley Holloway's husband (Willy Kuschel) recently described a weevil as a full species rather than subspecies just because a trinomen would be too unwieldly!

From: Kenneth Kinman <kennethkinman at webtv.net>
To: stephen_thorpe at yahoo.co.nz
Cc: taxacom at mailman.nhm.ku.edu
Sent: Sun, 4 July, 2010 4:08:57 AM
Subject: Geodorcus sororum

Hi Stephen,
      Before I respond to you and Richard on the subject of
subspeciation and speciation (which can be a semantic minefield), I'll
first comment on Geodorcus sororum.            
      It sounds like a very interesting problem.  I would have most
likely been inclined to name it as a separate subspecies (Geodorcus
capito sororum), and then see if evidence could be found to justify
raising it to full species level.  But I guess entomologists (especially
coleopterists) aren't big on naming subspecies. 
        Anyway, I wonder if this population is the result of having
evolved dimorphic males, and the big males simply won the war (instead
of both forms surviving by using different, but successful, strategies).
If so, I wouldn't be surprised if these larger males could easily
interbreed with females elsewhere in the Chatham Islands.  Is there any
sign of male dimorphism in genus Geodorcus or related genera?


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