[Taxacom] Paraphyletic species: Crocodylus niloticus

[John Grehan]

No doubt others will have their take on this that will be quite
different. With a widespread 'species' one might have a section of the
ancestral range differentiate sufficiently to call it another species.
One might then say that the part that did not differentiate is the
original species and it is therefore paraphyletic. Or one could say that
it is now a new species also, because the original species encompassed
the one that became different - all this assuming that when one became
different the other did not in any way change from the ancestral
condition. Either way, both entities, the one the changed, and the one
that did not, came from a unique common ancestor that encompassed both. 
 
A little more complex is the situation where there are multiple
differentiations over a widespread range. In theory, those that share a
most recent common ancestor within that range will be identifiable by
their sharing certain biological features in common (whether that be
morphogenetic or molecular in character) not shared by others. However,
one sometimes finds situations where two taxa are most closely related
by they are separated by more distantly related taxa. This phenomenon
has been identified in the literature and Croizat referred to it as
'wing dispersal'. The usually response is to say that the entities in
question are not really most closely related, or if they are efforts are
made to invent dispersal to connect them. The other possibility is that
the entities evolved through  recombination of ancestral characters such
that they are all as closely related to each other in time, but
biologically relationships vary according to the biological characters
that link them together regardless of their geographic location within
the ancestral range.
 
John Grehan

________________________________

From: Stephen Thorpe [mailto:stephen_thorpe at yahoo.co.nz] 
Sent: Friday, May 13, 2011 10:52 PM
To: John Grehan; taxacom at mailman.nhm.ku.edu
Subject: Re: [Taxacom] Paraphyletic species: Crocodylus niloticus


At any rate, there are some conceptual problems here: firstly, surely
species very commonly evolve from a widespread parent species, by way of
a small number of individuals becoming isolated in a new place (either
by vicariance or by dispersal)? Surely, that fact *alone* doesn't make
the parent species "paraphyletic"? Surely, if the parent species
diverges further *as a whole*, then it becomes monophyletic? But, it
could still retain a certain amount of original geographic variation, so
the daughter (sister?) species could still better match parent
individuals from near the source of the vicariance/dispersal event which
gave rise to it, thereby seeming "more closely related" to parent
species from that area? Alternatively, the parent species could diverge
geographically within its own range, and so individuals from some place
could become "different" ... I am still trying to figure out all the
possible scenarios and their consequences for the monophyly vs.
paraphyly of the parent species ...
 
Stephen