[Taxacom] Paraphyletic species: Crocodylus niloticus

[Michael Heads]

Hi Dan,

 
you say: 'one can only conclude that a C. niloticus traveled from Africa to NW in the Pliocene'. But there is another possibility. The fossils may represent minimum ages, i.e. the groups cannot be younger than the Pliocene, as shown by the fossils, but the groups themselves may be older than their oldest fossils. 

 
Michael Heads


Wellington, New Zealand.

My papers on biogeography are at: http://tiny.cc/RiUE0

--- On Sat, 14/5/11, Dan Lahr <daniel.lahr at gmail.com> wrote:


From: Dan Lahr <daniel.lahr at gmail.com>
Subject: Re: [Taxacom] Paraphyletic species: Crocodylus niloticus
To: "John Grehan" <jgrehan at sciencebuff.org>
Cc: taxacom at mailman.nhm.ku.edu
Received: Saturday, 14 May, 2011, 8:57 AM


Hi John,

Paraphyly: The article suggests that C. niloticus is paraphyletic
because eastern and western populations constitute independent
lineages in their reconstructions: eastern populations are more
closely related to New World species than to western populations. It
is only an indication of paraphyly as the reconstruction is based on
mitochondrial genes.  These are maternally inherited, thus an
alternative hypothesis may be coined that females stay more at home
than males (one can look at nuclear genes to test this hypothesis).
It is also worth noting that 6 specimens hardly constitutes
comprehensive sampling to definitively conclude paraphyly of the
species, in my view this is more of a particularly striking piece of
evidence.

DIspersal: The oldest C. niloticus fossils are Pliocene, and the
oldest New World Croc fossils are also Pliocene. Because the branching
pattern revealed by this study puts C. niloticus + NW crocs in a
well-supported clade, one can only conclude that a C. niloticus
traveled from Africa to NW in the Pliocene, i.e. around 3.6 Mya. No
molecular clock needed, and in fact the authors did not use any
implementation of molecular clocks in their analyses.

I do not understand how this example of phylogenetic evidence combined
with the interpretation of available fossil records does not
constitute a convincing case of transatlantic dispersal.

Kind regards,

Dan

On Fri, May 13, 2011 at 4:16 PM, John Grehan <jgrehan at sciencebuff.org> wrote:
> It's not an example of transoceanic dispersal at all. That's just the
> usual misrepresentation of molecular clock dates as actual or maximal.
>
> The article did not give any indication of paraphyly. Please describe.
>
> John Grehan
>
> -----Original Message-----
> From: taxacom-bounces at mailman.nhm.ku.edu
> [mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Kenneth Kinman
> Sent: Friday, May 13, 2011 4:03 PM
> To: taxacom at mailman.nhm.ku.edu
> Subject: [Taxacom] Paraphyletic species: Crocodylus niloticus
>
> Dear All,
>       A recent paper indicates that Crocodylus niloticus is
> paraphyletic with respect to the clade of Crocodylus species in the
> Americas.  Not only a great example of a paraphyletic species, but also
> transoceanic dispersal as well.  Link to news story is given below.
>               ----------Ken
>
> http://www.newscientist.com/article/dn20464-crocodiles-swam-the-atlantic
> -to-reach-america.html
>
>
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-- 
Daniel Lahr
-------------------------------------------------
PhD candidate
Organismic and Evolutionary Biology
U Massachusetts- Amherst
319 Morrill Science Center, Amherst
Amherst, MA 01003
413-585-3881

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