[Taxacom] very nice opinion article in today's Zootaxa

Richard Zander Richard.Zander at mobot.org
Fri Sep 30 15:20:50 CDT 2011


Compare:

A molecular data set of say 50 species, represented each by one
exemplar, and 1,000 phylogenetically significant sites gives 50,000 data
points. This is considered a big data set.

But, a morphological data set of 50 species may have each 50 traits
represented (over 250 years of study determining trait distributions) by
an average of 1000 specimens each, and we get 50 x 50 x 1000 = 2,500,000
data points. Say only half the traits were genuinely conservative
(resistant to adaptive change) and we get 1,250,000 data points.

The difference is that the molecular data set results in a cladogram
that does not cluster specimens. The 95% credibility distinguishes
single specimens, so one can easily imagine two or more samples
occurring in one broad morphological distribution of traits. Just
because the two samples differ at 95% or more in credibility is no cause
to postulate two cryptic species.

Only if the distribution of molecular variation of many samples matches
the distribution of morphological variation of many samples is there
genuine support of molecular data and morphological data. If there are
only small samples of molecular data, any match is mere corroboration
("not incompatible with") not support, for the morphological study. A
mismatch means nothing.

The burgeoning number of samples per taxon in barcoding is excellent
news. 

* * * * * * * * * * * *
Richard H. Zander
Missouri Botanical Garden, PO Box 299, St. Louis, MO 63166-0299 USA  
Web sites: http://www.mobot.org/plantscience/resbot/ and
http://www.mobot.org/plantscience/bfna/bfnamenu.htm
Modern Evolutionary Systematics Web site:
http://www.mobot.org/plantscience/resbot/21EvSy.htm


-----Original Message-----
From: James Whitfield [mailto:jwhitfie at life.illinois.edu] 
Sent: Friday, September 16, 2011 5:36 PM
To: Richard Zander
Cc: Doug Yanega; taxacom at mailman.nhm.ku.edu
Subject: Re: [Taxacom] very nice opinion article in today's Zootaxa
Not to argue for using only one sample to stand for a species, or only
one DNA barcode, as I agree with you that it is a bad idea.

But if people would bother to look carefully at the original papers,
they are based on LOTS of molecular samples from each species.  It is
the CONSISTENCY of the molecular or morphological differences that is
critical, not how major the differences are. In this case the molecular
and morphological differences are consistent across many samples within
putative species and do indeed support one another.

Most species descriptions based on morphology (probably 95% + of all
species descriptions?) do not have any convincing analysis associated
with them. We are in the habit of trusting the experts in this case
rather than requiring strong evidence.  Most of the time this works
really well, I'd say.

Jim




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