[Taxacom] very nice opinion article in today's Zootaxa
jwhitfie at life.illinois.edu
Fri Sep 30 15:59:41 CDT 2011
Another difference is that taxonomists (I am including myself here)
examine, and screen through, all those specimens, but virtually never
actually code them as data for the individuals, they merely look for
differences, so that we actually lack the distributional data of the
characters in a way that we do not for the larger molecular data sets.
So as Richard says, the large data sets in barcoding are welcome (although
why not sequence multiple genes so we'd get a decent phylogeny?), and it
would be nice to see the morphological data sets also treated in a
statistically rigorous manner.
We'll get there, eventually....
> 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
> * * * * * * * * * * * *
> Richard H. Zander
> Missouri Botanical Garden, PO Box 299, St. Louis, MO 63166-0299 USA
> Web sites: http://www.mobot.org/plantscience/resbot/ and
> Modern Evolutionary Systematics Web site:
> -----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.
James B. Whitfield
Department of Entomology
320 Morrill Hall
505 S. Goodwin Avenue
University of Illinois
Urbana, IL 61801
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