An answer to paraphyly?

Thomas G. Lammers lammers at FMPPR.FMNH.ORG
Mon Jun 16 07:00:17 CDT 1997


At 12:31 PM 06-14-97 +0100, Graham Thackrah wrote:

>In reply to the recent postings on TAXACOM here are a few of my thoughts.
>
>As far as I understand it, after a speciation event, two entirely new taxa
>are described, if such an event could be witnessed. The "old" species they
>evolved from ceases to exist as far as cladistic methodology is concerned.
>
>If one thinks about it this must be the case. If a species consists of a
>homogenous (with respect to their diagnostic features) population of
>individuals any group of organisms "budding off" would have to disrupt this
>homogeneity. If the entire population of species A is separated into two
>groups then the two groups will share some characteristics and not others.
>When it comes time to describe these two entities one would naturally
>describe two distinct species. Their common ancestor would be the homogenous
>population prior to the separation, NOT one of the extant taxa. There is no
>way to my mind that one of the two groups can be described as an ancestor to
>anything and certainly not to each other.

This is EXACTLY the sort of thinking I have been speaking about!   This
seems so divorced from REALITY.

Okay, let's get down to brass tacks, a concrete example.   We have a
sexually reproducing species of angiosperm growing in Chile, let's call it
Planta chilensis.  Lots of populations, all over the country, near sea level
up to a 1000 m or so.  Like any REAL population, it is genetically variable,
with different individuals variously homozygous and heterozygous at  various
loci.

A handful of propagules are dispersed to the Juan Fernandez Islands, 600
miles off shore, just once.  As a tiny random sample, only a minute fraction
of the species total genetic variation left with them.  Most importantly,
virtually any gene they took with them probably  is present somewhere else
in the populations remaining; it is unlikely that any unique alleles
disappeared with their dispersal.  In the islands, they establish, they
grow.  But conditions are different on the island than on the mainland.
Different substrates, different climates, different competitors, different
predators.   With a genetic bottleneck and strong selection (notv to mention
cessation of gene flow), it is probably not too many generations before the
island populations are significantly different, phenotypically and
genotypically, from their mainland sisters.

Were we to come across this situation after enough generations, we would no
doubt describe the island populations as a distinct species, Planta
fernandeziana, closely allied to mainland Planta chilensis.

BUT... what if we could travel back in time and collect specimens of tthose
mainland populations BEFORE dispersal and establishment of P. fernandeziana?
Back in the herbarium, we compare our time-machine-retrieved specimens to
current ones.  Do we say, "Jeepers!  These specimens from 500,000 B.C (or
whatever)  are a whole different species!  Look at how different they were
before Planta fernandeziana diverged"?

I think not.


Thomas G. Lammers

Classification, Nomenclature, Phylogeny and Biogeography
of the Campanulaceae, s. lat.

Department of Botany
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, Illinois 60605-2496 USA

e-mail:     lammers at fmppr.fmnh.org
voice mail: 312-922-9410 ext. 317

*******************************************************************
"In no affairs of mere prejudice, pro or con, do we deduce
 inferences with entire certainty, even from the most simple
 data,"
                -- Edgar Allan Poe




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