Farewell to Species - reticulation

Richard Jensen rjensen at SAINTMARYS.EDU
Thu Feb 3 12:15:23 CST 2000

On Wed, 2 Feb 2000, Thomas DiBenedetto wrote:
> But the name of the original taxon was meant to apply to all descendants of
> the common ancestor of the original taxon. So the original name appliesto
> all parts of this example; the singular branch as well as both sublineages
> after the divergence. The original taxon is a higher taxon. You are right
> that there are two species, but there is also a third taxon; the higher
> taxon which encompasses the two species. And the original name must refer to
> the higher taxon.
> Apomorphies are the evidence by which we recognize lineage divergences. A
> lack of evidence may make lineage identification difficult or impossible,
> but that doesnt change the underlying reality that the divergence may have
> happened. If we knew, independently of character evidence, that a divergence
> has occurred, surely we would want to recognize that,,no?


Maybe we're getting somewhere here (and thanks to Pierre Delaporte for a
private message that has provided additional insight).  Let me draw a
simple tree representing my example:

                Q. mar            NT            Q. ark
                   \               |              /
                     \             |            /
                       \ ----------           /
                         \                  /
                           \              /
                             \          /
                               \      /
                                 \  /

So, within the genus Q (Quercus, if you must have a name), mar and ark are
currently recognized as sister species.  Suppose an isolated
metapopulation of mar undergoes rapid evolutionary change producing a new,
reproductively isolated species (NT; perhaps an autopolyploid
segregate of mar). Mar is a geographically widespread species and, after
the origin of the NT, the other populations/metapopulatons of mar
continue as before; however, mar now has no autapomorphies to distinguish
it from NT; NT does have one (or more) autapomorphies.

We started with two species and now we have three.  Mar continues to exist
as a species, ark is unaffected by what has happened, and NT is
a species in its own right.  So, mar, which was to this point a valid
species, now becomes a metaspecies because it can be recognized only by
virtue that it is not NT?  And, what happens nomenclaturally?  Using the
existing system, I would recognize three species: Q. mar, Q. nt, and Q.
ark.  However, given that NT is derived from the lineage including mar,
whatever rank is assigned to NT, mar must now occupy a higher rank.  Given
that mar and ark were initially at the same rank, what happens to ark?
Does it now also "move" to a higher rank to maintain its sister status
with mar, or does it stay at the same rank as NT?

As I read the paper by Pleijel, which I believe started this thread, this
is partly why he has dispensed with species and recognizes only
taxa (based on lineages).  His suggested nomenclature would refer to the
lineages as (if I interpret it correctly) ark(Quercus), mar(Quercus),
and nt(mar,Quercus).

Is it just that I have constructed a convoluted strawman? Or is there
something important here?  We do have "species" of plants that have arisen
via autoploid segregates of another or as hybrids between extant
species, i.e., the new species and the ancestral species are

Richard J. Jensen      |   E-MAIL: rjensen at saintmarys.edu
Dept. of Biology       |   TELEPHONE: 219-284-4674
Saint Mary's College   |   FAX: 219-284-4716
Notre Dame, IN  46556  |

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