Farewell to Species - reticulation

[Hubert Turner]

On Mon, 14 Feb 2000 22:14:24 -0800, Curtis Clark <jcclark at CSUPOMONA.EDU> wrote:
>In the case of species that contain long-lived
>clones, *some of the same individuals may still be present*; it would be
>extremely inconvenient to say that they switched species just because some
>peripheral population diverged.
I would say that the lineage a specimen belongs to is decided at the
birth of that specimen. If a specimen (a gravid female A) disperses
to an uninhabited island, but her equally gravid twin sister B stays
behind, and a new population is founded on the island which is
isolated (and remains so) from the population on the mainland where
the founder originated, there is a permanent split in the
genealogical network: all descendants of A forming one lineage, all
descendants of B part of the other (A and B might even have mated
with the same male C). If the moment of splitting is taken as
decisive, all descendants of A belong to the island taxon while all
descendants of B (together with all their conspecifics) belong to the
mainland taxon, if we are willing to distinguish between the two.
Whether the island lineage, the mainland lineage, and the ancestral
lineage (containing specimens A, B and C) can be recognised and might
deserve separate status at some taxonomic rank or not is another
matter. Some time after the split, the island lineage and/or the
mainland lineage may acquire new character states (evolutionary
novelties, i.e. mutations, may have gone to fixation) allowing them
to be distinguished from each other, and maybe from the ancestral
lineage.  In term of the phylogeny, those states are the apomorphies
that should be assigned to the branches (and thus to the clades that
descend from them), even though the individual specimens that are
part of those branches do not all display those apomorphies (e.g. the
first-generation descendants of A and B may not yet all carry the
character states that are later to become fixed in the respective
lineages). Thus, it is not be possible to identify all specimens
correctly as belonging to either the island branch or the mainland
branch (or even to the ancestral branch) on the basis of the
character states they display, and not all specimens can be
classified correctly as far as their position in the genealogy is
concerned. For phylogeny reconstruction, only the fact that we can
assign apomorphic character states to branches (clades) matters.
Phylogenetic systematics is concerned with the relationships between
different branches, not with assigning specimens to their correct
place in the genealogical network. Because of this, the units
employed in a phylogenetic study (lineages characterised by
particular character states) are not identical to the units
taxonomers usually employ when describing a biota or making
identification  keys (sets of specimens characterised by particular
character states).
--

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Dr. Hubert Turner
EEW, Sect. Theoretical Biology & Phylogenetics
PO Box 9516, 2300 RA  Leiden, The Netherlands
Visiting address: Van der Klaauw Laboratory, Kaiserstraat 63, Leiden
Phone: +31-71-5274904    Fax: +31-71-5274900
E-mail: turner at rulsfb.leidenuniv.nl
WWW: http://wwwbio.leidenuniv.nl/~turner/index.html

FROM 18 JANUARY TILL MID-APRIL I WILL BE AT THE NEW YORK
BOTANICAL GARDEN AS VISITING SCHOLAR, DOING MOLECULAR
PHYLOGENETIC RESEARCH ON ANACARDIACEAE. MY REGULAR E-MAIL ADDRESS
WILL REMAIN FUNCTIONAL DURING THAT PERIOD.
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