BSC/PSC

[Doug Yanega]

Matt Buys asked:

>1. One of Ernst Mayr's objections to the phylogenetic species concept
>is its failure to recognise so-called sibling species which by
>definition look identical externally. In the light of advances in the
>molecular field, is there published research on whether two sibling
>species that look identical externally can be grouped into two taxa
>based on molecular evidence?

I know of at least two recent cases in insects of exactly this: (1) a
revision of North American Cryptocercus (wood roaches) in which one
nominal taxon was split into five based on molecular characters; the
species diagnoses, in the original descriptions, were molecular
sequences rather than moprhological characters. The descriptions are
validly published. (2) it was discovered that populations of the
sweat bee Halictus ligatus (the most widespread native bee species in
the Western Hemisphere) from Florida and adjacent mainland areas were
allozymically distinct from the rest of the country, and that there
was a narrow zone of overlap in which the two allozyme morphs did not
intergrade or recombine, suggesting an absence of gene flow.
Accordingly, it was decided that the Floridian allozyme morph was a
different species, and an old available name, Halictus poeyi (given
to a Floridian specimen), was resurrected out of synonym and applied
to it.

None of the above taxa can be effectively distinguished without
recourse to chemical analyses of some sort, so while they are good
species in principle (under both BSC and PSC, presumably), they are
an impractical nightmare for a traditional taxonomist.

>2. The BSC has to do with the ability to reproduce. The phylogenetic
>species concept (PSC) is based on the possession of unique characters.
>
>All individuals resulting from sexual reproduction are however unique
>(at least at the molecular level). So as I see it, to be a species, you
>have to possess characters that are unique (enter the PSC) to a
>population or group of populations, implying some form of gene flow
>(enter the BSC), but not unique to an individual. So, can I submit to a
>species concept for sexually reproducing organisms consisting of dual
>elements, viz. a spittle of BSC mixed with a bit of PSC?

I'm not so sure. For me to buy into this, I'd like to see some sort
of concrete evidence that the PSC is not over-splitting. That is, is
it possible for PSC proponents to demonstrate that NO populations
exist which have even a single diagnostic locus, and yet are capable
of 100% viable interbreeding with other, diagnostically different
populations? Only if no such thing happens can one claim that the BSC
and PSC are compatible, even in principle. In practice, of course,
there are probably very few species in existence where one *cannot*
find two conspecific populations which have *some* reproductive
barriers (be they pre- or post-zygotic), meaning most "species" are
in a gray area. The BSC, by its nature, is trying to draw a boundary
around a nebulous phenomenon - which is one of the main reasons the
PSC is often considered the more *objective* species concept (either
there is a single diagnostic difference or there isn't). The
molecular approach to the PSC, of course, still has its own practical
limitation based on thoroughness of sampling - it's easy to miss a
rare allelic variant in a population when one's sample size is three
specimens, and likewise easy to find differences when sampling two
populations that have numerous *unsampled* intervening populations
between them.

Again, however, since I'm a traditional taxonomist, I recognize
species by whether or not I can tell preserved specimens apart (not
whether they interbreed, or whether their genomes differ by a single
base pair), and find it all meaningless if you can't tell me how to
distinguish two insect species when I have them on pins in my
collection. I can only imagine how much worse it is for plant
taxonomists, dealing not only with hybrids, but ecotypes, in addition
to plain old natural variation. It really is a shame that we'll never
have those Tricorders from Star Trek, where you just point the device
at a specimen and it scans the genome remotely and tells you what
species it is. ;-)

Peace,
--

Doug Yanega        Dept. of Entomology         Entomology Research Museum
Univ. of California - Riverside, Riverside, CA 92521
phone: (909) 787-4315 (standard disclaimer: opinions are mine, not UCR's)
              http://cache.ucr.edu/~heraty/yanega.html
   "There are some enterprises in which a careful disorderliness
         is the true method" - Herman Melville, Moby Dick, Chap. 82