Mayr on semi-species & speciation

[Ken Kinman]

    Not surprisingly, I follow Ernst Mayr's version of semi-species.
Although many cladists probably prefer a more morphological species concept,
Mayr's biological species concept is a little more rigorous, and a
semi-species is therefore not regarded as a good species (but on the way to
getting there).  Theoretically, circumstances could force the two
semi-species to "amalgamate" back into a state in which speciation is no
longer incipient.  As long as this potential remains, I would not regard
them as two separate species.
    Anyway, here is a quote I found about semi-species from Ernst Mayr's
1996 paper "What is a Species, and What is Not?" (Philosophy of Science,
63:262-277):
    "Speciation, as Darwin has shown,is normally a gradual populational
phenomenon.  Sudden, saltational speciation as in the case of
allopolyploidy, seems to be virtually absent in most groups of sexually
reproducing organisms.  Owing to the gradualness of the speciation process
one should find in nature populations that are on the way to becoming
separate species, but have not yet quite completed the process.  Such
"semi-species" are indeed found.  They are documented, for instance, by the
so-called zones of secondary hybridization.  Here two incipient species,
usually expanding from a Pleistocene refuge, hybridize along a more or less
long contact line, but the hybrid zone stays narrow, often less than 100 km
wide, even though this contact zone may have existed for 5-10,000 years.
Both of the two semi-species discriminate against introgressing genes of the
other semi-species, as documented by the lowered fertility of hybrid pairs."
     Although the biological species concept of Mayr is sometimes too
difficult to use, and can be impractical in many situations, I feel that it
is the concept one should probably aspire to for most taxa.  The more
practical morphological species concepts (like the phylogenetic species
concept) tend to have been historically used in the initial studies of a
given taxon, but whenever possible this should be followed up using Mayr's
more rigorous "biological species concept", in my opinion.  We should NOT
throw out such a concept just because it is difficult and not always
practical.  To completely overthrow it in favor of an across-the-board
phylogenetic species concept would probably lead to a lot more instability
(and perhaps even chaos, once the genetic data really starts to flood in).
                            --------Ken Kinman

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