The Development of the Philosophy of Species 321
The Phylotype
One attempt to resolve the issue is an operational approach. The cluster of genomes
of asexual organisms forms what is called a “phylotype,” a term coined by C. W.
Cotterman in unpublished notes dated 1960.^118 Phylotype is a rank-neutral term,
though, that is determined entirely by the arbitrary level of genetic identity chosen.
For example, “species” in asexuals might be specified as being 98%+ similarity of
genome, or it might be 99.9%+ (both are in the literature). A phylotype of, say 67% or
80% might be used for other purposes (such as identifying a disease-causing group
of microbes):
A genomic species is one described by DNA sequence homology. Here the most com-
monly adopted standard is a collection of strains the chromosomal DNA sequence
homology of which is greater than 70% measured under optimal renaturation condi-
tions with ΔTm <5%.^119The phylotype concept, while useful in other respects, reinvents, or preinvents,
phenetics—1960 predates most of the work of Sokal and Sneath. A phenetic taxon
was called the operational taxonomic unit (OTU) and it used an arbitrary measure
of similarity and difference of morphological traits: an 80% “phenon line” was
the arbitrary measure for species based on phenotypic similarities. Different phe-
non measures would give different taxa, and there was no principled reason other
than convention for adopting one over another. Moreover, if “barcode” genes like
16sRNA are used for diagnosis of taxa,^120 then just as with phenetics, the taxa will
vary according to the components of the analysis.Branching Random Walks
It might be thought that clustering in genome space doesn’t need explanation.
Clustering of phylotypes might in the first case simply be due to stochastic processes
of elimination and divergence. To reject that assumption, I will appeal here to a study
on null models in morphospace.^121 Given that the morphological traits here are heri-
table, and that they are in effect asexually transmitted (because the morphology here
is typical of a species that diverges in a branching random walk), it is applicable to
our case. This is an argument by simulation, and lacks as yet any empirical founda-
tion, but as we have no clear prior assumptions about what the default expectations
for asexually reproducing lineages may be, this may help to give us some. Further
simulations might change these expectations. The authors understood their simula-
tion to apply in the case of asexual lineages.^122
Pie and Weitz simulated speciation in four cases (Figure 13.3). In the first case,
they allowed speciation without extinction (A), which generated the expected carpet
(^118) Denniston 1974.
(^119) Denniston 1974; see also Johnson 1973.
(^120) Hebert et al. 2003, Blaxter 2004, 2005, Wheeler 2005.
(^121) Pie and Weitz 2005.
(^122) Pie, pers. comm.