Taxacom: On species diagnoses and DNA-based descriptions in taxonomic practice

[Thomas McCabe]

Hello. I am a physician biologist with an interest in biological taxonomy
and the discussions at Taxacom. May I add a comment and questions about the
journal article referenced by Thomas Pape ("Tightening the requirements for
species diagnoses would help integrate DNA-based descriptions in taxonomic
practice", *PLoS* Biol 21(8): e3002251)? The authors of this paper go
beyond nomenclature to advocate for taxonomic species diagnoses that are
both contrastive and state-specific. The authors also attend to the
interesting possibility and problems of defining species solely with
genome-wide DNA sequences.

Regarding defining species, I request thoughts on two questions, the first
skeptical, the second leading.

First, a question about using a genome-wide DNA sequence as sole diagnosis
for a taxonomic species. Suppose technology advances so that a description
of the complete sequence of all the nucleic acid macromolecules making up
the genetic complement of each potential specimen supposedly representing a
newly-discovered taxonomic species is attainable. Then, if each living
being has a unique genetic complement (at least one single nucleotide
polymorphism or indel making one of its DNA sequences unique among those of
other living beings), what will prevent a seeker of cryptic species from
using every specimen as a type for a separate taxonomic species?

Second, and related to the first question, if each natural species is a
lineage-segment, why is each taxonomic species diagnosis drawn from one or
more unrelated specimens instead of from a directly observed
lineage-segment of specimens? That is, cannot biological taxonomy be an
experimental, as well as a descriptive, science?

Respectfully,

Thomas McCabe, M.D., M.P.H.

Independent Scholar