As with any feature of biological organisms, DNA sometimes provides
nice clean distinctions between forms and sometimes doesn't.  Perhaps
the most consicuously genetically messy mollusks are those with hybrid
polyploidy.  Multiple sets of DNA, each with different histories but
potentially able to get mixed and matched.  Lasea, Campeloma,
sphaeriids, various thiarids, freshwater Corbicula, etc.  The apparent
ancient lineage sorting problems cropping up in freshwater
cerithioideans (and probably others) is another example.  If an
ancestral population has more than one version of a gene, over time
isolated descendant populations are expected to lose all but one
(ignoring mutation producing new ones).  However, that can take a long
time, so that a population has two very different versions of a gene
within it.  And an occasional hybridization event can put odd genes
into a population.  A definitive answer on DNA patterns requires
thorough sampling of multiple genes across the range of the species of
interest.

A further problem on subspecies is that, given the lack of official
standardization, old below-species names are to be credited as
subspecies.  Sometimes these were merely intended as recognition of
individual variation, the modern concept of variety, but they get
credited as subspecies.  Those wanting to have or to sell as many
kinds as possible like to recognize more forms.  Thus, there is a need
to distinguish between what is thought to be a type of individual
variant (such as an albino) and what is thought to be distinctive of a
large population that eventually intergrades with other populations.

--
Dr. David Campbell
Visiting Professor
Department of Natural Sciences
Gardner-Webb University
Boiling Springs NC 28017

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