As  publishers we have often this problem.
When I receive a paper describing a species on a single specimen I always
try to suggest the author to wait a little bit in the hope of receiving
some more material. Sometimes this happens. Sometimes somebody else finds
the further material and the first author is obviously upset with me...
However, we always get in touch with a couple of referees about the new
species, and often also the referees do agree on the description on a
single specimen base: but this is (and, I think, it must remain) more an
exception than a normal procedure.

Maria

Maria Antonietta Fontana Angioy
Evolver srl - La Conchiglia
Rome, Italy