That Confusing Pseudemys Group 

from a post on the Herp-L list by Darrel Frost

About 1964 Sam McDowell published a paper where he noted that three
evolutionary groups of turtles could be recognized in the Pseudemys
group, these were Pseudemys (cooters), Trachemys (sliders), and
Chrysemys (painted turtles). He noted that evolutionary relationships
among these three was difficult to determine and that either three
genera could be recognized (Trachemys, Pseudemys, Chrysemys) or one
genus (Chrysemys-the oldest name), with three subgenera (Trachemys,
Pseudemys, and Chrysemys). This was controversial, not because people
doubted the results of McDowell's study, but because they had
essentialist notions about what the name Chrysemys could be attached
to. 

The sad fact is, that if Pseudemys was an older name than Chrysemys,
there would have been no controversy, and we wouldn't have the names
Trachemys and Chrysemys to worry about (at least not as genera). There
was a long period where different authors used different nomenclatures,
and nobody made any clear contribution elucidating the central issue of
which two of the three groups were evolutionarily most closely related. 

Fairly recently, a paper discussed the issue and decided that the only
way to stabilize the taxonomy of the groups was to recognize three
genera. This was a political solution, but a sane one, and one sanctioned
by McDowell's 1964 paper. I doubt if there will be much more discussion
of this 'problem', since the Gordian Knot has been cut, essentialists
(nonevolutionary taxonomists) can have their search images and
phylogeneticists have a taxonomy consistent with evolutionary history.