Gary Rosenberg writes that species concepts are "1) taxonomic vs.
evolutionary, (2) theoretical vs. operational, (3) contemporaneous vs.
clade, (4) reproductive vs. cohesive."
 
So an old-fashioned paleontologist might favor a taxonomic operational
contemporaneous cohesive species concept, but a new-fangled molecular
biologist might prefer an evolutionary theoretical cladistic reproductive
species concept. With all shades of variation in between, and bearing in
mind that each of Gary's oppositions are also intergradational. Is that
clear, Art? Don't make me come over there!
 
Andrew K. Rindsberg
Geological Survey of Alabama
Tuscaloosa, Alabama, USA