Ducks and pandas. Makes as much sense as anything. I remember being given an example of the futility of trying to neatly classify species or subspecies, whether by genetic, anatomical, or any other criteria: Here are a bunch of pencils worn down to varying lengths. Divide them into two groups, the long ones and the short ones. Now three groups, long, short, and medium. You'll always be making a lot of arbitrary decisions at the margins of each group, and it's really all subjective, unless you accept that each one is an individual with its own unique variant traits, but all are part of a single, unified, larger, group, pencils, which can clearly be distinguished from pens or paintbrushes. So, ducks, pandas, and pencils pretty much do it for me for right now.