There are certain people who cling to the motto, neatness above all! Such people probably shouldn't delve too deeply into the biological world, for living things stubbornly resist being neatly lined up, each in its place. Mostly, this is due to the complexity of life. Why, we can't even have, or at least agree, on a single definition of "kind" that works well for everything.
Our definition of a biological species is based on who can successfully
breed with whom. By this criterion, two populations that can't interbreed freely
are separate species. But wait! Many organisms don't require a male/female pair for
reproduction. Some pretty advanced critters, such as our local whip-tailed lizards, get
along very nicely without any males at all. So we name some organisms as species
because they look alike—our so-called morphological species concept. And then
there's a whole bunch of populations busily evolving, but who haven't yet made
up their minds as to whether they are going to be reproductively isolated, look
different, both or neither! Sheesh!
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.