For horned lizard, horns alone do not make the species

A thorough study of the million-year evolution of California's horned lizards, sometimes referred to as "horny toads," shows that when it comes to distinguishing such recently diverged species, the most powerful method integrates genetic, anatomical and ecological information.
In the study, published this week in the early online edition of the journal Proceedings of the National Academy of Sciences, researchers from the University of California, Berkeley, and the U.S. Geological Survey consider all these criteria to show that when the coast horned lizard (Phrynosoma coronatum) moved north from Baja California and spread throughout the state, it diverged into at least two new species
 
for more detail please check the given link:http://www.eurekalert.org/pub_releases/2009-07/uoc--fhl072109.php