ScienceDaily: A new study of 2300 species of mammals and nearly 6700 species of birds from across the globe helps explain why there are so many more species of plants and animals in the tropics than at higher latitudes. In a study supported by the National Evolutionary Synthesis Center in North Carolina, researchers found that while the tropics harbor a greater diversity of species, the number of subspecies -- potential stepping stones in the process by which one species becomes two -- is actually greater in the...