A second theory posits that the tropics hold more species because tropical species are finicky. According to this line of reasoning, what’s important about the tropics is that temperatures there are relatively stable. Thus tropical organisms tend to possess relatively narrow thermal tolerances, and even slight climatic differences, caused, say, by hills or valleys, can constitute insuperable barriers. (A famous paper on this subject is titled “Why Mountain Passes Are Higher in the Tropics.”) Populations are thus more easily isolated, and speciation ensues.
