Species

A species is a difficult unit of life to describe biologically, except for those organisms that engage in sex. In those cases, their genetics, behavior, and geographic distribution defines the population. Geographic separation and/or the constant pressure exerted on it by mutations within that population leads to the eventual creation of new species by the process of natural selection, whereby the mutant best suited to its new environment stands the best chance of survival. The species concept is the lynch pin upon which the science of biology rests.

Links
  http://www.sp2000.org
http://www.all-species.org
http://www.saveamericasforests.org/news/EOWilsonIntro.htm