Species

A group of similar living things that are capable of interbreeding with each other and of producing fertile offspring that resemble themselves. Species are are the fundamental units of biological classification. Some species have distinct populations that vary from each other. Where the differences are significant and the populations biologically isolated, these forms are classified as separate subspecies, or races. In situations where two individuals belonging to different subspecies meet and form a mixed pair, they are still capable of interbreeding successfully.