The Concept of Species

The species is the most fundamental and most frequently used unit in the whole of biological classification — it is the basic "building block" from which every higher taxonomic category (genus, family, order, and so on) is ultimately constructed.

A species can be understood as a group of individual organisms that share a set of fundamental similarities in structure, physiology and genetic makeup, are capable of interbreeding with one another to produce fertile offspring under natural conditions, and are reproductively isolated from other such groups — meaning they do not, in nature, freely interbreed with individuals of a different species. Members of the same species may still show minor variation among individuals (in size, colour, or minor structural detail), but this variation stays within limits that keep them recognisably the same kind of organism.

Species are distinguished from their closest relatives on the basis of clear, consistent differences — most traditionally morphological (structural) differences that a trained taxonomist can observe and measure, though modern taxonomy increasingly also uses genetic, biochemical and behavioural differences to draw the same boundary. For example, the lion (Panthera leo) and the tiger (Panthera tigris) are both large cats belonging to the same genus, Panthera, and share many features in common — yet they are recognised as separate species because of consistent, heritable differences in appearance, and because, in the wild, they do not interbreed. …