Nominalism versus realism

When we classify the natural world into units such as species are we imposing categories devising on a natural continuum, or are the categories real divisions in nature?

The idea that species are artificial divisions of a natural continuum is called nominalism; the alternative, that nature is itself divided into discrete species, is called realism.

'Folk taxonomy'

The question is often posed in phenetic terms, but the answer depends on what species concept we are using. The most striking evidence that species exist as phenetic clusters comes from 'folk taxonomy'.

People working independently of Western taxonomists usually have names for the species living in their area and some do classify species much like Western taxonomists. Ernst Mayr investigated this in the 1950s (pictured opposite on the right) and found that the Kalám of New Guinea, for instance, recognize 174 vertebrate species, all but four of which correspond to species recognized by Western taxonomists. People elsewhere do not agree quite so closely with museum classifications. A reasonable generalization is that agreement is usually good for large vertebrates but it breaks down for smaller, and economically unimportant, species.

However, the fact that independently observing humans can see much the same species in nature does not show that species are real rather than nominal categories, except perhaps on the phenetic species concept; but, as we have seen, the phenetic species concept is rejected for other reasons.