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Evolution__3rd_Edition

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Biological species are real, not<br />

nominal units<br />

Folk taxonomy often matches<br />

formal taxonomy<br />

CHAPTER 13 / Species Concepts and Intraspecific Variation 375<br />

taxonomic categories, but has particularly been discussed in the case of the species category.<br />

The idea that species are artificial divisions of a natural continuum is called nominalism;<br />

the alternative, that nature is itself divided into discrete species, is called realism.<br />

On the biological species concept, species are real rather than nominal units in<br />

nature. If we take the set of all organisms currently classified as human beings and as<br />

chimpanzees, then these organisms do divide into two discrete reproductive units. A<br />

human being can interbreed with any other human (subject to provisos, such as that<br />

the two humans are of opposite sex and of reproductive age), but with no chimpanzee.<br />

Interbreeding between species does not blur out. Here is a thought experiment to illustrate<br />

what “blurring out” would mean. Take the set of all human plus all chimpanzee<br />

individuals. Then pick an individual at random. Now experimentally place that individual<br />

with a range of potential mates from across the entire set of other individuals. If<br />

reproductive output varied continuously from 100% to 0% across the full set of mates,<br />

then interbreeding could be said to blur out. In fact reproductive output would jump<br />

between 100% (or a high figure) and 0% with nothing in between. Human and chimpanzee<br />

interbreeding does not blur out. In a way, the strangeness of imagining what<br />

blurring out would mean illustrates how humans and chimpanzees form real, not<br />

nominal, reproductive units. In any case, humans in fact form a real reproductive unit.<br />

So too do most species.<br />

Species are likely to form phenetic units in consequence. Because interbreeding is<br />

confined to a certain set of individuals, an advantageous new mutation will spread<br />

through that set of individuals, but not into other such sets (that is, other species). If<br />

chimpanzees gain a favorable mutation, it will not spread to us even if we would benefit<br />

from it. For this reason, biological species often form real, rather than nominal, phenetic<br />

clusters. The most striking evidence that species exist as phenetic clusters comes<br />

from “folk taxonomy.” People working independently of Western taxonomists usually<br />

have names for the species living in their area, and we can look at whether they have hit<br />

on the same division of nature into species as have Western taxonomists working with<br />

the same raw material. Some people, it seems, do use much the same classification of<br />

species. The Kalám of New Guinea, for instance, recognize 174 vertebrate species, all<br />

but four of which correspond to species recognized by Western taxonomists.<br />

As we saw (Section 13.7.1), phenetic and reproductive units do not always coincide.<br />

In polytypic butterfly species, there are many discrete phenetic forms and “folk taxonomies”<br />

of these butterflies tend to recognize many forms rather than the single biological<br />

species. Likewise, folk taxonomies would probably not distinguish sibling<br />

species, though most sibling species are too obscure for this question even to have been<br />

asked. In summary, species in nature are real rather than nominal interbreeding units<br />

in most cases, but not in all.<br />

13.8.2 Categories below the species level<br />

Species in many cases form discrete phenetic units. This contrasts with subspecific<br />

units such as “subspecies” and “races.” (I put the words in quotes because, although<br />

the categories are sometimes used, biologists are skeptical about their utility for the<br />

reason we are about to look at.) Subspecies and races a the two terms are almost

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