Evolution__3rd_Edition

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Biologists disagree about how real
the higher taxonomic units are
can be made here. Evolutionary biologists who support the biological species concept
characteristically differ from those who support the ecological species concept in their
attitude to the reality of taxa above the species level. The biological species concept can
apply to only one taxonomic level. If species are defined by interbreeding, then genera,
families, and orders must exist for some other reason. Mayr has been a strong supporter
of the biological species concept and (in 1942, for example) duly reasoned that
species are real, but that higher levels are defined more phenetically and have less
reality; that is higher levels are relatively nominalistic. Dobzhansky and Huxley held a
similar position.
Simpson, however, favored a more ecological theory of species. The ecological
concept can apply in much the same way at all taxonomic levels. If the lion occupies
an adaptive zone corresponding to a single ecological niche, then the genus Felis
may occupy a broader adaptive zone, and the class Mammalia an even broader
adaptive zone. Adaptive zones could have a hierarchical pattern corresponding to (and
causing) the taxonomic hierarchy. All taxonomic levels could then be real in the
same way. The relative reality of the species, and of higher taxonomic levels, is therefore
part of the larger controversy between the ecological and reproductive species
concepts.
13.9 Conclusion
CHAPTER 13 / Species Concepts and Intraspecific Variation 377
In evolutionary biology, the interesting questions about species are theoretical. The
practical question of which actual individuals should be classified into which species
can on occasion be awkward, but biologists do not tie themselves in knots about it.
The majority a perhaps over 99.9% of specimens can be fitted into conventionally
recognized species and do not raise even practical problems. Other specimens can be
identified after a bit of work a or even left on one side until more is learned about
them.
The more interesting question is why variation comes in nature arranged in the
clusters we recognize as species. There are several possible answers, as we have seen.
Different species concepts follow from different ideas about the importance of interbreeding
(or gene flow) and natural selection. It is sometimes possible to test between
them, but the results so far have not been enough to confirm any one concept (or any
plurality of concepts) decisively. However, there is general agreement that phenetic
distinction alone is not an adequate concept, and that the key explanatory processes
are interbreeding and the pattern of ecological resources.