Evolution__3rd_Edition

350 PART 4 / Evolution and Diversity
Time
4
3
2
1
Form, or character state (e.g., beak size)
The evolutionary controversies
about species are not mainly
concerned with practical or formal
issues
Two species
Ambiguous zone
One ancestral species
Figure 13.2
Difficulties in species recognition are expected in the theory of
evolution, because variation exists within each species and new
species evolve by the splitting of ancestral species. During the
evolution of new species, the distinction between the species will
be ambiguous during times 2 and 3. At stage 3, for instance, no
phenotypic character can unambiguously distinguish between
two species; indeed two species do not yet exist.
division of the ring would also be theoretically meaningless: there really is a continuum,
not a number of clear-cut, separate species. Problems of this kind are exactly what we
should expect given that species originated by an evolutionary process. We should not
expect clear-cut defining characters to exist for all species; that is not the way nature is.
Species are in practice mainly recognized by phenetic characters, more or less successfully.
However, when evolutionary biologists discuss species concepts, they are not
usually discussing how species are recognized in practice. They are discussing deeper,
theoretical concepts of species, concepts that may lie beneath the practical procedures
that are used to recognize particular species. Is the bald eagle just the set of eagles that
have white heads and tails? Imagine that a parental pair of bald eagles with good white
heads and tails produced a nest of eagles of some different color pattern. Would they
have given birth to a new species? If the color of the head and tail was all there was to
being a member of Haliaeetus leucocephalus, then the answer would clearly be yes.
However, if the species have a more fundamental definition, and the coloration was
picked only as a practically useful marker, then the answer would be no. Indeed, the
new eagles without the white coloration would render that taxonomic character out of
date, and it would be time to start looking for some other characters to recognize the
species. Most of the discussion of species concepts that follows assumes that species
definition has some deeper meaning than the phenetic characters used to recognize the
species in practice. When biologists argue about species concepts they are not arguing
about how species are defined in practice.
13.2 Several closely related species concepts exist
A first distinction among species concepts is between horizontal and vertical concepts
(Figure 13.3). A horizontal concept aims to define which individuals belong to which
species at any instant in time. A vertical concept aims to define which individuals
belong to which species at all times. Vertical concepts are mentioned here mainly for
completeness; most of the interest in species concepts is in horizontal concepts.
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