Evolution__3rd_Edition

348 PART 4 / Evolution and Diversity
(a) Bald eagle
(b) Golden eagle
Species are formally defined and
practically recognized by phenetic
characters
Adult
Adult
Figure 13.1
(a) Adult bald eagle (Haliaeetus leucocephalus) and (b) adult
golden eagle (Aquila chrysaetos), seen from underneath. The
species can be distinguished by their pattern of white coloration.
13.1 In practice species are recognized and defined
by phenetic characters
Biologists almost universally agree that the species is a fundamental natural unit. When
biologists report their research, they identify their subject matter at the species level and
communicate it by a Linnaean binomial such as Haliaeetus leucocephalus (bald eagle)
or Drosophila melanogaster (fruitfly). However, biologists have not been able to agree
on exactly how species should be defined in the abstract. The controversy is theoretical,
not practical. No one doubts how particular species are defined in practice.
Taxonomists practically define species by means of morphological or phenetic characters.
1 If one group of organisms consistently differs from other organisms, it will be
defined as a separate species. The formal definition of the species will be in terms of
characters that can be used to recognize members of that species. The taxonomist who
describes the species will have examined specimens of it and of related species, looking
for characters that are present in specimens of the species to be described, and absent
from other closely related species. These are the characters used to define the species.
Almost any phenetic character may end up being useful in the practical recognition
of species. Figure 13.1 for example shows the adults of the bald eagle (Haliaeetus leucocephalus)
and the golden eagle (Aquila chrysaetos), seen from below. A bird guide will
1 Phenetic characters are all the observable, or measurable, characters of an organism, including microscopic
and physiological characters that may be hard work in practice to observe or measure. Morphological characters
are characters of the shape or observable form of the whole organism or a large part of it. Behavioral and
physiological characters are part of the phenetic description of an organism, but not part of its morphology.
However, taxonomic descriptions are usually made from dead specimens in a museum, and the phenetic characters
that are specified in taxonomic descriptions are usually morphological characters. The words “phenetic”
and “morphological” are therefore practically almost interchangeable here. Also, the word “phenotypic” could
be used instead of “phenetic.”
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