Evolution__3rd_Edition

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Phenetic classification is nonevolutionary
Phylogenetic classification
represents evolutionary
relationships
CHAPTER 16 / Classification and Evolution 473
“phenetic” throughout this chapter.) The phenetic method groups species according
to their observable phenetic attributes: if two species look more like each other than
either does to any other species, they will usually be grouped together in a phenetic
classification. The full classification consists of a hierarchy of levels, such that the members
of different groups at higher and higher levels look decreasingly similar to one
another. A wolf and a dog (same genus) look phenetically more similar than do a wolf
and a dolphin (same class).
In formal classification, phenetic similarity has to be measured. Almost any observable
attributes of organisms can be used for this purpose. Fossil vertebrates can be
classified phenetically by the shape of their bones; modern species of fruitflies by the
pattern of their wing venation; and birds by the shapes of their beaks or the color
pattern of their feathers. Species can be grouped according to the number, shape, or
banding pattern of their chromosomes, by the immunological similarity of their
proteins, or by any other measurable phenotypic property.
Nothing needs to be known about evolution in order for species to be classified
phenetically. The species are grouped by their similarity with respect to observable
attributes alone, and the same principle can be applied to any sets of objects, non-living
or living, whether or not they were produced by an evolutionary process. It could be
applied to languages, furniture, clouds, songs, and styles of art and literature, as well as
biological species.
The phylogenetic principle, however, is evolutionary. Only entities that have evolutionary
relations can be classified phylogenetically. The clouds in the sky, for instance,
cannot be classified phylogenetically (almost every cloud is formed independently, by
physical processes a though a few clouds may be formed by the division of ancestral
clouds, and those could be classified phylogenetically). The phylogenetic principle
classifies species according to how recently they share a common ancestor. Two species
that share a more recent common ancestor will be put in a group at a lower level than
two species sharing a more distant common ancestor. As the common ancestor of two
species becomes more and more distant, they are grouped further and further apart in
the classification. In the end, all species are contained in the all-inclusive phylogenetic
category a the set of all living things a which contains all the descendants of the most
distant common ancestor of life.
In most real cases in biology, the phylogenetic and phenetic principles give the same
classificatory groupings. If we consider how to classify a butterfly, a beetle, and a
rhinoceros, the butterfly and beetle are more closely related both phenetically and
phylogenetically (Figure 16.1a). The beetle and butterfly both look phenetically more
alike and share a more recent common ancestor with each other than either does with
the rhinoceros.
In other cases, the principles can disagree, for two main reasons. One reason is
evolutionary convergence. Adult barnacles superficially look rather like limpets. If we
were to classify an adult barnacle, limpet, and lobster phenetically we might well put
the barnacle and limpet together even though the lobster and barnacle share a more
recent common ancestor and are grouped together phylogenetically (Figure 16.1b).
The other reason is illustrated by groups like reptiles. The phenetic and phylogenetic
classifications of the reptilian groups differ because some descendants (such as birds) of
the common ancestor of the group have evolved rapidly. The rapidly evolving groups