Evolution__3rd_Edition

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16Classification and Evolution
B iological classification is concerned with distinguishing
and describing living and fossil species, and with
arranging those species into a hierarchical, multilevel
classification. The theory of evolution has a strong influence
on classificatory procedures. Chapter 13 was about species;
this chapter is about classification above the species level.
We begin by looking at the two principles a phenetic and
phylogenetic a that have been used to classify species
hierarchically into groups (such as genera, families, and
higher level categories), and see how the three main schools
of biological classification put them to use. We then look at
the conditions in which the two principles give the same, or
differing, classifications of a set of species. The main question
of the chapter is which (if any) of the two principles is the
better justified. The answer comes from an argument that
phylogenetic classification at its best is objective, whereas
phenetic classification always suffers from subjectivity.
We then look at some consequences of the strict use of
phylogenetic relations to classify species into groups.
We finish by considering why real evolution has resulted
in a tree-like diverging pattern of relations among species.