Evolution__3rd_Edition

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Classification 1
Classification 2
Classification 1
Classification 2
Evolution produces a hierarchical
tree of life
5 4 3 2 1
a
a, b
b
a'
b'
1
2
3
4
5
1
2
3
4
5
b'
a'
CHAPTER 16 / Classification and Evolution 487
Figure 16.7
Paraphyletic groups contain some but not all of the descendants
of a common ancestor. When they are defined, a decision has to
be taken about which descendants to exclude. The decision is
phenetic. a, a′, b, and b′ are character states; a′ and b′ are derived
from a and b, respectively. In the figure, the question is whether
to exclude species 1 (and define the paraphyletic group of 2–5 by
ancestral character a) or species 1–2 (and define the group 3–5
by ancestral character b). Notice there is no guarantee that shared
ancestral characters can define such paraphyletic groups as 2–5
and 3–5, because a or b could in principle have undergone any
number of changes within the branches leading to species 2–5
and 3–5.
16.8 The principle of divergence explains why phylogeny is
hierarchical
All three schools of classification a phenetic, cladistic, and evolutionary a aim at
hierarchical classification. In the case of cladism, that is unsurprising. The phylogenetic
tree is a hierarchy and phylogenetic classification will be hierarchical too. It is less
obvious whether a phenetic classification has to be hierarchical. Nature presents us
with an infinity of phenetic patterns. Some indeed are nested hierarchies, but others are
overlapping hierarchies or non-hierarchical networks. If we aim at a phenetic classification,
we have no strong reason to classify hierarchically. Biological classifications
are hierarchical because evolution has produced a tree-like, diverging, hierarchical
pattern of similarities among living things. Indeed, the hierarchical nature of biological
classifications has since 1859 been part of the evidence for evolution (Section 3.9,
p. 61). Evolutionary descent produces, in Darwin’s words a pattern of “groups within
groups.”
But why should evolution proceed in this form? The question has an important place
in the history of Darwin’s thinking. He thought up natural selection in the late 1830s, as