Evolution__3rd_Edition

258 PART 3 / Adaptation and Natural Selection
Figure 10.1
Two lineages of titanotheres
showing parallel body size
increase and the evolution of
horns. Only two of many
lineages are illustrated.
Reprinted, by permission of
the publisher, from Simpson
(1949).
The fossil record shows some
apparently directed trends ...
. . . but they are unlikely to have
been driven by directed variation
Early Oligocene
(most forms with large
horns)
Late Eocene
(small horns arising in
several different groups)
Early Eocene
(no horns)
Megacerops
Dolichorhinus
Eotitanops
Protitanotherium
Brontotherium
lineages, the earlier forms lacked horns whereas later ones had evolved them (Figure
10.1). Osborn, and others, believed that the trend was orthogenetic: that it arose not
because of natural selection among random mutations but because titanotheres were
mutating in the direction of the trend.
Directed mutation could explain a simple, adaptively indifferent trend. If a titanothere
was equally well adapted no matter what size its horns were, then a trend toward
larger horns might be generated by directed mutation. In fact, the horns are thought to
be adaptive, and that makes directed mutation an implausible explanation. Mutation is
random with respect to adaptation (Section 4.8, p. 88). If mutation is directed, it is in a
non-adaptive way. Thus, if someone explains a trend by orthogenesis (or directed
mutation) we can ask how the “orthogenetic” mutations could keep on occurring in
the direction of adaptive improvement. If the reply is that variation just happens to be
that way, then adaptation is being explained by chance a and chance alone cannot
explain adaptation, almost by definition.
This objection is not all that strong for titanothere horns, because their adaptive
function is little understood. The trends might have been possible by simple increases
in size. However, for other known trends in the fossil record, such as the evolution of
mammals from mammal-like reptiles (Section 18.6.2, p. 542), the objection is much
more powerful. Mammals evolved over about 100 million years, during which time
changes occurred in the teeth, jaws, locomotion, and physiology. Almost every feature
of the animals was altered in an integrated way. Directed mutation alone would
be highly unlikely to drive a complex, multicharacter, adaptive trend of this kind. A
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