Evolution__3rd_Edition

278 PART 3 / Adaptation and Natural Selection
Expansion rate (W)
1
10
102 104 106 Helicoid forms
1.0
axis
C
0.8 from
0.6 curve
D
0.4
of generating
0.2
D) (
4 3 2 1 0
Translation (T ) Distance
Figure 10.9
The three-dimensional cube describes a set of possible shell
shapes. Around the outside of the figure, 14 possible shell shapes
are illustrated as drawn by a computer. Only four regions in the
cube are actually occupied by natural species: A, B, C, and D. All
Constraint and selection can be
alternatives ...
A
B
Planispiral forms
other regions in the cube represent theoretically possible
but naturally unrealized shell shapes. The space is called a
morphospace. reprinted, by permission of the publisher, from
Raup (1966).
against and eliminated. Alternatively, the empty parts could be regions of constraint: the
mutations to produce these shells have never occurred. If the constraint was developmental,
it would mean that for some reason it is developmentally impossible (or at least
unlikely) for these kinds of shells to grow. The non-existent shells would be embryological
analogies for animals that disobey the law of gravity a they are shells that break the
(unknown) laws of embryology. The absence of these shells would then be no more due
to natural selection than is the absence of animals that break the law of gravity.
Just as natural selection and constraint are hypotheses to explain the absence of any
form from nature, so they can both hypothetically explain the forms that are present.
Faced with any form of organism, we can ask whether it exists because it is the only
form that organism possibly could have (constraint), or whether selection has operated
..