Evolution__3rd_Edition

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Figure 10.7
Developmental asymmetry in
genotypes of the Australian
sheep blowfly (Lucilia cuprina)
that are, or are not, resistant
to the insecticide malathion.
(a) Developmental asymmetry
in genotypes when the resistant
gene RMal first appeared, soon
after malathion was first used.
+ is the original, non-resistant
genotype. RMal disrupts
development, producing
greater average asymmetry; and
is selectively disadvantageous
in the absence of malathion.
(b) Developmental asymmetry
of RMal flies after modifiers
(M) have evolved to reduce the
developmental disruption; it is
now reduced near to the level of
the original +/+ flies, and in the
absence of malathion RMal has
little selective disadvantage or is
neutral relative to +. The sample
size is 50 flies for each genotype.
Redrawn, by permission of the
publisher, from McKenzie &
O’Farrell (1993).
A morphospace for shells shows all
the shell forms that could possibly
exist
Coiling axis
Mean asymmetry ± s.e.
3
2
1
0
Resistance
genotype
Modifier
genotype
(a) Without modifier (b) With modifier
CHAPTER 10 / Adaptive Explanation 277
+/+ +/RMal RMal/ RMal +/+ +/RMal RMal/ RMal
+/+ +/+ +/+ M/M M/M M/M
growth. The explanation for the quantum jumps is a developmental constraint:
growth, by molting, is dangerous and to grow with a smooth curve would require
frequent risky molts. It is better to molt more rarely and grow in jumps.
Developmental constraints have been suggested as an alternative explanation to natural
selection for two main natural phenomena. One is the persistence of fossil species
for long periods of time without showing any change in form (Section 21.5, p. 606).
The other is the variety of forms to be found in the world. We can imagine plotting a
morphospace for a particular set of phenotypes and then filling in the areas that are and
are not represented in nature.
Raup’s analysis of shell shapes is an elegant example. Raup found that shell shapes
could be described in terms of three main variables: translation rate, expansion rate,
and distance of generating curve from the coiling axis (Figure 10.8). Any shell can be
represented as a point in a three-dimensional space, and Raup plotted the regions in
this space that are occupied by living shells (Figure 10.9).
Large parts of the shell morphospace in Figure 10.9 are not occupied. There are two
general hypotheses to explain why these forms do not exist: natural selection and constraint.
If natural selection is responsible, the empty parts of the morphospace are
regions of maladaptation. When these shell types arise as mutations, they are selected
Initial generating curve
Generating curve
after one revolution
Figure 10.8
The shape of a shell can be described by three numbers.
The translation rate (T) describes the rate at which the coil moves
down the coiling axis: T = 0 for a flat planispiral shell, and is an
increasingly positive number for increasingly elongated shells.
The expansion rate (W) describes the rate at which the shell size
increases; it can be measured by the ratio of the diameter of the
shell at equivalent points in successive revolutions; W = 2 in
the figure. The distance from the coiling axis (D) describes the
tightness of the coil; it is the distance between the shell and the
coiling axis, and in the figure it is half the diameter of the shell.
See Figure 10.9 for many theoretically possible shell shapes with
different values of T, W, and D. Redrawn, by permission of the
publisher, from Raup (1966).