Evolution__3rd_Edition

304 PART 3 / Adaptation and Natural Selection
(a)
Day 1
Day 37
Breeds for
37 days
48 new colonies
(16 beetles each)
(b)
Mean number of adults
300
250
200
150
100
50
48 colonies
(16 beetles in a colony)
0
0 1 2 3 4 5 6 7 8 9
Generations
Group-selected beetles evolve
lower fecundity
High
Control
Low
Figure 11.2
Wade’s experiment with Tribolium beetles. (a) Experimental
design: 48 colonies were bred for 37 days, and then a new round
of colonies was formed from the colonies that had grown to a
low (or high) population density. (b) Results showing population
densities in lines selected for high or low population densities and
in unselected controls. The results are the means for 48 colonies.
Redrawn, by permission of the publisher, from Wade (1976).
Not surprisingly, the population density in the “low” lines decreased relative to the
“high” lines (Figure 11.2b). The decrease in the low lines is due to group selection. Presumably,
within the 37 days of any one cycle, the beetle types with high fecundity were
increasing within each colony relative to the less fecund beetles. However, between
cycles, Wade’s group selection for low fecundity more than out-weighed the individual
selection and average fecundity declined. The group selection was strong enough to work.
In a way, the group selective structure of the experiment is superfluous. We could
simply breed from beetles with lower fecundity. Artificial selection of this kind would
reduce beetle fecundity without their being kept in groups for 37 days. But Wade’s purpose
was to illustrate group, not artificial, selection and his experiment does so. It has
alternating rounds of individual and group selection and the experimental group selection
is strong enough to produce the effect that Wynne-Edwards thought to be common
in nature. Box 11.1 shows how the group selection design of Wade’s experiment
has had a practical application.
The fact that group selection can be implemented in an experiment does not mean
that group selection is important in nature. Biologists doubt group selection for theoretical
reasons, and because of the kinds of adaptations seen in nature. The experiments
are instructive, however. They show what group selection means, and how individual
selection can decrease the efficiency of a group. Muir’s experiment (Box 11.1) also has a
commercial interest.
..