Evolution__3rd_Edition

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Box 11.1
Group Selection for Egg Laying in Hens
Muir (1995) found a practical application
of the basic experimental set up
used by Wade to illustrate group selection.
Muir repeated the experiment on
egg-laying chickens. Farmers have
selected chickens to lay the maximum
possible number of eggs, but in farms
chickens are often kept in groups. The
individual chicken that lays the most
eggs may then be an “antisocial” individual
who lays extra eggs by reducing
the productivity of other chickens in
the group. The bird that lays the most
eggs may, for instance, take a disproportionate
amount of resources, or be
aggressive to other birds. Muir kept
chickens in groups of nine birds, and
selected the most productive group.
The consequence was that the chickens
evolved to be less selfish, and mortality
declined. Productivity, in terms of numbers
of eggs laid, went up dramatically
from less than 100 to over 200 a year per
Most adaptations appear to benefit
the individual organism
hen, in five generations (Figure B11.1).
The experiments show how individual
and group selection can conflict. A
selective regime that prevents individual
selection can improve the average output
of the whole group. The improved
Eggs per hen housed
250
200
150
100
50
CHAPTER 11 / The Units of Selection 305
egg laying, combined with the improved
quality of social life for the chickens,
has considerable agricultural interest.
Further reading: Sober & Wilson (1998,
pp. 121–3).
0
2 3 4
Generation
5 6
Figure B11.1
Group selection for increased egg laying. Hens were kept in nine-hen groups,
and the groups that laid more eggs contributed more to the next generation,
in an experimental design similar to Figure 11.2. From Muir (1995).
11.2.6 Which level in the hierarchy of organization levels will evolve
adaptations is controlled by which level shows heritability
Adaptations can exist for the benefit of genes, cells, organisms, kin, or groups of
unrelated individuals. Genic adaptations like segregation distortion are rare; cell line
adaptations are very rare, at most, in Weismannist species, but may be found in non-
Weismannist species such as plants; organismic adaptations are common; the number
of examples of kin-selected adaptations is increasing; and group adaptations are probably
rare.
Why do adaptations mainly appear at the organismic level, with a few additional
cases for groups of kin? Earlier in this chapter, we have already discussed the answer to
this question, but a more general answer can be given. The units in nature that show
adaptations are the units that show heritability (Section 9.6, p. 235). Mutations that
influence the phenotype of a unit (whether a cell, organism, or group) must be passed
on to the offspring of that unit in the next generation; if this happens natural selection
can act to increase the mutation’s frequency.