Evolution__3rd_Edition

11The Units of Selection
A daptations clearly benefit something in the living world,
but to understand exactly why adaptations evolve we
need to know in theory what it really is that adaptations
evolve for the benefit of. This chapter begins by explaining
the problem. The first main section of the chapter considers a
series of adaptations which benefit increasingly higher levels
of organization of life: we start with adaptations that benefit
only a small clusters of genes, and move through the cell,
organism, and family levels, up to possible adaptations to
benefit whole groups. The examples illustrate the conditions
for adaptations to evolve to benefit the different hierarchical
levels, and reveal why adaptations at most levels other than
the organism (and family group) are rare, though not nonexistent.
We finish this section with a general criterion that
an entity must satisfy in order to evolve adaptations: it must
show heritability. The second main section of the chapter
asks the more fundamental question of what entity natural
selection operates on, and describes an argument to suggest
that the entity is the gene, though defined in a special sense.
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