Evolution__3rd_Edition

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Adaptation and
Natural Selection
Part three
The three chapters of this part are about adaptation a the fit of organisms to life in their
environments. We begin with a conceptual chapter. Adaptation was known about
long before Darwin lived, and pre-Darwinian thinkers had tried to explain the existence
of adaptation. Chapter 10, however, argues that only natural selection can explain
adaptation. Some characters, particularly molecular characters, have evolved by processes
other than natural selection, but they are not adaptations. Not all evolution proceeds by natural
selection, but all adaptive evolution does. The chapter also looks at “gradualism” in
Darwin’s theory a it looks at the way new adaptations evolve by modifications of previously
existing parts, and at the size of the genetic changes that occur during adaptive evolution.
The chapter discusses how perfect the adaptations of living species are, and what constrainsts
there are on adaptive perfection. The chapter ends by considering various definitions
of adaptation.
In Chapter 11, we move on to ask what the entity is that adaptations evolve for the
benefit of. Evolution by natural selection happens because adaptations benefit something,
but what is it exactly a genes, whole genomes, individual organisms, groups of organisms,
species, or what? This is the question of “What is the unit of selection?” Adaptations, the
chapter suggests, usually benefit organisms, but there is a deeper criterion that can be used
to understand the exceptions as well as the rule: more fundamentally, adaptations evolve
for the benefit of genes. Only genes last long enough for natural selection to be able to
adjust their frequencies over evolutionary time. Organismal adaptations usually result
because gene reproduction is more closely tied to the reproduction of organisms than
any other entity, and gene reproduction is maximized if adaptations are at the organismal
level.