Evolution__3rd_Edition

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4 Natural
Selection and
Variation
T his chapter first establishes the conditions for natural
selection to operate, and distinguishes directional,
stabilizing, and disruptive forms of selection. We then
consider how widely in nature the conditions are met, and
review the evidence for variation within species. The review
begins at the level of gross morphology and works down to
molecular variation. Variation originates by recombination
and mutation, and we finish by looking at the argument to
show that when new variation arises it is not “directed”
toward improved adaptation.