One of the unresolved-and rather bitter-disputes in evolutionary biology is between the creeps and the jerks. The creeps (so dubbed by the jerks) think that evolutionary change is gradual. The jerks (so dubbed by the creeps) think it happens in sudden jumps that are separated by long periods of statis. Probably, both are true. Work done a couple of years ago by Mark Pagel of Reading University, in England, suggests that about a fifth of evolutionary change happens jerkily at around the time new species form. The rest creeps in gradually over the millennia.
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