Evolution__3rd_Edition

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7 Natural
Selection and
Random Drift in Molecular
Evolution
T his chapter discusses the relative importance of two
processes in driving molecular evolution: random drift
and natural selection. We begin by looking at what it means
for drift to be a general explanation for molecular evolution.
We then go on to some features of molecular evolution and,
in particular, its relatively constant rate (the “molecular
clock”). We see how certain details of molecular evolution
have led to the development of the “nearly neutral” theory.
We then look at the relation between functional constraint
on molecules and their rate of evolution. Evolution in the
non-coding parts of DNA, and for synonymous changes
within genes, is probably mainly by drift. The relative
contributions of selection and drift to non-synonymous
(amino acid altering) changes are less clear. Natural
selection can leave its signature in the statistical properties
of DNA sequences, and the modern genomic era of biology
has made it possible to study selection and drift in new ways.
The chapter finishes by looking at four of these.