Natural selection explains many of the most fascinating things in nature, from
the genetic code to the complexities of the human brain. Natural selection is
fundamentally a simple concept. But its explanatory power expands when we
see how it acts, and how the evolutionary outcome depends on genetics and
development. When we understand these sides of the evolutionary process,
we can begin to answer a cascade of fascinating questions: How do coopera-
tive and selfish behaviors evolve? Why do some species reproduce sexually
and others asexually? Are different populations of a species likely to evolve
the same way if they experience the same environmental challenge? What
explains the extraordinary display of the peacock, the immense fecundity of
oysters, the brevity of a mayfly’s life, the pregnancy of the male sea horse, the
abundance of transposable elements in our own genome?
Darwin realized that a complete understanding of evolution requires under-
standing the mechanism of inheritance. One of the biggest frustrations of his
life was that he never was able to learn that mechanism. The field of genetics
began after Darwin died, when Mendel’s work was rediscovered in 1900. Mod-
ern evolutionary genetics started to develop with the synthesis of Mendelian
genetics and Darwin’s theory of natural selection, and the discovery of other
forces that can also cause evolutionary change. The keystone to understanding
how evolution works is the “genetical theory of natural selection,” to quote the
title of a famous 1930 book by the pioneering population geneticist R. A. Fisher.The Genetical
The Genetical Theory of Natural Selection
Species selection
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The brilliant color patterns of tropical butterflies in the genus Heliconius warn predatory birds
that they are poisonous. Butterflies with color patterns that are common have an advantage
because birds quickly learn to avoid them after tasting a few. Rare color patterns (as seen in
this hybrid individual) are disadvantageous because birds have not yet learned to avoid them.05_EVOL4E_CH05.indd 103 3/23/17 9:01 AM