Darwinian evolution involves the change over time of the genetic composition of populations and is thus a genetic theory. Although Mendel published his genetic studies of pea plants in 1866, not long after The Origin, Darwin never saw them, so a key component of the theory was missing. The rediscovery of Mendel’s work in 1900 unexpectedly provoked a major controversy among evolutionary biologists. Some argued that Mendel’s discoveries did not apply to most genetic variation because the traits studied by Mendel were discrete, meaning that they had clear alternative states, such as either yellow or green color in peas. Most of the variation we see in natural populations, in contrast, is continuous, meaning that variation occurs across a spectrum (Chapter 18). Human height, for example, does not come in discrete classes. People are not either 5 feet tall or 6 feet tall and of no height in between. Instead, they may be any height within a certain range.

How could the factors that controlled Mendel’s discrete traits account for the continuous variation seen in natural populations? This question was answered by the English theoretician Ronald Fisher, who realized that, instead of a single gene contributing to a trait like human height, there could be several genes that contribute to the trait. He argued that extending Mendel’s theory to include multiple genes per trait could account for patterns of continuous variation that we see all around us.

Fisher’s insight formed the basis of a synthesis between Darwin’s theory of natural selection and Mendelian genetics that was forged during the middle part of the twentieth century. The product of this Modern Synthesis is our current theory of evolution.