I flog R. A. Fisher's The Genetical Theory of Natural Selection a fair amount on this site. You don't need to understand everything in the book, nor do you have to agree with everything in it, but it is a great point of departure toward understanding evolutionary genetics. I've noted that you can get it free in PDF format. But if you want to browse it online in a easier format, here you go: (original link courtesy of Unz.org)
The Genetical Theory of Natural Selection
Explore the Genetical Theory of Natural Selection by R. A. Fisher to deepen your understanding of evolutionary genetics.
More on Discover
Stay Curious
SubscribeTo The Magazine
Save up to 40% off the cover price when you subscribe to Discover magazine.
Subscribe