"In a species with a million individuals," writes John H. Gillespie, "it takes roughly a million generations for genetic drift to change allele frequencies appreciably. There is no conceivable way of verifying that genetic drift changes allele frequencies in most natural populations. Our understanding that it does is entirely theoretical. Most population geneticists are not only comfortable with this state of affairs, but revel in the fact that they can demonstrate on the back of an envelope, rather than in the laboratory, how an important evolutionary force operates."

Longer than the back of an envelope but more concise than many books on the subject, this brief introduction to the field of population genetics offers students and researchers an overview of a discipline that is of growing importance. Chapter topics include genetic drift; natural selection; non-random mating, quantitative genetics; and the evolutionary advantage of sex. While each chapter treats a specific topic or problem in genetics, the common thread throughout the book is what Gillespie calls "the main obsession of our field," the recurring question, "Why is there so much genetic variation in natural populations?"