You can find the PDF version of this book on various online platforms, but I must inform you that some of these sources might not be officially affiliated with the publisher or authors. However, here are a few options:
and first published in 1970, is widely considered the foundational text for modern theoretical population genetics. The book bridged the gap between purely descriptive biology and rigorous mathematical modeling, establishing a quantitative framework for understanding how evolutionary forces—such as natural selection, genetic drift, and mutation—shape the genetic composition of populations over time. Science | AAAS Core Concepts and Structure an introduction to population genetics theory pdf
"The theory of population genetics is the foundation of evolutionary studies. Without it, evolution is just stamp collecting." You can find the PDF version of this