Organic Evolution - Definition
'Organic evolution' was a common term. It is just rarely used today. Today, we just say 'evolution' or 'evolutionary biology' when referring to the field of study of 'evolution' (or of study of 'evolutionary processes').
As you are used to work in applied mathematics, you'll definitely be interested in an intro book to population / evolutionary genetics. Population genetics is the mathematical basis of evolutionary theory. You will find recommendations at the post Books on population or evolutionary genetics?.
For you, I'd recommend Population Genetics: A Concise Guide as a start (or another one presented in the first section of the top answer to the post I referred to above). If then, you like coalescent theory (branching process), you might want to look at Coalescent Theory: an Introduction or some other more specialized books.
Not Math Oriented
Maybe you'll want something more introductory with much less math such as the Evolution: Making sense of life by Zimmer and Emlen or Evolution by Futuyma. These books are classic textbooks for intro class to evolutionary biology in Bachelor. These books will have the advantage to present you some other subfields of evolutionary biology such as the diversity of life, reading phylogenetic trees, ecological relationships and many other things that the books of population genetics don't go over.
Most common fields of math used in evolutionary biology
The most common fields of mathematics used in evolutionary biology are system dynamics, game theory, approximation theory, probability theory, branching processes and diffusion equations.