Reddit reviews Nietzsche, Biology and Metaphor

Plato, Kant, Schopenhauer. These are the philosophers he is engaged with/indebted to the most. His references to their ideas and works are sometimes direct, often not, so understanding the background here is essential to be able to read and appreciate what Nietzsche is getting on about in several places.

A major influence of Nietzsche was Friedrich Lange, especially his History of Materialism. Nietzsche is known to have read and re-read this book over two dozen times, and reading it you will discover where Nietzsche's inspiration came from. This book's presentation of Kant must have had a major impact on Nietzsche's conceptualization of Kant (it's not clear exactly how many primary sources of Kant's works Nietzsche actually read).

There are a number of minor figures to read, but this is more for specialists or really deep-diving. This resource on Nietzsche's library is great to find out what he was reading when he was working on various projects, and the primary sources can add a lot of depth and missing context to Nietzsche's more obscure aphorisms.

Edit: I decided to add some more stuff.

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• Eternal recurrence, Nietzsche's problems with Plato and Kant, and his basic indebtedness to late 19th century European thought is covered in this excellent book by George Stack. It's really expensive, but you could probably get a copy through inter-library loan or something.
  
• Nietzsche's use of physiological metaphors and concepts are derived mainly from the works of William Roux. These connections are explored in a good book by Gregory Moore. This book also explores the Nietzsche's understanding of Darwinism and evolution generally. Highly recommended.
  
• The notorious concept of will to power may have been inspired in part by Nietzsche's readings of Boscovich, especially his Theory of Natural Philosophy. Nietzsche's thoughts on 'becoming' are indebted to both Heraclitus and to the ideas on dynamics found in Boscovich.
  
• A little known philosopher named Afrikan Spir and his best known book Thought and Reality was read by Nietzsche a few times. The Neo-Kantian ideas of Lange and Spir seemed to have made more of an impact on Nietzsche than Kant himself.

>As someone within an evolutionary field, Nietzsche has never been discussed meaningfully to my knowledge within the purview of evolution.

If you mean that no evolutionary biologists have talked about Nietzsche's thoughts on evolution, I don't know whether that's true or not. But lots of philosophers definitely have talked about Nietzsche's thoughts on evolution. Here are a few:

https://www.amazon.com/Nietzsches-New-Darwinism-John-Richardson/dp/0195380290

https://muse.jhu.edu/article/517309

https://muse.jhu.edu/article/517310/pdf

https://www.amazon.com/Nietzsche-Biology-Metaphor-Gregory-Moore/dp/0521812305

http://www.counsellingeastlondon.org/wp-content/uploads/2012/07/The-Blackwell-Companion-to-Nietzsche.pdf#page=524

http://brianleiternietzsche.blogspot.de/2009/02/nietzsche-and-lamarck.html