Choice that was natural for Darwin (9)
I believe the answer is:
selection
'choice that was natural for darwin' is the definition.
'selection' can be an answer for 'choice' (synonyms). I am unsure of the rest of the definition.
'choice that was natural' is the wordplay.
'choice' becomes 'select' (synonyms).
'that was natural' becomes 'ion' (I am not sure about this - if you are sure you should believe this answer much more).
'select'+'ion'='SELECTION'
Can you help me to learn more?
(Other definitions for selection that I've seen before include "Horse tipped for betting" , "Choice, pick" , "Act of choosing" .)