Make iPhoto overwrite my photos I'm new to iPhoto. Before importing any photos I unchecked the option "Copy items to the iPhoto library".

When I rotate and edit photos in iPhoto (e.g. Enhance, Straighten etc), the original photo is unchanged and iPhoto creates its own copy of the photo which it stores somewhere else.

Can I force iPhoto not to do this, and to modify the original photo?
No.

iPhoto is designed to make non-destructive edits, preserving the original photo file. Overriding this feature is rather contrary to the basic nature of the program, which attempts to be careful to preserve, and never to modify the original photo files.