yank-pop-change-selection

yank-pop-change-selection is a variable defined in `simple.el'.
Its value is

nil

Documentation:
Whether rotating the kill ring changes the window system selection.
If non-nil, whenever the kill ring is rotated (usually via the
`yank-pop' command), Emacs also calls `interprogram-cut-function'
to copy the new kill to the window system selection.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.