Should you wish to prevent systemd being installed when updating from Debian Wheezy to Jessie create a file like
Package: systemd-sysv Pin: release o=Debian Pin-Priority: -1
and ensure you uninstall the Gnome desktop and choose another as that has a "requires" dependency on systemd. Do this before changing the wheezy entries in your
/etc/apt/sources.list or the more modern
/etc/apt/sources.list.d/*.list components form to the jessie ones.