There's nothing formally in law requiring it. It's left to the parties, and they use various stratagems to try to achieve it. They're all committed to achieving greater diversity and ethnicity, but it really comes down to the parties to say how they do that.
Evidence of meeting #20 for Electoral Reform in the 42nd Parliament, 1st Session. (The original version is on Parliament’s site, as are the minutes.) The winning word was constituency.
A video is available from Parliament.