You know that joke-ish, brain-teasery thing about the doctor who says "I can't operate on my own son," but you're told the doctor isn't the patient's father, and the answer turns out to be that the doctor is a woman? It works, to the feeble extent it does, because it rests on our unconscious, culturally programmed preconceived notions—the kind of sexist background radiation that bombards us every day. You just assume the doctor is a man, for no clearly definable reason.
That's how the venerable British science-fiction adventure show Doctor Who worked for decades, despite the fact that its central character is defined by the ability to regenerate into an entirely different body (once the actor undertaking the role tires of all that running around and shouting technobabble and patiently Who-splaining the dire situation at hand to the nearest uncomprehending Earthling).
With the announcement that Jodie Whittaker would be assuming the role of the 13th Doctor, the show about time travel finally set foot in the 21st century. Something we've always sort-of known is now canon: The Doctor can be any sex, any gender, any race. The only thing that matters is the character's heart(s). As one wise cultural observer once opined, the show is about the triumph of intellect and romance over brute force and cynicism.
It would be a mistake to think that Whittaker's hire means there are now no limits on who can play The Doctor, however. As "The Woman Who Fell To Earth," the emotional and (literally) down-to-earth season opener on which Whittaker made her debut, amply demonstrated, the Doctor can be anything, as long as it's British.