quoting @za419: Actually, who she is changes all the time - If a woman who's carrying the last copy of a mitochondrial branch (imagine Eve has two daughters, and one of their female lines has only one surviving woman) dies daughterless (either doesn't have surviving children or only has sons), then the title of Mitochondrial Eve might move down a generation (if Eve had two daughters and one line dies out, then the daughter who's female lines survive is the new Mitochondrial Eve).