h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This entry has, as far as I can tell, grasped completely the wrong end of the stick in two important respects. First, there is no bottleneck effect and secondly, Mitochondrial Eve is a phenomenon of chance, not selection.

There is no particular value to the mutations that accumulate in mitochondrial DNA. It is precisely for this reason that we can use the mtDNA clock to date "Eve". If there was a selective advantage, the ticking of the mtDNA clock would not be at a constant rate.

One of your other correspondents had it right when he or she said that what is important aobut mitochondrial Eve is not that her genes were especially useful, but that she had at least one daughter, and that her daughters had daughters, and so forth for all the generation between Eve and now. Mitochondrial Eve is the last person whose DNA we can trace to all modern people. Obviously Eve's mother, one of her grandmothers, one of her greatgrandmothers, etc also gave rise to the mitochondrial DNA we all have today. But she was the youngest person we can trace.

There is a woman alive today who may one day inherit Eve's mantle. It gets a little more difficult to imagine how one person can pass on a set of genes to everyone who is alive subsequently. But really its just a statistical thing. There is a chance in every generation that a woman will, in effect, not pass on her mitochondrial DNA. If she has no children or only has sons, that is the end of her mitochondrial DNA lineage. In time - say some 200,000 years or 10,000 generations from now - it will happen that every mitochondrial line, except one, will end in this way. In the year 202003 or thereabouts, everyone on earth will be be able to trace a mitochondrial root to one woman living in 2003. She will be the new "Mitochondrial Eve". Clearly she is descended from the mitochondrial Eve of 200,000 years ago.

I could offer a clearly copyrighted explanation (from an as yet unpublished book) which goes into more depth. But having read the T&C of the site, I'd better not.