Contrary to their Biblical counterparts, mitochondrial-eve & Y-chromosome-
adam do not represent the singular founders of the human species. The
mitochondrial-eve/ Y-chromosome-adam phenomena are more like winning a
lottery than anything else. It's the natural outcome over time in any
group of reproductively linked organisms with a finite population. At some
point in any population, only one, unbroken, female lineage will emerge.
This will happen whenever the last of a "rival female dynasty" fails to
produce a daughter in the current generation.
The same goes for male lineages. All these takeovers can happen while the
population size remains constant (The average number of generations between
a lineage's progenitor and the "takeover time" in the population being
strongly dependent on things such as population sizes and modes of
inheritance). At some point the title of mitochondrial-eve will be passed
on to one of the daughters in her lineage, just as she "won" the title
from one of her mothers in the past.
This is a statistical, rather than Biblical phenomenon. It is, in fact,
a simple prediction -- not of evolution, per se, but a prediction of
what happens when a species of a certain size lives a certain number of
years. There will be parts of the genome which have older progenitors
than mito-eve or y-chromosome-adam (& there may be parts which are
younger). I thought about putting together a quick Excel macro to
demonstrate this outcome but I think the latest computer virus scare
put a damper on that. ;^)
But here's how to model it (crudely) with a coin toss: Take a sheet of
paper and draw four "Y"s across the top (We'll model "adam" here...).
For each figure flip a coin twice(two kids/individual). If a coin lands
"heads", draw a "Y" under the "father"; for each "tail" an "X". Do that
for all four "fathers". Repeat the two-coin-flip process for each "Y"
(son) in the each following generation (Two "X"s from a father ends
that particular lineage). Eventually, all but one of the original "Y"
lineages will terminate with two "tails". (There is an alternate
possibility that the last lineages will simultaneously "crap out" in
the same generation but that's an artifact of small sample sizes
and the somewhat unrealistic set-up of only two offspring per