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Re: Be fruitful and multiply... within reason

Posted by Bob on December 5, 1999, at 9:31:35

In reply to Be fruitful and multiply., posted by CC on December 3, 1999, at 16:45:53

> Mutations can increase the ability of a species to survive although it would be a rare event.

I don't think you're taking it far enough here. It would be an exceedingly rare event, but we're talking about rare in terms of probability. For example, an "exceedingly rare" event may have a 1 in 1 billion (0.0000001%) chance of occuring. Just like playing Lotto -- if all you've got is one shot (or five or some other low number in comparison to the P-value of the event), then the Law of Probability is going to kick your arse in all likelihood. However, if instead of one shot at producing a particular result you have more like a number of shots equal to several orders of magnitude larger than the entire "population" of events (1 billion, for comparison to the 1 in a billion shot) then the Law of Large Numbers starts coming into play, providing some assurance that that one in a billion event WILL happen. I'm not talking about a billion events ... more like a billion of a billion events.

> For example, chimpanzees and humans are 98% genetically similiar.

... but don't forget that it's not humans who have been drifting apart from chimps, but rather its both chimps and humans who have been drifting apart from some common ancestor -- two more complex genomes from one common, simpler genome. And are chimps even our closest relative? If not, you have to reach even further back in our genetic history to a less-evolved common ancestor.

> How many genes do humans have, ~10^100?

One googol of genes?! Now there's a number I haven't seen used in quite some time....

No, humans have less than 10^5 (100,000) genes. In terms of base pairs, it's more like 3 billion in the human genome. So, even though matters are 2^(10^93) times less complex than you suggest, the odds are still pretty daunting ... particularly since the more complex aspects of our functioning are prboably determined by, to be conservative, more than one gene.

OTOH, Toss into the mix that adult humans have *trillions* of cells in their bodies. Toss in again how many cell divisions happen to those trillions of cells over the average lifespan of an individual. Now take into account that we are not talking about my genome or your genome, but the HUMAN genome. For all individuals alive at this moment in time -- how many opportunities for mutation are there? 5.5 billion individuals times several trillion cells times some probably large number of cell divisions over a lifetime. Toss into that mix the variety that can be produced not by cell division but, instead, by sexual reproduction. And now toss in all the individuals that we can consider to be homo sapians over the last, what?, twenty thousand years ... or is it more or less?

Are you getting some idea of just how staggeringly high the number of events are compared to the chance for any one given mutation?

If we're going to throw in googols, turn the numbers around. It's more like there have been one googol (give or take a dozen orders of magnitude) of possible events to produce mutations, rather than that being the chance of getting one (and I'm being generous here by not *raising* the stakes to a geometric (power of 2) level).

Now, the real kicker is this. The genome isn't looking for that one mutation, nor is Mother Nature or any other mystical force trying to add a sense of direction to what might be complete chaos. Any given gene may mutate. Maybe someone out there can tell me, with a ballpark figure, how many ways any individual gene CAN mutatue ... I doubt there's only one path for mutation per gene. So, we're not looking for one single event and how many times it gets reproduced in all those possible events listed three paragraphs above ... we must talk about ALL mutations that occur in those events. Again, there is no way of telling a priori, given our knowledge of how the genome produces us, whether one particular mutation is going to produce, say, a digestive system better able to handle a diet of processed foods laced with BGH-like chemicals and pesticides or a brain capable of faster speeds in addition to all the parallel processing it already does.

Heck, somedays I'm just happy to have an opposable thumb (one genetic point I like to torment cats about whenever possible ... just imagine how much more mischief they could cause with opposable thumbs!)

(Source for my figures:, a Lawrence Berkeley National Lab publication for the Human Genome Project)

> ... does it seem reasonable that this much genetic change could occur through random events, with or without biological selection, within the time frame generally accepted?

I fint it reasonable that there should be far more variation that there is. I also find it a gift from God that there is so little genetic variation within our species ... we have such a hard time, given our level of "civilization" and "humanity", dealing with difference as it is.





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