Monday, June 20, 2016

Questions of Evolution

Evolution’s details remain a puzzle as the problem of archeogenetics, the figuring out of the pathway that evolution took from simply chemicals to advanced organism, is extremely complicated. Over the billion years or two or three that it took, there were literally hundreds of thousands or even tens of millions of mutations which were compared using a wide variety of fitness tests. Almost all of the losers have disappeared, leaving no trace, no fossils even, as all organic chemicals are decomposed in short periods compared to the length of evolution. To determine what came first, what came second, and what came hundred and sixtieth is a challenge that no one has figured out how to solve. Yet, it may well be that one or more difficult hurdles for life was met and passed on our planet, but for one reason or another, it was failed on most other planets. Without knowing what these hurdles were, it is hard to know if it was planetary conditions that limits the number of aliens visible from Earth, transmitting signals hither and yon, coming to visit us, or leaving monuments here for us to find when we evolve far enough to recognize them. We don’t know if instead it was some evolutionary jump that is just so rare that only we have made it, unknowingly, that allows us to approach so close to star travel, a few centuries perhaps, and the other hundred million planets in the Milky Way to be nowhere near as lucky.

One more aspect of evolution is that it proceeds by small steps. To get from chemicals to roosters, evolution cannot do five or ten mutations to some genetic coding and then we hear the cocks crowing. Each mutation makes a small change in the total picture of a cell or an organism, and it has to be compared, in whatever fitness competition exists for this cell or organism, so that the older original one can be displaced and the new version take over. If the cell is numerous, say quadrillions or quintillions of them exist, and that mutation occurred in one of them, how long is it going to be before all the quadrillions and quintillions are the new version? This of course is a multiple of the generation time, but remember that those original cells were doing just fine, surviving and propagating well enough to produce stupendous numbers of copies, and they are all not going to just up and go extinct because somewhere there is one cell which is better in some way than they are.

Thus there are two key times for each mutation, one is the time it takes for some cosmic ray or chemical mutagen or coding error to get lucky and make this specific change, and the other is the time for the fitness competition to eliminate the old versions in favor of the new. Recall that fitness is chancy, and the first cell might just happen to get eaten before it had a chance to even make its first copy, and evolution would have to wait the same time, all over again, for a second shot at this particular mutation. There are many perils facing any individual cell, and any one of them can do an extinction event on a just mutated version of a cell.

As for the elimination process, in most cases it is not the new version taking on the original version, vanquishing it and eliminating it, and them moving on to the rest of the quadrillion copies. Instead, it is a multiplication rate question coupled with a limitation on total numbers. In a situation where there was bountiful supplies for both the original and the mutated version of the cell, there is no fitness competition, and both just keep merrily producing descendants of their own particular type. Only if there is some limit to the number of them does fitness play a role. In that situation, the rate of multiplication of one has to be just a bit larger than the other. Perhaps this single mutation puts the replication rate of the new version at a tenth of a percent higher than the old version. This is a tremendous advantage. In less than a hundred thousand generations, the old version will disappear. If the cell is in an environment where it can make a copy of itself in a day, this is zero time in the evolutionary time scale. Even if the environment is harsh, and it takes a year, it is still zero time. But if the mutation is not so great at changing the replication rate, so it is only a ten-thousandth of a percent difference, then evolution doesn’t last long enough for the mutation to successfully displace all the old version, at least in more difficult environments.

This means that if we want to make some progress in unraveling the mystery of archeogenetics, we need to think about fitness comparisons and the time taken in the competition, and about mutation rates. Either a very rare mutation or a very slight increase in replication rate could mean that some mutation here doesn’t happen on all other planets where life is trying to evolve.

For mutation rates, mutagens are working all the time, but perhaps on planet X there are not nearly so many cosmic rays, because of some stellar magnetic shielding, or because of the galactic environment. Mutations from this source are needed to penetrate into some nook of the genetic code to make a change worse. So, do we have skyscrapers and hydrofoils here because of our cosmic ray dosage and planets without it do not? For fitness comparisons, is the replication rate closely linked to temperature or salinity or the presence of magnesium ions or something else in the oceans, which just happen to happen here and nowhere else? We get a hundredth of a percent for some critical change and everybody else gets a hundred thousandth of a percent change? Again, those skyscrapers and hydrofoils are only going to be on Earth.

The first thing that has to be done is some overview of evolution, so that a comparison of which mutations are important and which ones are incidental can be made. Then perhaps some insights might be generated on only those important ones, and a bit of progress in the evolution mystery can be obtained. Then, it might be possible to make some good guesses as to whether evolution has any chokepoints for alien worlds, or whether it just works everywhere like a well-oiled machine and doesn’t provide any distinction between Earth and other solo planets.


  1. I'm Loving your blog Stan, great stuff to think about. Keep up the good work.