The Atheist's Guide to Reality_ Enjoying Life Without Illusions - Alex Rosenberg [32]
When chemical reactions happen billions of times in small regions of space and time, even a small percentage of exact copies quickly come to number in the millions, as does the percentage of slightly varied copies, with one or two atoms of different elements in place of the original atoms. Most of the time, the outcome of this is process is wastage—a molecule that doesn’t replicate itself or that falls apart, just as the second law requires. But sometimes—very rarely—variation produces a molecule that is slightly better at replicating or one that is just a little more stable.
Now we have replication and variation. What about fitness differences, the last of the three requirements for evolution by natural selection? Fitness is easiest to understand at the level of molecules bouncing around in a world controlled by the second law. Molecules that form from atoms are stable for a certain length of time. Some break apart right after forming because their bonding is too weak to withstand the force of other atoms that bounce into them or even just pass by. Some “fragile” molecules will remain intact for a while. They just happen by chance to avoid bouncing up against other molecules, ones with stronger charges that pull atoms away from their neighbors. Here again the second law rears its head. As molecules bounce around, any amount of order, structure, pattern almost always gives way to disorder, to entropy. Hardly any molecule is stable for extremely long periods, with the exception of the bonded carbon atoms in a diamond crystal.
There are differences in stability among molecules owing to the variations that inexact replication permits. Differences in stability have an impact on the replication of different types of molecules. A template molecule produces copies just by random interaction with atoms that bounce into it or pass close enough to be attracted. The longer the original templating molecule stays in one piece—that is, the more stable it is, the more copies it can make. Most of its copies will be just as stable as the original template molecule, since they will be exact duplicates. They will serve as templates for more copies, and so on, multiplying copies of the original.
Of course, just as there are differences in the stability of different molecules, there are differences in their rates of replication. Both the number of copies of their templates that can be made and their stability will depend on their “environment”—the temperature, the local electric and magnetic fields, the concentration of other atoms and molecules around them. Consider two molecules that differ from one another along the two dimensions of stability and ease of replication. The first remains intact on average twice as long as the second; the second templates twice as many copies per unit of time as the first. Over the same period, they will produce exactly the same number of copies. What will be the long-term proportions of molecules of the two types? It will be 1 to 1. As far as producing copies are concerned, the two different molecules will have equal reproductive fitness. And of course, if their differences in stability and replicability don’t perfectly balance out, then after a time there are going to be more copies of one type of molecule than of the other.
Molecules randomly bouncing around a region of space and bonding to form larger molecules will eventually, randomly, result in a few stable and replicating structures. Their structures will vary, and the variations will affect