I've been wanting to see if I can do a better job on my original topic here:
Natural Limitation to Evolutionary Processes . As I've reread that old thread I see I got overwhelmed and defensive and didn't do a very good job of thinking through my answers. In some cases I never got back to answering some people at all, Pink Sasquatch in particular, and I'd like to try again.
In a nutshell: Evolution is said to be powered by natural selection, and some say other processes contribute as well, such as genetic drift. You will also find in definitions of evolution on some websites the statement that variability is necessary for evolution to occur. My argument is that natural selection and genetic drift, all the processes that select or isolate a portion of a population, do bring about the change called evolution but also always reduce genetic variability, which is the opposite of what evolution needs.
Paul K answered my opening post on that thread this way:
If you accept for the sake of argument that new variation is entering the population through mutation then what happens depends on the rate at which new variations appear against the rate at which variation is lost through selection. Only if the rate at which variation is lost is greater than the rate at which new variation enters the population will there be a net decrease in variation.
This is the answer that is always given to this argument, and Pink Sasquatch gives another version of the same answer in her post that follows PaulK's. I answered in turn many times that increase in variation (sometimes called "information") doesn't prevent this ultimate reduction -- you can have as much variability as mutation or any other source of genetic variability can provide, but the processes that select and isolate, which are considered essential to evolution, inevitably work to reduce the needed genetic variability, and this spells the end of evolutionary processes. It seems to be generally overlooked that for evolution to occur, alleles must be eliminated, thus reducing genetic diversity.
But I realize this has to be demonstrated.
Genetic drift is demonstrated at Wikipedia with a series of jars representing subsequent generations of a population through which drift occurs, demonstrating that while the population starts out with equal proportions of blue and pink alleles, over time drift completely eliminates the pink ones leaving only the blue,
a complete elimination of one allele.
The same thing happens but more systematically if
natural selection is doing the eliminating.
Then if you think
mutation can save the day, all that happens is that the mutated allele gradually eliminates all the other alleles, once again eliminating genetic diversity.
This is only what happens to one gene of course, but the trend is inexorable. There is no way to get a trait established in a population if alleles in competition with the allele for that trait are not eliminated.
However, there's usually more going on than just the establishment of a single allele. Even in natural selection for a single trait you have to remember that individuals carry these traits and their alleles, and its the individual that is selected for its trait. This individual's genome is full of other genes for other traits, so when it passes on its selected allele it also passes on all the rest and over time if this trend continues the population will simply become more like that individual all around and alleles for that individual's traits will all have been reduced or eliminated, substantially reducing the genetic variability in the gene pool as a whole.
However, it is possible of course for the single allele to work its way through the population without remaking the whole population in the image of this individual, if it is that particular trait that has special reproductive value from generation to generation. This seems to be the point Pink Sasquatch was making. Then you will only have eliminated alleles for that trait. You'll have a population of frogs with better ability to catch flies as she was saying, but you'll be pretty far short of a new species. Small changes occur in populations all the time for various reasons without getting anywhere near speciation.
With a bottleneck, or domestic selection, on the other hand, where a phenotype is selected (or randomly isolated in the case of bottleneck) and is completely reproductively isolated, then you get the dramatic changes in many traits at once, and when you have that you also have dramatic genetic reduction for all those traits. Often this situation leads to inability to interbreed with former populations and is known as a new species.
I've always liked the cheetah example because it is a case of a wonderfully selected animal that demonstrates extreme genetic reduction, to the point of fixed loci for many traits.
But whether we are talking only about a change in a single trait or in many traits at once, the trend is ALWAYS toward genetic depletion. You can add as many new alleles as you think mutation can come up with at any point in this progression, but when these selection and isolating processes go to work on them the very same thing happens. You may get a new trait but you'll always get it at the expense of all the other genetic possibilities, and when this occurs with many traits you eventually get speciation, fixed loci, and such limited ability for further variation evolution is for all intents and purposes at an end.
To my mind this absolutely spells the end of evolution. Evolution itself defeats evolution.
Edited by Faith, : to fix url
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Edited by Admin, : Fix quote: [q] => [qs]
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