I'm sort of curious what you mean by "variability." Like, what is it, and how would we measure it?
Hmmmm? I don't know if you can really quantify variability in linear terms, because with every passing generation, there is always going to be some shuffling of extant genes, some mutations along the way, and so on. And since almost every organism is unique in some capacity, there is always some variability.
Do you consider it a property of individuals or of species or populations?
Well, it starts on the individual level, and we could have different expressions found in a peripheral population that finds itself isolated from the main populace. The question is whether enough favorable mutations x natural selection can support or account for the amount of diversity we currently see found on earth.
In a static population where the reproduction rate is slow (by slow, I mean they don't produce much progeny: i.e. humans) the ration of fixed genes would be 1:300 because of cost substitution. Organisms of "bad stock" or "low fitness" would prevail, even in spite of natural selection.
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Imagine a population of 100,000 of those organisms quietly evolving their way to humanity. For easy visualization, I'll have you imagine a scenario that favors rapid evolution. Imagine evolution happens like this. Every generation, one male and one female receive a beneficial mutation so advantageous that the 999,998 others die off immediately, and the population is then replenished in one generation by the surviving couple. Imagine evolution happens like this, generation after generation, for ten million years. How many beneficial mutations could be substituted at this crashing pace? One per generation -- or 500,000 nucleotides. That's 0.014 percent of the genome. (That is a minuscule fraction of the 2 to 3 percent that separates us from chimpanzees)." -Ted Holden
How then can we account for all of the diversity we find?
"A man can no more diminish God's glory by refusing to worship Him than a lunatic can put out the sun by scribbling the word, 'darkness' on the walls of his cell." -C.S. Lewis