Natural selection at the level of organisms is just the "regular" form of evolution that we are familiar with. I used the word "organism" simply as a contrast to other levels at which natural selection can act.
Note that although the DNA is the genetic material, a selective pressure such as the threat of predation acts on organisms. Very rarely would selection act directly on the DNA. Rather, the correlation between carrying a snippet of DNA and not getting eaten causes that snippet to become overrepresented in the population over time.
Got it, thank you. I'd love to see a good pointer to worked out examples of natural selection happening at non-organism levels if you have one handy. It sounds pretty fascinating.
Well, one of the best cell-level examples I can think of is somatic hypermutation, the mechanism by which your immune system refines a good antibody into an excellent one. Briefly, your body starts out with the B-cell that produces the good antibody, and randomly mutates the critical regions of the antibody gene, generating a large population of B-cells that each produce slightly different antibodies. The B-cells with the best antibodies (highest affinity for the antigen) are selected and given the "do not die" signal. The rest of the B-cells do not receive this signal and self-destruct. The end result is a population of B-cells that produce higher-affinity antibodies than the original. Repeat while ill.
I may misremember some of the details, but that's the gist. Your body basically generates antibodies via an evolutionary algorithm. Wikipedia has a good explanation. Keep an eye out for the word "selection": http://en.wikipedia.org/wiki/Somatic_hypermutation
Wow. That is nothing short of amazing. Thank you, your comment motivates me to learn more about biology. For one, I have been labouring under the illusion that the brain holds a monopoly in terms of organizing intelligent behaviour. The immune system is clearly another. Got any more examples?
This suggests that it takes a complex organism to fight disease. Once multicell organisms developed do you think an immune system was the first type of organized intelligence or that foreign invaders and parasites served as a catalyst to ever more complex systems?
It doesn't necessarily take a complex organism to fight disease. Pretty much every organism, including unicellular ones such as bacteria, possesses some form of innate immunity against that organism's most common pathogens. Simpler organisms fight disease by simply reproducing faster than the disease can kill them.
As for the brain having some sort of monopoly on "organizing intelligent bahavior", that is a complex issue. I wouldn't say that the adaptive immune system necessarily possesses intelligence. It consists of many small and relatively simple components that interact to produce complex behavior. Realize that during somatic hypermutation, the immune system is not intelligently choosing which mutations to generate. It simply generates a random sample of all possible mutations and then sets up a simple selection process that produces a result that looks intelligent to us.
On the other hand, one could plausibly make a similar argument about the brain's intelligence being the product of the interaction of individual neurons. Many people believe that our brains are no less deterministic than a silicon-based computer. Complexity is complex.
Anyway, to give you another example, cancer cells are subject to greatly increased mutation rates, and mutations that result in faster growth or better recruitment of nutrient-supplying blood vessels (vascularization) will quickly become fixed in the tumor as the cells with that mutation overtake the growth of others. This also provides an example where selection at the cellular level is at conflict with selection at the organismal level. Within the tumor, selection favors faster-growing cancer cells, but these same cells are clearly the most detrimental to the organism as a whole.
Note that although the DNA is the genetic material, a selective pressure such as the threat of predation acts on organisms. Very rarely would selection act directly on the DNA. Rather, the correlation between carrying a snippet of DNA and not getting eaten causes that snippet to become overrepresented in the population over time.