As much as we know, everything in our Universe at the present time is in process of undergoing changes both evolutionary (smooth, and up to the point, mostly quantitative change) and revolutionary (abrupt and typically qualitative changes).
The cosmic level of natural selection: Imagine the beginning of our Universe, whether it was Big Bang or the coalescence of the matter. Probably there were multiple formations and systems, some more stable and some less. Less stable ones disappeared in explosions, implosions, black holes, fusions, annihilations and in many other unknown yet processes. The stable ones survived longer, and we can watch them in the sky, including those that were "living" 5 billions years ago in another Metha-Galaxy, whose light is just reaching the Earth as we speak.
Similarly, the Solar system underwent many changes, some planets survived, some not, and by now almost all possible collisions with vast majority of massive asteroids on majority of orbit have happened already. So, the chances of another cataclysms is pretty low (which is encouraging, right?), and the Solar system so far is in the state of dynamic stability.
Atomic/particles/field level of natural selection:
Imagine the nuclear fission or fusion bomb explosion. The very first moments there is an immense diversity of different isotopes, particles and rays/fields. The most unstable ones are extremely short lived, in tiny faction of a second they disappear, being transformed into other forms also with various "life span". The natural selection gradually sorts them out– for some of them it takes seconds, for some year, for Uranium half-life is 2.5 billion years. Of course, both intrinsic and extrinsic factors (such as cosmic rays) keep influencing the existing equilibrium, but nevertheless we can call it relatively stable stage of dynamic stability.
Chemical & physical natural selection and dynamic stability:
When our Earth became solid, probably, 6 billion years ago, it was affected by different kinds of energy of extremely high levels , including but not limited to temperature, irradiation, physical processes and chemical reactions which resulted a huge diversity of compounds and chemicals. The Moon was circling around the Earth in a very close proximity of 20,000 km (today almost 20 time farther away), causing tidal waves of lava tsunami splashing through the broken by the gravitational effects Earth crust rather than today's water tides.This variability and diversity presented multiple opportunities to combine them in the nature laboratory later on, which 4.5 billion years ago resulted in the evolving first life forms.
Biological level of natural selection:
My definition of life is rather broad :
"Life is a complex system in the dynamic stability state that in the ever changing environment maintains its dynamic stability by using self-regulation processes with the feed-back loop and reproduces itself". The key word in stability is "dynamic": as Heraclitus noted, you cannot twice step into the same river because the waters keep changing even though the entity considered to stay the same. We ourselves also keep changing– in a period of several months majority of atoms in our body are completely replaced by new ones and yet we consider ourselves "Yes, I am the same person even though I keep changing". One might find inorganic, electronic and even cosmic structures that will meet this definition, and I would consider them a life form too. Computer viruses can be an example. Imagine a self-learning program that will evolve high enough to be self-aware. Well, you've seen those in Sci Fi movies already, but I read about Japanese program like that in a LAN. Hopefully, it will not get sophisticated enough to find a way from LAN (local area network) into WEB (world wide web). We have enough problems without Terminators. In a way, all evolving and self-regulated systems may be considered a living organism: a language and a society too.
There were many life forms in the primary ocean that were as alien to us as another planet life forms. We are not likely to learn about them in a forseeble future because majority of them are gone forever whether due to competition or because of the everchanging environment. We know only about tiny faction of the ancient life forms and species. Primary ocean was a hot place, there was no ice on the planet, so all the fresh water was in the ocean with resulting diluted salt concentration about 0.9%, and since then all the life forms on Earth, whether living in the 24% salt solution or on the Everest mountain, still have the same 0.9% in their blood. Life is very conservative, it almost never re-shuffles the existing structures preferring rather building another "store" on the multi-story building. That's why "social engineering" of human being will never work– we are what we are: the end result of 4.5 billion year natural selection. We share with gorillas 98% of genes, but majority of them are inactive ones that just like uninhabited ground floors with only the upper floor, built on the top, in use. When you compare only working, expressed genes, then the difference between us is significantly bigger. Those of our genes that code for gills, tails, scales or sea worm eyes and brittles normally are repressed.
Almost 100% of the archaic life forms and species are gone, and this process of disappearence of the old species and evolving new ones keeps going on now. Therefore, whining about species going extinct is not always justified– it may be just a part of the natural process. Some of the oldest bacteria are thermoacidophilic Archeobacter, that nowdays can live only deep under water close to volcanos, enjoying hot bath with several hudnred degrees temperatures. Majority of them were competing and destroying each other. Some life forms formed successful symbiosis combining best of both worlds. For example, absorbing the main source of energy on Earth chloroplasts very likely originally were a separate life form. Ancestors of today's plants (and some protozoa-single cell animals like Euglena) incorporated them into their cells providing shelter and substrates while chloroplasts provided with Sun's energy. Another similar example of successful symbiosis is our ancestors' cell that incorporated mitochondria. Our cell provided, again, shelter and material substrates while mitochondria generated our main source of energy– ATP (adeno-tri-phosphate). Since then our cells have 2 sets of genetics: our famous double helix DNA and mitochondria's circular DNA that has different altogether coding sequences. Male sperm cell, just like original cell, has no incorporated mitochondria. Thus, when the sperm cell uses up all its ATP energy store (typically in 3 days), then it dies. That is why mitochondrial mutations and diseases are passed along only via maternal line, since the ovum has incorporated mitochondria, and genetic studies of our maternal ancestors are also based on the mitochondrial DNA analysis. Paternal studies are based on the short Y-chromosome, and that's how scientists learned that 18% of males in Asia carry genes of Mongolian Genghis Khan. Each night he had a different woman, and if the female offspring was also about 18%, then 36% of Asians are his descendants. As the story goes, he was stabbed to death in the middle of the night by Tibetian princess after he raped her too. Since for such a great warrier was a shame to be killed by a woman, so his court officially announced that he died due to a hunting accident.
Another life form that has survived is a mycoplasma, which unlike us doesn't have a firm cell membrane but rather a thin layer. This life form causes atypical pneumonia and STD with PID, possible infertility and premature births. Slime mold is also pecular life form that can be seen as a single cell organism, as a fungus-like form, and as a multicellular slowly moving slime.
Existing life forms broken down to Kingdoms, Subkingdoms, Classes, Orders, Family and Genus. I personally see natural selection working by 2 mechanisms: evolution and revolution.
Evolution of life forms is slow process of changin existing species, and it is capable of creating other species, but, probably, not other classes.
Revolution of life forms is a rapid change typically due to the catastrophic events such as a huge asteroid hit that unearths radioactive Uranium to the Earth surface. First it kills majority of the life forms with greatly reduced diversity, then it causes massive mutations. Under normal circumstances majority of mutations are detrimental or lethal, while very few are beneficial, and they are relatively rare, so the chance of 2 beneficial mutants meeting each other and starting new Class or Order is exceedingly low. However, when the catastrophe causes massive mutations then the beneficial mutants do have a chance to meet each other and start a new Class or a species with drastic changes. That's how the feathers and the spines have evolved.
Competition exerts a natural selection pressure, frequently selecting the most adapted to the current environment and conditions individuals. They may be the fittest, but under scarce food conditions they might be the smallest, or the slowest metabolism-wise, or the strongest, or with highest indurance : whatever it takes to survive better in this particular and everchanging environment. Seemingly the same piranha fish schools in one part of the Amazon river might devour an animal in a matter of seconds, and in another part they are peaceful herbivours, nibbling immersed into the water berries. On one of 2 isolated by the ocean corall islands the same fish can be edible, but on another one deadly poisonous. About "most adapted ones": it is true frequently, but not always, because there are 2 different goals of survival: individual survival and species survival. These might be conflicting goals. Mating selection also might lead to the improved or worsened chances of individual survival: huge antlers might be beneficial during mating, but in densely forrested area the antlers might get stuck during chase by the predator. The same is true for the "peak shift" factor, when exaggerated features of the desired kind might attract more mating behavior than it is worth by survival needs. This "conflict of interests" also can be seen in any multicell organism where the individual cell and the whole organism survival might be at odds in the situation like cancer: one cell rebels against the body and whether gets killed by the body defense mechanism or kills the body and then dies too. Of course, if the cancer is caused by a virus then the virus doesn't care– it still spreads around, infecting other organisms. It's like totalitarian ideology (socialism or islamofascism, for instance) affects a bunch of people,who then try to ruin their own country, and it is also lose-lose situation for everybody, except the virus of the totalitarian ideology that spreads further when it kills the infected ones or the body.
Society is analogous to the whole body, the individual cell is analogous to a person, and the rule of law coordinates the survival of the society, while minimally interfering with the self-regulation at the cell and other levels. The government is analogous to the self-consiousness tiny spot in the left frontal lobe of the brain which is aware of only tiny faction (like 0.00000001%) of the total self-regulated processes, going on in the body at any given time, but it is very arrogant and overconfident, just like Obama is. Remember French king who declared "I am the state!"? Majority of our body is regulated by the old mechanisms, build-in in each cell, in the organ/tissue self-regulation, in the hormonal and autonomous nervous system, in the subcortilal structures of the brain and in the subconcious centers of the cortex. Our conscious ego, "I", a sentinel in the middle of the brain camp on the watch tower, is like a tiny spot of light that glides on the surface of the brain cortex from a watch tower light projector. It can focus only on a one spot in the time, while the rest of the developments are in the dark even in the most superficial cortex, not mentioning deeped sub-cortex structures, autonomous nervous system, organ/tissue self-regulation and cellular metabolism. When the brain gets mad, affected by psychiatric disiease (like todays Washington, DC, is affected by the "progressive" totalitarian mental disorder), then it interferes with everything it can reach, hurts and ruins anything it can, and it can push the whole body of the cliff in the national suicide. That's today's course of action of American enemies in Washington, DC. Obama and his leftist "progressives" in the Congress push really hard the whole nation to the path of national suicide, on the railroad to serfdom, where the next station is Eurosocialism like in Greece, and the final destination is Islamofascism like in Sudan. Ideally there is a check and balance system that mainly leaves the cells (Americans) alone with some tissue/organ (municipalities, communities associations, churches, ) self-regulation, autonomous nervous system self-regulation (state level) and minimal interference, just general coordination at the cortex level (the feds). The problem in all the countries, even in America, that mental illness ot totalitarianism is spreading to the head of the nation, to the government, and we must take care of that in November,2010 reducing size, budget and powers of the feds to about 1% of what it is it now. Can you imagine when the head is half-size of the body? But it is exactly the case with the feds now– feds expanded to the more than 50% of the country, forming huge-headed squid with its tenticles everywhere, in every pocket. We must remove the federal squid from Washington, DC and restore the humane essence of the government, chained to its place by the American Constitution. Normal head surface area in humans, as anyone in the burn center will tell you, is about 9%, which means that all levels of government (municipal, state and the feds) must not exceed that 9% size in their total combined budget and decision making powers. 91% of the power to the people – that's the goal in November !
We tend to have an antropocentric view on the nature, which frequently fails us. A female canguroo being chased by the dogs throws them her cub in order to escape while they are chewing on her cub. The nature's rationale is that the canguroo cubs are born just 1 cm long, and for a very long time they cannot survive on their own. If she will sucrifice herself then both will die. Meanwhile the mom can have another cub to assure the species survival. Female spiders and scorpios eat their male mates in the end of the honey moon, and the males actively provoke this outcome because well fed female is more likely to nurture successfully their offspring. If the food is scarce, then female spider might let her offspring to eat her. Whatever it takes to survive– that's the mother nature's moral code. That's how self-regulated processes with the feed back loop work in the natural selection.