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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 ca
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 a
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.