What is the theory of evolution? How are survival of the fittest, natural selection, capability to adapt, and reproduction related in the theory? What is the role of adaptation?Ever wondered why there's so many breeds of dog? Or in the wild, so many different types of animals that can't be found anywhere else in the world?
When organisms reproduce, their offspring follows the set of DNA given by its parents. The result is something very similar to the parents...but not quite.
There's always something slightly different about the offspring. Most times you can't even see it. Most times it just doesn't matter. In sexual organisms, genetic recombination plays a huge part here. Every time two gametes (sperm and egg) meet, their chromosomes combine to form something based on the two, but with an entirely new set of information. Your own personal genome is unique in the entire universe. Mutation also serves a role. These can be good, bad or benign. Both of these serve to either enhance, suppress or express a trait.
Now, the process above is very mostly random. But it's not the only thing happening here. All organisms are copies derived from their predecessors with slight changes. But after those slight changes are made and it's time for the organism to play the game of life, it's time for the second half of evolution to begin: Selection.
Now, selection is nothing new to us humans. We've been doing it manually for thousands of years, and have only recently begun to understand how it worked. Take dogs for example. With every litter, there's always going to be some dogs that grow to be the largest (or smallest). If you were to rinse and repeat this process and only breed the largest (or only the smallest) dogs you see (preferably some from other litters too), you will eventually have a very large breed of dog. Well done, you have now selected a dog for size. This is a very simplified example of what goes on in real life. Modern dog breeders select for a great deal of things and wind up producing a wide variety of things.
But nature can do this too. Each organism is identical to its species but slightly different. In what way it is different can affect the rest of the species. If the trait is bad, then the organism and any progeny it makes is at a disadvantage to the rest of the species and its lineage usually dies out shortly. If it's benign, not doing anything particularly good or bad then the trait lingers around for a bit and slowly spreads to other organisms in the region after they breed for long enough. If it's a good trait, we get to see some really interesting stuff. The organism and its children are now at an advantage to the rest of the species. The trait starts to expand greatly and eventually reaches the entire species in the area where it is beneficial. It can be anything from being able to have more babies, to having a set of slightly longer legs, or males being more (or less) aggressive to other males, a slightly different fur pattern, pretty much anything you can think of and if it works, it's put in; if it doesn't work, it gets thrown out eventually.
Animals wind up finding suitable niches in nature with the help of natural selection. Long ago, our ancestors had a stable niche of living in the trees. Natural selection shaped them thusly; the animals that were better at living in trees bred and the ones that were worse did not. Eventually there was an opportunity for us to benefit off of the plains, so many left to take advantage of them like some chimpanzee troops do today. Natural selection started to shape us into forms most suitable for plains life. Out in the plains, there's no trees to hide up, so you have to use your wits. Especially since you're already not exactly shaped for plains life, your apelike forest body is rather disadvantaged. The animals that stuck together, worked together and used their intelligence to solve their problems had greater success of breeding. We started to rely more and more on tool use, using rocks to kill our prey or process plant food. Natural selection shaped these populations into forms more suitable for tool use. And here we see another facet of evolution: Natural selection giveth, and natural selection taketh away. As we relied more and more on intelligence, we relied less on our physique. As we relied less on our strength, it stopped getting selected for, for the most part. As it stopped getting selected for, the trait started to deteriorate. More and more generations began to have slightly weaker builds as they focused more on intelligence to survive. This just wound up having us need intelligence even more to survive, which made us less physically capable, which forced populations to be even smarter. This started a massive snowball effect of intelligence leading to a less advantageous build leading to greater demand for intelligence, etc. Eventually, just a few hundred thousand years ago there came to be multiple species of humans that can talk, work together in large numbers and use advanced tools. Our species was capable of adapting in any situation we were faced with and out-competed every other species. Unfortunately, this means they all went extinct as we took over their lands and sometimes interbred with them. The end result of these groups of apes deciding the plains have lots of food is a very prolific species that is also very intelligent, but also not as strong as some other animals. We're built quite well for endurance walking and running, but that's one of the few physical advantages we have as natural selection did not favor the rest.
In the span of a few thousand years of artificial selection, we have hundreds of wildly different types of dog, fish, cow, horse, sheep, and plants like corn, potato, lettuce, carrot, etc.
Now imagine what paths life can take in millions of years with natural selection!