he principle behind natural selection as originally explicated by Darwin and then elaborated by successive evolutionary biologists is relatively simple. Imagine you have 1000 mice living in a field. Not all of the mice will be identical. Various processes including imperfect replication of DNA and omnipresent low levels of radiation (due to things like cosmic rays) cause random mutations, and so some of the mice will be bigger and stronger, or have other adaptive advantages and other mice will be weaker. The stronger mice compete more successfully for food and thus are more likely to successfully raise baby mice to adulthood. Thus slowly the advantageous characteristics will be transmitted to future mouse populations.
Yes, and all organisms produce more offspring than can live in their environment. Only the individuals that fit into their niche like a key in a lock or a hand in a glove will survive. Also the environment changes. It gets hotter or colder. Volcanoes, storms, and meteoroids destroy habitat. Erosion destroys mountains. Earthquakes make mountains. A change in one species changes the environment of another species.
Also, (maybe somebody else can confirm this) with one possible exception the process is unpredictable. There is no direction or goal.
The one possible exception is that in every geologic age, the organisms tend to get bigger.
I think the answer in post #1 is the basic idea, but I think Dawin would say that "some of the mice are fitter," not "some of the mice are stronger" because a stronger mouse might not be the fitter mouse. Depending on the niche in which the mice live, a weaker mouse might be fitter.
The mice that fit well into the mouse niche will reproduce more than the mice that fit less well into mouse niche.
Are you stating that natural selection is made possible due to "Various processes including imperfect replication of DNA and omnipresent low levels of radiation (due to things like cosmic rays) cause random mutations"?