What is the theory of natural selection, and why does it occur in an organism?

Natural Selection is survival of the fittest aka Darwinism. It basically states that the strongest survive and the weakest don't. Maybe it's through food shortages, bad weather, or predators. It occurs in an organism for population control. If there is too big of a population, then there will be a recourse drought and natural selection will do its thing again.

volcanoes (an event in the geosphere) release a large amount of particulate matter into the atmosphere. these particles serve as nuclei for the formation of water droplets (hydrosphere). rainfall (hydrosphere) often increases following an eruption, stimulating plant growth (biosphere).

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: