Evolution is an observable fact where the heritable traits of a population of organisms change over time. What that statement lacks is a description of the mechanisms that cause that. The “Theory of Evolution” explains how the fact of evolution happens. The theory of evolution states that those heritable traits of a population of organisms change over time as a result of natural selection or genetic drift. (Note that there are slight variations of this.) So natural selection is one of the two driving forces of evolution. Natural selection is the differential survival and reproductive success of individuals in response to the environment due to differences in phenotype. For example, and herbivore may have a random mutation that gives it better camouflage from predators, and thus will survive longer and reproduce more. To summarize, evolution is what we observe, the change of a population of organisms over time. Natural selection is one of the mechanisms that drive it.
The ultimate source of energy (for most ecosystems) is the sun<span>. The ultimate fate of energy in ecosystems is for it to be lost as heat. Energy and nutrients are passed from organism to organism through the food chain as one organism eats another.</span>
