In order for natural selection to take place we must have one individual. a cloned species. the same traits. variety in a population.

In order for natural selection to take place we must have "variety in a population". This variety of traits is what allows natural selection to "pick" the most favorable traits.

Food chains show energy transfer between organisms. The size of a food chain can vary, yet there is a limit to the number of levels that a food chain can reach. Why is there a limit to the number of levels that a food chain can reach? ... Energy is lost at each trophic level.