In the genetic sense, it can't. Evolution really has one direction, and one direction only: forward. However, evolution may cause organisms to evolve traits that resemble traits of some of their precursors. For instance, whales are sea-mammals that descended from land-mammals, which in turn evolved from earlier aquatic organisms like fish. But due to the demands of their environment, whales evolved an overall morphology that resembles those of these earlier fish.
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Natural selection leads to evolution by giving the organism that is best suited for the area to thrive. Thus those who not suited for the area die off and evolution moves forward.
This process is called natural selection. It is the mechanism by which traits that provide a survival or reproductive advantage to an organism become more common in a population over time.
Natural selection can only work on genetic variation that already exists. So mutation comes first, then natural selection.
It hasn't. Natural selection is a key part of the ecosystem itself.
Natural selection is the mechanism primarily responsible for the development of today's biodiversity.