Natural selection occurs in all populations of living things. All living things have variations or differences due to their different DNA. Any adaptations an organism possesses that gives it a selective advantage may reproduce and pass along those adaptations to offspring. This is what Darwin meant by survival of the fittest. Over time, as enough differences accumulate, a new species may evolve. Even in asexually reproducing organisms, there can be natural selection. Different strains of the same species will have genetic variation. Those with the best traits for that environment may pass along their DNA to offspring. Simple organisms can even exchange their DNA via the process of conjugation, with other cells, thus perpetuating the beneficial trait within the population. Nature is always putting pressure on organisms and those best adapted are selected for.