Alfred russell wallace devised the theory of natural selection at about the same time that. The voyage of the beagle: a 5 year journey for research, examined organisms from many different places. Variation and competition determine reproductive success: variation: the differences found across individuals of any given species (genetic, biological, physiological, competition: different animals/species competing for food, mates, territory. Although evolution occurs over the long run, natural selection can produce important changes in the short run- on the space of only a few years. Phenotypic variation (differences in beak size) can produce important selective advantages that affect survival. Natural selection works because the members of any species have different phenotypes/ because these phenotypes are caused by different genotypes, successful individuals will pass on their genes to the next generation. Over time, competition for food and resources will allow only the best adapted phenotypes to survive, thereby producing evolutionary change.