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Natural selection is the driving force behind evolution.
Organisms tend to over populate their environment and this overpopulation forces the offspring to compete for the limited resources necessary for survival and reproduction. The size and natural history of salmon is controlled by water temperature. The size and natural history of guppies in different populations is determined by the food preference of the predators that eat the guppies. The size and natural history of guppies is determined by the availability of adequate amounts of food for the guppies. Guppies are the major predators in Trinidad.