Final answer:
Competition for limited resources is essential for natural selection because individuals with advantageous heritable variations are more likely to survive and reproduce, leading to the evolution of populations over time.
Step-by-step explanation:
Natural selection can only occur when there is variation among individuals in a population, and when these variations are heritable. This is because natural selection depends on the principle that more offspring are produced than can survive, leading to competition for limited resources. With a plethora of offspring and limited resources, individuals must compete to survive and reproduce. Survival and reproductive success are thus not random; they are influenced by the variations individuals inherit that make them better suited to their environment. This competitive process guides the direction of evolution by selecting for traits that enhance survival and reproductive advantage, therefore causing populations to change over time. Ultimately, natural selection results in populations that are better adapted to their local environment and this is considered the main mechanism for adaptive evolution.