According to Darwin's theory of evolution, organisms compete for resources such as food, water, mates, and territory. This competition is a driving force behind natural selection, the process by which certain heritable traits become more or less common in a population over time. Traits that give an organism an advantage in competition are more likely to be passed on to offspring, while traits that are disadvantageous are less likely to be passed on. Over time, this can lead to significant changes in the characteristics of a population.