Most populations are stable in size. Characteristics are inherited as genes on chromosomes. Natural resources are limited. The fittest individuals leave the most offspring. There is differential reproductive success within populations.
Variation exists within populations.
Individuals must adapt to their environment. You have been observing a sparrow population on campus. You have noted that some of these sparrows are darker than others. In particular, you have noticed that darker colored females choose to breed with darker colored males within the population. There is differential reproductive success within populations.