Darwin was able to formulate his theory of evolution based on several facts. All populations have the potential to increase in size. Individual organisms in a population are not alike. 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.