Which of the following statements is NOT one of the principle observations upon which Darwin based his theory of natural selection?
a. The population size of a species would increase exponentially if all individuals that were born reproduced successfully
b. Populations tend to remain stable in size, aside from seasonal fluctuations
c. Environmental resources are limited
d. Individuals in a population vary extensively in their characteristics
e. Most of the variation observed among individuals in a population is due to environmental causes; thus, very little variation is heritable (passed on from parent to offspring)