Consider a population where the number of dorsal spines on a fish is determined by a single gene, which is defined by two different alleles. The dominant A allele encodes a long spine and recessive a allele encodes a short spine. The fish is diploid and reproduces sexually. If the number of AA individuals is 500, the number of Aa individuals is 100, and the number of aa individuals is 400, which of the following statements are true:
I. The population is in equilibrium.
II. The population is not in equilibrium.
III. The observed number of fish with short spines exceeds the expected.
IV. The expected number of AA individuals exceeds the number of observed.