Which of the following statements best expresses the concept of punctuated equilibrium?
A. Small variations gradually accumulate in evolving lineages over periods of millions of years.
B. Stability is achieved when selection favors the heterozygote, while both types of homozygotes are at a relative disadvantage.
C. Random mating ensures that the proportions of genotypes in a population remain unchanged from generation to generation.
D. Evolutionary changes consist of rapid births of speciation alternating with long periods in which species remain essentially unmodified.