19. Which description BEST represents the gradual model of speciation (i.e., the formation of new species)? (A) Speciation occurs regularly as a result of the accumulation of many small changes. (B) An isolated population differentiates quickly from its parent stock as it adapts to its local environment. (C) Speciation occurs under unusual circumstances and therefore transitional fossils are hard to find. (D) Species undergo little change over long periods interrupted only by short periods of rapid change.