What does metamorphosis signify in an organism's development?

Metamorphosis signifies a striking change in appearance or character in an organism's development, particularly in the life cycles of many insects and amphibians. This biological process typically involves a complete transformation from one developmental stage to another, such as from a caterpillar to a butterfly or from a tadpole to a frog. During metamorphosis, not only does the external morphology of the organism change significantly, but internal physiological changes also occur, enabling the organism to adapt to a new ecological niche or life stage.

In contrast, the other options do not accurately capture the essence of metamorphosis. Gradual changes in size are more associated with growth rather than a transformative process. An increase in population size relates to reproduction and ecology, rather than individual developmental processes. Lastly, a gradual transition between larval stages refers to the progression within certain stages of development but does not encompass the dramatic shifts that define metamorphosis. Thus, the key characteristic of metamorphosis is its dramatic and comprehensive nature of change rather than gradual or incremental shifts.