Learn English – A word for changing shape/size

You could try one of these:

crossroads
pivotal point
defining moment
critical juncture
cusp

Since this is in the domain of biology, the word might be evolve. Evolve (v.): to change or develop slowly often into a better, more complex, or more advanced state. [ref] Example: The North-American crows have evolved to adapt to urbanization.

To describe a change over time of a part of an animal (e.g. beak), you can try the word morph. Example: The wings of these birds morphed over millenniums as atmospheric pressure changes.

To describe a change in habitat location over time, you can try the word shift. Example: After Seattle became a city, the Pacific wren and the Swainson’s thrush shifted their habitat. [ref]

Suggestion after asker added an example: trend. Example: The trend of population size over the last decade.