Adaptation can happen quickly: either by need or to take advantage of opportunity.
In 1971, biologists moved 5 adult pairs of ruin lizards from their home island in the south Adriatic Sea to a neighboring island. 35 years later the lizards were quite different: head size and shape, bite strength, and greatly adapted digestive tracts, along with dramatic changes in population density and social structure.
Lizards on barren Pod Kopiste island, the old homestead, were well-suited to snagging mobile prey; feasting mainly on insects. Life next door, on Pod Mrcaru, offered lusher vegetation.
The lizards on Pod Mrcaru adapted larger heads and a stronger bite to tear fibrous plants. Depending on the season, 2/3rds of the new diet was vegetarian. Digestive tracts adapted to slow food passage, thus giving more time for microbial digestion of plants, including creation of fermentation chambers: an extremely novel adaptation for lizards.
This change in diet provided a larger and more predictable food supply. Whence came increased population density. Because foraging became the norm rather than predation, the adapted lizards gave up defending territories.