Answer:
The correct answer would be speciation.
Speciation refers to the gradual process by which the population gets evolved into different species.
As the lizard population went to the treeless area, they faced new areas and niches.
The selection pressure of this area is different from that of their previous habitat. As there were no trees thus, they did not require the ability to climb the trees.
In addition, different variations accumulated over several generations which led to the formation of new species which was not able to climb the trees.
