The gene pool for this population consists of only green lizards.
A mutation has occurred which resulted in a lizard being born yellow instead of green. This is also known as genetic variation since a lizard has a different gene for color.
Predators searching for food see the visible yellow lizard which leads to yellow lizards being eaten more and struggling to survive. But on the other hand, overpopulation starts to take place since the green lizards are able to reproduce more because of their beneficial trait.
The environment goes through a change which benefits the yellow lizards now instead of the green. So, the yellow lizards are now able to blend in with the environment and survive because of their trait being the best adapted to the change.
Overtime, the gene pool has changed to a mostly yellow lizard population because the yellow lizards are living to reproductive age unlike the green ones because they're more visible in this new environment. Since predators can see them more, the green lizards are being eaten at a higher rate.
Thus, more lizards are being born with the beneficial trait while the population of green lizards is slowly declining.