Organisms do not adapt much over their lives, but in a large population there will be many different individuals. Some taller, some shorter, some may have a longer beak or a slightly different colour pattern etc etc.
if one of these individuals is better suited to fit the environment, say its colouring is slightly more similar to the background, it will be better able to hide from predators, or sneak up on prey. This means that it is more likely to survive and reproduce successfully, meaning that there will be more individuals with its colouring in the population. Slowly the colour pattern could become more common, and like that, over many generations, species slowly come to adapt to their environment.
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