How can environmental change sometimes lead to adaptations and evolution, and other times lead to extinction?
Some species adapt better to their environment than others.
Let's say there are ten green grasshoppers and ten yellow grasshoppers. When a bird comes and looks for grasshoppers, the yellow ones stick out more than others. When this happens, the green grasshoppers become more common than the yellow ones. In a similar way, if some organism has mutated or gained a better advantage, then it will continue on. The same organism with the bad mutations will become extinct.