The environment is always changing to greater or lesser degree. Happens when changes are either too rapid or too widespread for a species to either migrate or adapt.
Small populations face bigger risk of extinction than large ones.
How slow is slow enough to allow adaptation depends on the organism. The fact that species respond to these changes individually, rather than as groups, means that the ecological communities we see today represent novel combinations of organisms that may or may not have evolved together. This is the basis for the rest of the semester.