What must happen for a species to evolve?
A selective pressure is a situation where some individuals are more likely to reproduce than others.
For example, a pair of red(/(original?)polar arctic) foxes have a litter of kits, but one of them is born with a genetic mutation that causes its fur to become white during wintry months.
When winter comes, the (now) white fox finds it much easier to hunt for food, as its white fur allows it to blend in with the more snowy landscape, helping it to survive the winter and not freeze to death.
This gives the white fox a better chance of survival during the harsh winters, and will increase its chance to find a mate to pass on its 'white fur for winter' to it's offspring, whom will hopefully have a better chance of survival and so will pass on further their 'white fur for winter' characteristic.