Picture an example: a group of primates has adapted to live in the African rain-forest. A group of said primates, for whatever reason, become separated and stranded hundreds of kilometers away on the Savannah. Somehow, by some fluke of nature, they manage to eke out an existence. The former group is not subjected to any new selective (environmental) pressures; the latter group stranded in an environment to which they have not had time to adapt face new environmental pressures. Those pressures reward certain members, and their future offspring (those with beneficial mutations), with increased success in surviving and passing on those mutations. Eventually, beneficial mutations such as tallness and more upright walking (ability to spot predators from further away), tool-making (increases strength/efficiency which contributes to hunting success), and finer motor control (increases tool-making and tool-yielding ability) are adaptations that the environment selects for and rewards. After a million years, the Savannah group are taller, bigger, and possess fine motor control (and perhaps intelligence as a result). The rain-forest group, having no selection pressures out of the ordinary remain, more or the less, the same (although gene flow would be likely to change them somewhat also). The latter group can be said to have evolved form the former group, yet the former group persists.
