It can also be caused not by change to the environment itself, but by the organism's movement to a different environment. Erupting volcanoes cause sudden, drasitc change in an area, forcing organisms to evolve rapidly to adapt to the new environment. This penguin's ancestors looked much like other birds. Change in an organism's environment forces the organism to adapt to fit the new environment, eventually causing it to evolve into a new species. Isolation means that organisms of the same species are separated, and happens when there is something between the organisms that they can't cross.
Organisms become isolated as a result of environmental change.
The cause of isolation can be gradual, like when mountains or deserts form, or continents split apart. When organisms become isolated the two groups are also not able to reproduce together, so variations and mutations that occur in one group are not necessarily found in the other group. The longer the groups are isolated, the more different they are. They eventually become different species. Moreover, if there is a change in the environment of one group it does not necessarily occur in the environment of the other.