This penguin's ancestors looked much like other birds. As their environment became colder and wetter (possibly because of their migration), the birds evolved many traits to help them survive in the changed environment.
The common giraffe (top) and the reticulated giraffe (above), used to be from the same population. Over generations of isolation, separated by the Tana River in Kenya, the two groups have become distinct.
Environmental change and isolation of groups of organisms play an important role in evolution.
Environmental change is any change in an environment to which an organism must adapt. Change can be gradual, such as when mountains or deserts form, other species die out, or new species evolve. These things can take millions of years. Change to an environment can also be quick, such as floods, volcanoes, or earthquakes. 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, drastic change in an area, forcing organisms to evolve rapidly to adapt to the new environment.
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. For example, if a species of animal is mostly limited to eating one kind of leaf, and a change occurs: a fungus attacks and kills most of that kind of plant, the animal has to evolve either to fight the fungus or to eat something else.
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. It can also be quick, such as organisms being blown to different places by a storm or tsunami (tidal waves).
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. So they will evolve and adapt differently.
The finches and other organisms that Darwin found on the Galapagos Islands are examples that demonstrate the effect that environmental change and isolation can have on a species.
Koalas (above) and kangaroos (right), both live only in Australia, which has been isolated from the rest of the world for billions of years. This long-term isolation has resulted in species existing there that are very different from those found everywhere else in the world.