A group of penguins in Antarctica. These organisms are all the same species but not identical, some have thinner blubber than the other penguins. They can produce fertile offspring.
Random mutations in the genes that code for the thickness of skin causes some individual organisms to have thinner blubber than the normal penguin.
The penguins that have thicker blubber can survive the cold climate of Antarctica. This means they can catch more prey and live longer to produce more fertile offspring. Which is the basic meaning and purpose of an organisms life.
The land/ice that the penguins are inhabited on splits and causes a geographical isolation between two groups of penguins. One side of the ice travels towards a warmer climate and those penguins swam to the nearest island which was warmer. The other group of penguins stayed in the cold temperatures.
On the habitat where one group of penguins stayed, the environment is too cold for the penguins with the thinner blubber to survive, so those individuals die.
On the warmer habitat where the other group of penguins travelled to, the environment is too hot for the penguins with thicker blubber to survive, so those individual organisms die. The penguins with the thinner blubber begin to adapt and evolve to their environment which causes them to develop beaks, wings and smaller bodies to get away from predators and catch prey more easily.