After intensive observations and study, English naturalist and biologist Charles Darwin argued that environmental pressures, such as changes in climate, the presence of predators, and competition for food or mates lead to the survival of those better adapted. As the weaker and less adaptable die out, the species changes. This is natural selection.
While on his five-year voyage on the HMS Beagle, Darwin visited the Galapagos Islands. One of his important observations was that of the marine iguanas, who were the only aquatic lizards of their species. The amazing adaptation of these iguanas to their environment convinced Darwin of his theory of natural selection. These "imps of darkness" that Darwin described are unique. Because of this uniqueness, Darwin became convinced that they had adapted to their present conditions which are so different from those on the other side of the island where more typical iguanas live.
The marine lizards live on a side of the island that has a much different environment from the other side. These iguanas differ from others in several ways. One distinction is that they are dark gray and black in order to better absorb the heat from the sun. Because of their coloring, they can retain enough warmth to swim in the cold waters. Also, their claws and legs are much stronger than those of other iguanas so that they can hang on to rocks when the waves crash upon them. In addition, their long limbs and flattened tails help the lizards to swim well in the water. Their noses have become flattened and much shorter than others of their species. These flattened noses allow the marine iguanas to bite and eat the seaweed on the rocks more easily.
Unique to only this one side of the Galapagos Islands, the marine iguanas demonstrate natural selection. Those who have had longer legs, longer claws, and flattened noses have survived and reproduced, thus altering the species.