Copyright © 2007 Dorling Kindersley
The process by which changes occur in living things over time is known as evolution. The changes are passed from one generation to the next in genes. NATURAL SELECTION is one process by which evolution may occur. In nature, individuals with an ADAPTATION that helps them survive are more likely to reproduce. More of these individuals pass on their genes than their rivals, so the adaptation is more common in the next generation and builds up in the species.
Although the forelimbs of mammals, birds, and reptiles are modified in different ways, the basic design is the same, suggesting they all descended from a common ancestor. The basic design includes one upper arm bone, two lower arm bones, and five fingers.
Some animals become extinct as a result of evolution. They are replaced by other animals that are better able to survive. Dunkleosteus was an armoured fish with powerful jaws that lived about 350 million years ago. It may have become extinct as larger, faster sharks evolved, out-competing it for the fish they both hunted.
Today’s elephants are the result of a long process of evolution. Over millions of years, small changes were passed from one generation to the next. The first fossil elephant species were small, but over time they increased both in size and weight. The three species alive today are the sole survivors of a once much more widespread group.
Not all offspring survive to become adults. Those with favourable variations, such as long, thick fur in a cold environment, are more likely to survive than those without. This effect of different characteristics on survival is what the scientist Charles Darwin called natural selection. Natural selection is a cause of evolution but it is not the only cause.
Darwin spent years gathering evidence to support his idea of evolution by natural selection. He travelled the world on expeditions aboard the ship, HMS Beagle. When he reached the Galapagos Islands in the Pacific, he was inspired by the number of unique species he found there.
During the Industrial Revolution in the 1880s, pollution blackened trees in parts of England. Previously rare black peppered moths began to increase, as they were harder for birds to spot than their speckled counterparts. By 1900, most moths in industrial areas were black. Now, with pollution controlled, the black population has fallen again.
Adaptation is an outcome of natural selection. It is the gradual matching of an animal to its environment over time. It applies to everything about an animal from its anatomy and behaviour to its life cycle. It is important in evolutionary terms because the better adapted an animal is, the more likely it is to survive and produce offspring.
The marine iguana lives on the Galapagos Islands and feeds exclusively on seaweed. It shows a number of adaptations to this lifestyle. Because it feeds underwater, the marine iguana is a good swimmer and has a long tail, flattened from side to side, to help propel it through the water. It also has special glandular structures in its nose to help it get rid of excess salt.