Evolution and extinction of the animals
The rich diversity of animal life is the result of a continuous process of evolution—the change in living things over time as they adapt to their environment. As the world changes, animals that are best equipped for survival—the “fittest”— prosper and multiply, while others become extinct, vanishing forever.
Natural selection is the process by which the fittest organisms survive and produce more offspring. Over time it means that animals with a helpful adaptation become more numerous than those without, so the species gradually evolve. For example, in 19th-century Britain, a rare dark form of the peppered moth became common in places where trees were blackened by industrial pollution because it was better camouflaged and less visible to hungry birds.
When animals breed,the offspring inherit their parents’ features.
For example, some individuals may have the same coloring as their mother and others the same as their father, or a shade in between the two. If an unexpected color arises as a result of a mutation (change), it may give that individual an advantage,such as better camouflage.
Life on Earth is changing all the time as new species appear and others become extinct. Because fossil evidence is very incomplete, it is impossible to know which animal is ancestral to another, and the further back in time you look the more difficult it becomes. Scientists look for similarities between animals but this does not always mean they are closely related.
Branching family tree
The ancient animals shown here are not the direct ancestors of the modern elephant. However, they are all thought to be descended from a common ancestor, making them members of the same branching family tree.
Living things may be perfectly adapted to their environment, but then the environment itself changes. This is happening to the polar bear, which lives for part of the year on Arctic sea ice. Climate change is making the ice melt, and the polar bear is so specialized for life on the ice that it cannot adapt. As a result, it may become extinct.
Sometimes a cataclysmic event changes the world so radically that very few animals survive it. About 66 million years ago, an asteroid or comet is thought to have crashed on Earth, causing the mass extinction of the giant dinosaurs and flying pterosaurs. The survivors included the ancestors of modern mammals and birds.
Everything that we know about extinct animals is deduced from fossils—the remains or traces of long-dead organisms that have survived the normal processes of decay. Most fossils preserve shells and bones, which were buried in mud or sand that turned to rock. Minerals in the rock then turn the animal tissue to stone. Other types of fossils preserve impressions of soft-bodied animals or soft tissues such as feathers.
Animals that move into new habitats face new problems, and features that were once useful may lose their value. If birds start living on islands where there are no predators, they have no need to fly to escape them. Flying uses a lot of energy, so birds that cannot fly may thrive. Over time this can result in a flightless species, like this Galápagos flightless cormorant.