Diversity and Adaptation
The physical environments of Earth's different surface areas are constantly changing. Plate tectonics is the main agent of change, though slight oscillations in Earth's orbit and modifications of the air and ground by living things have played a role. Plants and animals must adapt to these changes or become extinct. The process of adaptation leads to the evolution of new species which leads to diversification of life forms on Earth. The ability to adapt and diversify enables life to continue in the face of change.

To adapt, species must be able to change at the molecular level--in the genetic code. Changes in the genetic code are called mutations. They can be caused by cosmic or man-made radiation, chemicals, or even spontaneous changes in chemical bonds. Population dynamics and selection effects enable mutations to spread through a population. From the point of view of survival, mutations may be

• helpful,
• harmful, or
• of no consequence to the survival of the organism.

Helpful mutations spread through populations because their owners survive to pass them on. The cumulative effect of numerous helpful mutations is adaptation: changed body structures and new species well adapted to their changed environments. For example, a long tongue on an anteater is an adaptation that enables it to probe into ant and termite colonies. The long tongue of a dog does not serve the same purpose (it is also not shaped the same as an anteater's); instead, it helps the dog cool itself through panting.

The images of the four bird feet shown on the student Physical Adaptation page are of a heron, an eagle, a canvasback duck, and a sparrow. The tall heron needs a wide base for holding up its tall body, so its toes are widely spread and long. The eagle is a predator that needs sharp, curved claws to pick up small animals and fish as it swoops down out of the sky. The canvasback is a duck whose webbed feet enable it to paddle water efficiently. The tiny sparrow uses its curved claws to grab onto a branch for perching.

Dinosaurs show similar adaptations. As carnivorous (meat-eating) theropods (the group that includes Tyrannosaurus rex) became larger, they evolved adaptations to accommodate their increased weight. Bones became thicker for support, holes in the skull became larger to make the head lighter, and necks became shorter to make the head easier to support.

Despite the positive examples, mutations, being largely random changes in the genetic code, usually are not helpful. In fact, in most instances, a significant mutation leads to the death of the mutated individual. However, in some instances, a mutation may be minor and/or not have any effect on the survivability of the individual. Like helpful mutations, inconsequential mutations can spread through a population. For example, humans include a mutation that shortened the canine teeth. We do not need long canine teeth to survive because we use knives to cut our food, and because we are omnivores (capable of eating both animals and plants).