Evolution in the long run depends on environmental change. This can happen by actual change in the environment, or by organisms being displaced into new environmental conditions. Without environmental change, natural selection would eventually perfect each species to be maximally adapted to its respective environment and lifestyle—and then little if any further change would occur. Examples:

1. Lingula—the oldest living genus of animals is 400,000,000 years old, and fossils from this age look      essentially like modern day Lingula living in marine muds. The marine mud environment has      apparently not changed much over this long span of time. Lingula is a Brachiopod—a shelled animal      that looks something like a clam.

2. Horseshoe crabs have also remained unchanged for almost as long as Lingula. Fossils from      360,000,000 years ago look basically like the modern horseshoe crabs along the Atlantic      shores—again, an inhabitant of shallow marine bottoms, another habitat which has remained relatively      stable.

3. The Chambered Nautilus—yet another marine animal, this time from the deep sea--also basically      unchanged for over 350,000,000 years.

Actually, there are many other organisms that have seemingly changed little over vast spans of time, and some of them have experienced some environmental change. Bacteria have changed little in their structure from those billions of years ago. The amoeba is probably not too different today from those of a billion years ago. Even cockroaches and scorpions are little changed over many millions of years. It seems that some body plans are just good at handling a wide range of environmental conditions. In fact, the first scorpions in the fossil record were marine organisms, yet they looked much like the modern scorpions, which are all now terresterial.

Traits and organisms which change little or none over vast numbers of generations are under a type of natural selection called Stabilizing Selection. That is, the trait is being maintained because no alternate form has proven better. For most living species, most of the traits in that species are successful, otherwise the species would have gone extinct. Usually only a few—if any, of the traits present in a species are under selective pressure to change. In short, natural selection is largely of the stabilizing type, keeping species basically the same over very long spans of time.

BUT with environmental change, what was a perfect adaptation may no longer be perfect, and change will occur—assuming that the species has the genetic variation necessary to bring about the required change. This type of natural selection is called Directional Selection. Examples:

1. Most mosquitoes are killed by DDT—DDT is sprayed over large areas of the globe—a few mosquitoes      that are resistant to DDT survive and give rise to the new generation of mosquitoes—eventually DDT      is worthless because all the mosquitoes in an area are resistant.

2. The Peppered Moth changes from light coloration with black specks to totally dark coloration as      environmental changes and bird predation drive the change (in your book).

3. South American Iguanas arrive in the Galapagos Islands where conditions are quite different from the      forest of S. America. One group of these iguanas adapts to feeding on marine algae in the cold waters      offshore. Their bodies evolve to be stouter than the S. American ancestors (conserves heat) and their      coloration darkens to better absorb the sun’s heat when they come ashore from their feeding periods.

4. Some fish and salamanders are accidentally displaced from surface waters into subterranean waters in      caves. Over many generations the animals lose their eyes and their skin pigmentation (no light in the      cave). They also typically become small and “frail” of body (far fewer food items to fuel big bodies).

5. Some birds accidentally end up (get lost or blown by storms) on isolated oceanic islands where no large      land predators exist. Over many generations flight is reduce or lost completely because food is      available on the ground and there are no predators to fly away from. The Dodo bird of the Mascarene      Islands, the Kakapo (a parrot) of New Zealand, and a flightless Cormorant on the Galapagos are good      examples.

In all but the first two examples above, the changes that took place were significant enough to produce new species, which were quite different from the parent line of iguanas, fish, salamanders, or birds. Given enough time, together with enough accumulated change—and a group of small dinosaurs evolved into birds, social roaches evolved into termites, a group of land mammals evolved into whales, etc.