It explains not only the similarities in the chemistry and structure of organisms but also their marked diversity. Although the likenesses are due to a common origin, differences result from the evolution of various species to fit into particular ecosystems. Furthermore, the complex relationships among the species of an ecosysem exist because these species have evolved ogether.
Evolution Is Change
In the simplest sense "evolution" implies a process of gradual change. The reader must understand hat this change occurs not within individual orga:isms but, rather, only between generations that are ·.;Jrts of populations. This is true because each orjCillism retains throughout life the same hereditary nakeup. In other words, the cells of each organism :ontain genetic information that, under proper en-rronmental conditions, permits that organism to ievelop specific structures and to function in certain ways. This genetic legacy, together with the ~nvironment, determines the characteristics of the few individual.
Most organisms result from sexual reproducion, whereby parents produce young that possess a combination of parental traits. This is one source of variability within a population. Another-and ill ultimately more important-source of variation . mutation, the sudden appearance of character·,tics totally different from those of the parents.
lutations are rare and usually lethal. Occasionally, however, under the prevailing conditions, a mutation can be harmless or even beneficial.
Obviously, the genetic endowment an orga. m receives must be reasonably compatible with its environment if the new individual is to exist at all. But the better the fit between these two factors, :he greater the chance that the organism will surive to reproduce, and the larger the number of its Jffspring that are likely to survive. Because the mvironment favors the growth and reproductive ;uccess of individuals whose genetic makeup best ;uits them to the circumstances, certain inherited :raits are favored. As a result, these traits are more :ommon in the next generation. Changes in inherited characteristics may be small for any gen~ration, so evolution occurs very slowly. Never:heless, the role of the environment in determining :he reproductive success of individuals lies at the :ore of the evolutionary process. This role is known is natural selection.
Although most of the history of life can be only nferred, sometimes we can observe evolution directly. English moth collectors have documented a iny but telling example of evolution in the peppered moth. Prior to the industrial revolution, the peppered moth was common in England, where it was characterized by a blotched or "peppery" pigment pattern. This mottled pattern camouflaged the moths as they rested on tree trunks during daylight hours. After the introduction of coal as a major energy source, soot from the smoke of coal fires blackened all the trees and buildings in industrial regions. In the following years, dark moths of the same species, which previously had been found only in small numbers, gradually replaced the once-typical "peppery" ones. Why? Because the original peppery-patterned moths had become more visible to birds or other predators, but uniformly dark moths blended with the sooty surfaces. In some areas, recent reductions in air pollution have been accompanied by an increasing reappearance of the lighter, peppery moths.
Small environmental changes may cause minor evolutionary changes, but marked environmental alterations can lead to widespread extinction. This happens because organisms are complex and delicately balanced systems. Hence radical changes are not likely to be compatible with life.
Species Formation
A species maintains its characteristics because its members can breed only with others of their own kind. The separation of one species into two requires some barrier that prevents interbreeding between two populations. For example, a geographical barrier may come to separate a species into isolated populations. Because no two environments can be identical, each exerts different selective pressures. Chance can also alter the genetic makeup of small populations. One way or another, two initially identical populations can become quite different. After a long period of time, enough changes accumulate in isolated populations to preclude interbreeding even if the opportunity should occur. Thus two populations of one species can evolve into two separate species.
We will return to the subject ot species formation later, when we discuss evolution in terms of specific genetic mechanisms. For now we ask you to remember that a species is a group of organisms that can interbreed (and produce fertile offspring) only among themselves.
The web of interdependence within an ecosystem causes change to sweep through like a wave. People sometimes view the world as stable, but this view is taken only because humans live for such a short while. In the long run we are all part of an ever-changing biosphere. With this change can come extinction as well as new species. The less the biosphere changes, the longer our kind is likely to endure.
