Evolution works through natural selection. The individual organisms that produce the most offspring who survive to maturity to reproduce pass on their genes. Individuals whose genetic characteristics reduce their chances of surviving and reproducing do not pass on their genes. As a result the ‘fittest’ genes survive while the least ‘fit’ disappear.
Fitness is not absolute, but is instead dependent on the ecological niche and environment of a particular organism. For example, most people would consider themselves to be higher on the evolutionary hierarchy than monkeys. Humans are said to have evolved from monkeys (a more accurate statement would be that modern humans and modern monkeys had a common ancestor millions of years ago). This would seem to imply that human beings are more highly evolved than monkeys.
But, although human beings are certainly more intelligent than monkeys, they are not superior to other primates in every way. Human beings and monkeys are specifically adapted to different ecological niches. Many species of non-human primates are specifically adapted to living in trees. They are vastly superior to human beings in this respect.
Further back on the evolutionary timeline, mammals may have evolved from fish-like marine organisms. Again this does not mean that mammals are superior to fish, even if most mammals may be more intelligent than most fish. Mammals can not breathe underwater without artificial help the way fish can. Terrestrial mammals, like human beings, can not move around efficiently underwater, the way fish can, either.
Evolution works to create organisms that are best adapted to specific environmental conditions and ecological niches. Many species of monkeys and apes are specifically adapted to living in the trees of tropical rain-forests. Fish are specifically adapted to living underwater.
When an organisms’ environment or ecological niche changes then the definition of ‘fitness’ changes for that organism. The characteristics that helped the organism to survive in the past will no longer be as favorable as they were before. New characteristics that fit the changing environment will now be favored by natural selection. As a result, the genotypic and phenotypic characteristics of the species will begin to change- in other words evolution.
There are many well-recorded examples of evolution and natural selection producing changes in various organisms in modern times.
One often cited example is that of the peppered moths of England. Originally the most common variety of peppered moths in England were mottled grey in color. There was also a less common dark grey variety. The extensive burning of coal during the industrial revolution, however, caused a great deal of smog and air pollution. As pollution increased, the darker variety of peppered moth become predominant and the lighter variety became more rare. As clean air acts were put in place and air quality improved in England, the lighter colored moths have again become more common while the darker variety have become less common. It is not entirely clear why the darker variety of moths thrived in more polluted conditions. In the past it was theorized that darker moths were better camouflaged from predators in dark, smoggy, polluted air conditions. Whatever the reasons, the genes of darker moths were the fittest in polluted conditions while lighter moths had an advantage in cleaner air conditions.
Another, more recent example, is the medium ground finch in the Galapagos Islands. In 1977 a drought killed many of the plants that produced the soft seeds that these finches fed on. The finches were forced to depend on larger, harder seeds for food. As a result, many of the individual birds with smaller, softer beaks, who were unable to handle the harder seeds, died. The individual finches with stronger, larger beaks survived. The drought decimated the medium ground finch population of the Galapagos, but those individuals that survived tended to have larger beaks than the average finch did before the drought. Because of the drought the average beak thickness of the finches increased. If lower rainfall had become a long-term trend in the islands, then the larger beaks would have likely become a permanent characteristic among the birds. The finches may have eventually evolved into an entirely different species. After heavy rains in 1982-1983, however, soft seeds were again abundant and the average beak size of the finches began to decrease again.
A final example of evolution has serious implications for human health. In the mid-20th century antibiotics were developed. These new drugs were able to cure many micro-organism infections that had previously been a serious threat to human health. As bacteria and micro-organisms were exposed to antibiotics, however, they began to develop resistance to them. A small percentage of a particular bacteria may have some resistance to an antibiotic like penicillin. The antibiotic may kill 98 % of the bacteria. But the 2 % that survive because they have some resistance to the antibiotic will then go on to reproduce, replacing the non-resistant bacteria. As a result of the widespread use (and misuse) of antibiotics, antibiotic-resistant bacteria have become more and more common over the years. Human activity has made antibiotic resistance a favorable trait for many forms of bacteria and other micro-organisms.
The process of evolution works through natural selection. Individuals whose genetic characteristics allow them to survive and reproduce pass on their traits to the next generation. When a species’ environment or ecological niche changes, then a new set of genetic characteristics become favorable to the organisms’ survival. As a result the organism evolves, in some cases producing entirely new species.
Robert Jurmain, Lynn Kilgore, Wenda Trevathan and Russell L. Ciochon. Introduction to Physical Anthropology. Eleventh Edition. Belmont, California: Thomson Higher Education., 2008., p. 28-31.