How does Darwin's theory of natural selection explain the origin of species?
If evolution was a car, the theory of natural selection would be the engine. The basic ideas of evolution were discussed long before there was any scientific research done to support them. The evolutionary concept was never able to gain any real steam because it lacked a mechanism. That is, scientists wanted to believe that species evolved from one form to another, but had no plausible process to make it happen. The theory of natural selection provides that reasonable method of evolution.
Natural selection essentially states that "the strong survive." The basic idea is that when change occurs, those organisms best suited to the new circumstances will thrive. Those who are not ideally suited will not be able to compete. Charles Darwin proposed this principle after observing some population variations in birds. He noticed that animals within a species often had slightly varied traits, and that those traits made some more suited to certain conditions. Darwin's theory was that, over time, the better suited animals would thrive and the others would die out completely. The resulting population would be entirely made up of those animals with the "better" trait. Over time, he reasoned, this could result in a species changing enough traits to eventually become a totally different creature, like a fish becoming a frog.
There have been some concerns expressed about the real meanings of the theory of natural selection. There is no doubt that variations within a single species make some members better suited to handle different circumstances. For instance, there's a popular story in science texts about moths. These moths lived in cities around the time of the industrial revolution and had to deal with increased pollution. Lighter-colored moths stood out on soot-stained buildings and trees, and thus, were easier targets for birds. The darker moths found it easier to survive, because they blended into the darkened environment. As a result, the population of light-colored moths dwindled over time, and the darker-colored moths increased. The dominance of the darker moths is used as an example of natural selection.
There is an important point to be made about the theory of natural selection, however. Once conditions return to "normal," the balance of that species will return to "normal" as well. Birds with unusually heavy beaks may become dominant during dry years, since they can more easily break open nut shells and tree bark. The "normal" birds, with regular beaks, will struggle and diminish. Yet, once the drought is over, the population tends back to normal-beaked birds. The darker moths who were more suited to the polluted times made up most of the moth population, but when the pollution began to fade, the moth population returned to its "normal" state.
Why does this happen? Species have shown to be genetically stable. In fact, genetic defects that change the form or function of creatures usually result in death. The examples of the moths and birds show that each species has some variations, and that those variations can favor different animals at different times. However, they also show that the same variations are possible generation after generation - which is why the populations can change right back to where they were. There are no new species or new variations being produced, just more or less of those that already existed.
There has been no scientific observation of any permanent change in species. There are plenty of proven cases of adaptation, which involves non-genetic changes. There are examples of natural selection changing the balance of populations within a species. Yet there are no known instances of a natural population experiencing a permanent, meaningful change. Observed genetic mutations are, in the natural world, crippling and usually fatal. While there is no doubt about the short-term function of natural selection, its long-term effects are not fully understood. While scientists prefer to point to the examples of birds and moths as proof of the theory of natural selection, they often refuse to see the same examples as contradictory to evolution itself.
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