|MadSci Network: Evolution|
Before we discuss the specific points that you bring up, let's briefly overview natural selection and how it contributes to evolution. Natural selection is the process by which favorable mutations are "selected," while detrimental mutations are removed from a population. Selection occurs when the mutation affects the organism's ability to survive. Mutations that cause phenotypes that are favorably suited to the environment will likely enable the organism to live longer and have more offspring. Over time, the organisms with this beneficial mutation will begin to outnumber the others. Likewise, mutations that threaten survival to an organism will often limit reproduction, thus preventing the mutation from being passed on to any offspring.
It is important to note that natural selection is a gradual process. In addition, only mutations that occur within one's sex cells can be passed on to the next generation. This means that a mutation in an individual cell in the body, such as one that causes cancer,will not be passed on and are therefore "immune" to the affects of natural selection.
Let's assume that an individual does happen to develop a mutation in the DNA of a sex cell. Most of the time,the body's complex system of DNA repair will repair the damage. Although the repair system is astoundingly precise, it is not perfect and mutations do pass through the "inspection". It is these mutations that are passed on to offspring, and if they affect the offspring's ability to survive, will be open to the affects of natural selection.
A vivid example of natural selection can be seen when one places a colony of bacteria on a nutrient plate that contains an antibiotic such as penicillin. This particular drug kills the bacteria and most of the cells will subsequently die. However, by random chance, a few individual bacteria may have a mutation that makes them resistant to penicillin. These bacteria will be "selected" because of their unique ability to reproduce on the plate. Within a few days, the entire plate will be covered with bacteria that all have the penicillin resistance.
The reason that this experiment is able to illustrate natural selection so clearly is that the addition of pencillin causes a tremendous change to the environment where the only bacteria that can survive to reproduce are the penicillin resistant "mutants." It is very important to keep this in mind when discussing the effects of natural selection on evolution. Most of the time, favorable mutations yield only a very slight increase in the likelihood of reproduction. Because of this, the effects of natural selecion generally take an extremely long time.
The peppered moth example that you brought up allows us to look at another imporant point regarding natural selection. Over time, this particular moth has changed into a color that offers better camoflage. The moths that were better able to hide themselves were more likely to avoid predators and thus reproducde more efficently. Over time, the entire species adapted this color. Now, in your question, you note that the moth is still a moth. This is exactly right. The change in color was a mutation that offered a marked increased chance of survival. As a result, it was incorporated very quickly into the species. Remember, that because the body repairs most mutations, the process of incorporating favorable mutations into an entire species takes a very long time. However, you could very easily image a series of mutations that give the moth larger wings, enabling them to fly faster. Other mutations may eventually yield a smaller body, enabling quicker movement. Over millions of years, these little mutations could add up to a species of moth that no longer resembles the moth of today. In fact, it may be so dissimilar that you would not ever refer to it as a moth. I hope this illustrates at least the possibility of how natural selection can contribute to evolution.
For more on transition fossils, go to http://www.talkorigins.org/faqs/faq-transitional.html
Try the links in the MadSci Library for more information on Evolution.