Darwin's theory of
natural selection is inarguably one of the most important and
revolutionary theories in biology. It is the
fundamental unifying theory of life. What this means is that it is pervasive throughout all of biology and is relevant to all its areas serving an explanatory function in such diverse subjects as
cell biology,
genetics,
ecology and even other related fields such as
psychology. Regardless of its importance, natural selection is quite uncomplicated and a relatively easy
concept to grasp, yet it is one of the most frequently misunderstood by the general
populace. Natural selection is not evolution, as many may believe; it is merely the mechanism through which evolution occurs.
Evolution can be defined specifically as the change in gene frequencies of a population, which accumulates over generations. This, of course, is a more modern notion since Darwin himself had no knowledge of genetics (or even Medellian inheritance for that matter). However, that does not mean that Darwin was wrong, in fact, it simply adds more credence to what he has already postulated by giving his theory the physical means with which to work. What Darwin saw as evolution is what he referred to as descent with modification; the term evolution was not used in the first edition of The Origin of Species. Our initial definition states that it is a change within a population, meaning that a population is the smallest group that can evolve; individuals do not evolve (despite what crappy self-help books will tell you!). This should be self-evident if you consider that it is the propagation of certain inherited genes through successful reproduction that causes change over time and not changes that occur to an individual during its lifespan. The idea that acquired changes in an individual, during its lifespan, are carried over to subsequent generations is misguided because it gives no real device for the trait to be passed on since the genes themselves have not been changed. This is known as a Lamarckian view of evolution. Evolution can be seen to be in action, as previously stated, if the frequency of genes changes. Natural selection is one of various ways in which the frequencies are changed.
In The Origin of Species, Darwin wrote, "I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection". Natural selection is the differential success in reproduction that occurs when certain heritable traits give individuals an advantage over others when leaving productive offspring. Having already stated that evolution does not occur within individuals, the inverse is true for natural selection. It is precisely the individual that natural selection is exerting its force upon. If a trait ensures that the individual will leave more offspring than others within the same population, the frequency of this beneficial trait (or more accurately, the frequency of the genetic material responsible for the trait) will be increased, causing evolution to proceed. This hinges upon the concept of Darwinian Fitness, which is the relative contribution an individual gives to the next generation's gene pool. Thus, natural selection operates only through differential reproductive success and not differential mortality, as commonly misconstrued. So, while it may be somewhat accurate to say that natural selection is the survival of the fittest, that does not necessarily mean survival of the strongest or fastest. It doesn't matter how physically adept an individual is, if it cannot produce viable offspring then it has a Darwinian fitness of nil.
Natural Selection is what pushes evolution on. It pokes and prods at it telling it to "hurry up" or "whoah, slow down there, cowboy". They are intrinsically and intimately connected, but they are not one in the same...and don't you forget it!