In the SparkNote on Darwin's evolutionary theory, we looked at Darwin's proposed mechanism for evolution, natural selection. It had five postulates:
Darwin knew that individuals were variable, that is, each individual in a population carried a unique set of traits. What he did not know is what produced this variability, namely genetic differences. Variation in the genes of individuals arises from several sources. Mutation, the alteration of existing genes to form new alleles, can arise from copying errors during DNA replication, DNA damage, and repair or recombination during cell division. Varation also arises from sexual reproduction, wherein new combinations of DNA are created through the independent assortment of genes.
This statement was a truly unique portion of Darwin's theory. In 1856, he did not know about DNA. He did not know about recombination events. He did not even know about genes. He merely understood that for selection to occur, variations must be transmittable from parent to offspring. We now know, that variation is caused by differences in genes and genes are passed on to offspring. More importantly, different genes are passed on to offspring independently of each other (independent assortment) and intact.
In most generations, more offspring are born than can survive to reproductive age given selection pressures such as predation and limited food supply. For example, many fish lay hundreds or even thousands of eggs at once, yet most of the young will be eaten or will starve before they can produce young of their own and pass down their genes (and the genes of their parents).
This portion of Darwin's theory is what we know as "survival of the fittest." Since more offspring are produced than can survive, some must die. Those that survive are those that have the greatest fitness. A trait that increases an individual's fitness is called an adaptation. Those individuals with the genes that convey traits that are best adapted to the environment in which the organism lives (those with high fitness) are more likely to survive and reproduce than those that are less adapted to their environment (those with low fitness).
On the genetic level, a specific allele for a trait can produce an adaptation and convey greater fitness. An individual with greater fitness is more likely to reproduce and pass this allele on to the next generation. Since more fit individuals produce more offspring, the percentage of individuals in the next generation with the fit allele--the allelic frequency of that allele--will increase. As this process repeats itself over many generations, evolution occurs, since the beneficial allele comes to exist within most of the population.