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.
Not all cases of survival of the fittest are genetically simple. Two ways in which alleles can convey a variable degree of fitness are through heterozygote advantage and balanced polymorphism.
Sickle-cell anemia is a disease in which the red blood cells have a defective kind of hemoglobin, the molecule that carries oxygen. This defective hemoglobin is the result of one particular allele of a gene coding for a part of the hemoglobin molecule. One would expect that an allele that caused such a disease would have a very low fitness and would be acted on by natural selection until its frequency in the population was practically zero. However, the allele for sickle-cell anemia remains in the population, especially among groups of people who live in areas affected by malaria. This is because the anemia-causing allele also conveys a protection against malaria. The maintenance of this allele in the population is an example of heterozygote advantage. A person who is homozygous for this allele will have severe sickle-cell anemia and will be selected against. However, in areas were malaria is prevalent, people who are heterozygous for the allele (have one sickle-cell anemia allele and one healthy allele) are more fit than those lacking it because they are protected from malaria and still have one functional allele to produce the appropriate kind of hemoglobin to prevent severe sickle cell anemia. Because heterozygotes have an increased fitness, the allele is maintained in the population.
A balanced polymorphism occurs when two phenotypes of a given trait occur with equal frequency in a population over many generations. An example of balanced polymorphism is seen in the scale-eating fish Perissodus microlepis. These fish attack another species of fish by sneaking up behind them and eating scales off their flanks. To help them do this, the scale-eaters have mouths that open to one side. Populations of Perissodus have equal numbers of left- and right-mouthed fish. This is because prey species have adapted to guard themselves against attacks from the scale-eaters. If all Perissodus had mouths that opened to the same side, they would all attack their prey on the same flank, and the prey species would adapt to guard that side more carefully, making it harder for Perissodus to attack. With a balanced population, prey species must split their guarding attention between both flanks, making it easier for either left- or right mouthed Perissodus to attack.