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.
Heterozygote Advantage
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.
Balanced Polymorphism
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.
