Evolution is based on changes in allele frequencies. For most genes, natural selection is by far the most
important factor in bringing about changes in allele frequency. In many cases, natural selection will mean
that one of two or more possible phenotypes will have a slightly better chance of producing fertile offspring
and will therefore have a greater contribution to the gene pool in the next generations. In cases of more
severe selection, the less advantageous allele may be lethal or prevent reproduction all together. In that
case there will always be genetic death of the individuals with the deleterious phenotype.
In humans, cystic fibrosis is an inherited disorder due to an autosomal recessive gene. Affected individuals
carrying two of the recessive alleles for the disorder (genotype ff) and as a result form extremely thick
mucus in their respiratory systems and elsewhere in the body. Their lungs are susceptible to frequent
infections and the disease is progressive and eventually fatal. Usually the victims die in their teens or early
twenties and so do not reproduce. This is changing with the advent of better treatments and successful lung
transplants but there is still not cure and survival rate past 30 years of age is rare. Among Caucasians of
European descent in the U.S., one person in 29 is a carrier, i.e. heterozygous for cystic fibrosis (Ff).
Severe natural selection has operated on the gene pool for the cystic fibrosis (f) and normal alleles (F) over
the centuries. The affected individuals do not reproduce and do not pass on their genes. New cases arise
only when both parents are heterozygous.
A test for the presence of the gene in a heterozygote would allow some persons with the “f” allele to decide
about reproducing, particularly if their partner was also a carrier. If they refrained, their alleles would not be
passed on to the next generation and the frequency of the “f” allele would decline in the population.
In the following exercise you will observe the effects of natural selection on the frequency of the “f” allele in
a population over four generations.