A "perfect" population would not carry any deleterious genes but
as we've already seen, natural selection does not produce a perfect
We would expect natural
selection to remove genes with negative effects from a population.
Individuals who carry those genes would not reproduce as much,
so the genes should not be passed on. And yet we see cases where this expectation is
not met. For example, human populations generally carry some disease-causing
genes that affect reproduction.
Why might deleterious genes exist in a population?
They may be maintained by heterozygote advantage
||Normal red blood cells (top) and sickle cells (bottom)
When carrying two copies of a gene is disadvantageous, but carrying only
one copy is advantageous, natural selection will not remove the gene from
the population the advantage conferred in its heterozygous state keeps
the gene around. For example, the gene that causes sickle cell anemia is
deleterious if you carry two copies of it. But if you only carry one copy
of it and live in a place where malaria is common, the gene is advantageous
because it confers resistance to malaria.
They may not really reduce fitness
Some genetic disorders only exert their effects late in life, after
reproduction has taken place. For example, the gene that causes
Huntington's disease typically does not exert its devastating
effects until after a person's prime reproductive years. Such
genes will not be strongly selected against, because an organism's
fitness is determined by the genes it leaves in the next generation
and not its life span.
They may be maintained by mutation
The mutation may
keep arising in the population, even as selection weeds it out.
For example, neurofibromatosis is a genetic disease causing tumors
of the nervous system. Natural selection cannot completely eliminate
the gene that causes this disease because new mutations arise relatively
frequently in perhaps 1 in 4000 gametes.
They may be maintained by gene flow
The gene may be common, and not deleterious, in a nearby habitat. If migration
from the nearby population is frequent, we may observe the deleterious
gene in the population of interest. For example, in places like the U.S.,
where malaria is not a problem, the gene that causes sickle cell anemia
is strictly disadvantageous. However, in many parts of the world, the gene
that causes sickle cell anemia is more common because a single copy of
it confers resistance to malaria. Human migration causes this gene to be
found in populations all over the world.
Natural selection may not have had time to remove them yet
The direction of selection changes as the environment changes what was
advantageous or neutral ten generations ago may be deleterious today. It
is possible that some of the deleterious genes that we observe in natural
populations are on their way out, but selection has not yet completely removed
them. For example, although there is debate about the issue, some researchers
have proposed that the relatively high frequency in European populations
of the gene causing cystic fibrosis is a historical holdover from a time
when cholera was more rampant in these populations. It is proposed that carrying
the cystic fibrosis gene provided some resistance to cholera and so increased
in frequency in earlier European populations. Now that these developed nations
are no longer threatened by cholera and the selective environment has changed,
natural selection may be slowly weeding the cystic fibrosis gene out of those