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How is genetic variation maintained?
Under natural selection we would expect that the advantageous alleles would be selected for and
the disadvantageous alleles would be selected against, resulting in stabilising selection and a
monomorphic population. It is therefore hard to see why polymorphisms exist, and indeed why any
genetic variation exists within a population. However, it is clear that this variation does exist, and it
is the raw material which natural selection uses to shape the evolution of new species and new
polymorphisms within a population.
The origin of genetic variation lies in mutation. Mutations can either be at the level of the gene or at
the level of the chromosome. Gene mutations can either be base pair substitutions, additions or
deletions which will affect one or a few nucleotides. Base pair substitutions may have no effect if
they occur in an intron, they also may have no effect due to the redundancy of the genetic code, the
same amino acid may still be coded for. However, if a new amino acid is coded for it could affect the
tertiary structure of the protein and it could be made redundant, these mutations will be selected
against. However, if the mutation has no negative effect it will remain within the population,
providing variation. Insertion or deletion of base pairs, frameshift mutations, have a greater effect
on the genetic code as all the amino acids coded for after the mutation will be affected and not just
the one, like in substitutions.
Mutations at the level of the chromosome could be due to a number of reasons. Deletion or
duplication of a section of the chromosome is most likely to occur during meiosis if unequal crossing
over occurs, and a part of one chromosome is transferred to the other chromosome. Sections of
chromosomes can break and rotate one hundred and eighty degrees before reattaching, called an
inversion. The number of chromosomes can also change. Two chromosomes can fuse together,
centric fusion, or one chromosome can separate into two, centric fission. Whole chromosomes can
either be missing or be in excess, or whole genomes can be duplicated in polyploidy events.
Mutations are the source of genetic variation, however, this genetic variation is maintained by
various mechanisms and not just purified by natural selection. Sexual reproduction is one
mechanism by which variations are maintained. During meiosis genes can recombine as a result of
chiasmata forming due to crossing over of chromosomes. Independent assortment can also cause
variation as each homologous pair of chromosomes assort independently of each other. Finally the
fusion of gametes from two different organisms allows different alleles to move more quickly
throughout populations than if sexual reproduction was not present.
In diploid organisms, the fact that there are two alleles of every gene present means that even if one
gene is non-functional, as long as the other gene is normal then the organism will be able to function
normally as the normal gene will be expressed. This means that the non-functional alleles will not be
selected against as they will be sheltered due to heterozygosity. Therefore, mutations can be
harboured and diploid organisms have a greater genetic load than haploid organisms, which in turn
can lead to greater variation.
As well as deleterious and advantageous mutations occurring, there will also be neutral mutations.
Some biologists have argued that the majority of genetic variation is due to neutral mutations
occurring at parts of the genome which are not under strong selection pressures. Genetic drift will
therefore occur, which is a random process by which the genetic material mutates but is not under
strong selection pressures and so the mutations are neither selected for or selected against. The
new mutations may eventually become fixed within the population, reducing genetic variation, but
this could take a long time.
There are several mechanisms of natural selection itself which can lead to genetic variation within a
population. These forms of natural selection lead to polymorphic populations where traits do not
become fixed over time because depending on the context or time different traits are favourable.
Negative frequency dependent selection occurs when the fitness decreases as it becomes more
common.
Batesian mimicry: A species which is palatable mimics a species which is toxic or unpalatable and so
gains protection. If the mimic species becomes too vast in numbers predators may begin to not
associate the species with being distasteful as there may be more mimics than the original
distasteful species and so the mimic species will drop in number. In some butterfly species there will
be polymorphisms, for example, within one species there may be butterflies mimicking multiple
different other toxic species and this will be maintained because if any of the mimics become too
popular they will be selected against, and so this is also known as balancing selection. (viceroy and
monarch)
Plant self-incompatibility alleles: Ovules cannot be fertilised by own pollen (to encourage crossfertilisation). Many of these systems based on a single gene and compatibility is determined in one
of two ways. Either the pollen is rejected because it is carrying an allele that is shared with the ovule
or the pollen is derived from a parent that shares the allele with the ovule, in this case it is the
genotype of the diploid anther in which the pollen was made which determines compatibility. A rare
allele will have a strong advantage because it can fertilize more plants. When the allele becomes
common then the plant will waste energy creating pollen which cannot fertilise other flowers
because it is now the common allele that is recognised. This can lead to high levels of polymorphism
at the molecular level.
Although the batesian mimicry example is at a phenotypic level and the plant self-incomatibility
example is at the molecular level, both will be determined ultimately by genetic variation. So in this
way, a monomorphic population is not selected for and polymorphism is maintained.
Another way in which natural selection favours genetic variation is when there is a heterzygote
advantage. If the heterozygote genotype has an advantage of the homozygous genotype then
variability will be maintained because both alleles will be maintained. The most well understood
example of this is sickle-cell anaemia. If a human has two sickle-cell anaemia alleles, which are
incompletely recessive, then the human will have sickle-cell anaemia. If the human has two normal
alleles then they will not have sickle-cell anaemia but will be more susceptible to malaria. If the
human is a heterozygote then they will have an intermediate phenotype, due to the incompletely
recessive nature of the sickle-cell alleles. They will have some blood cells which are sickle shaped but
not enough to cause the disease, and they will have an increased resistance to malaria. Due to the
advantage of the heterozygote, they will have the most Darwinian success within the population as
they will be most likely to survive to sexual maturity and therefore pass on their genes, and so the
genetic variation is maintained within the population.
It is highly unlikely that the organisms within a population will all be living in exactly the same
conditions, abiotic and biotic. Some organisms may live in slightly damper conditions or some may
live in slightly brighter conditions. Therefore natural selection will not be acting in the same context
and so exactly the same traits will not be favoured. An example of this could be found when looking
at different colour polymorphisms within a population of grasshoppers. In the winter brown
grasshoppers would be better camouflaged and so would be predated less, in the summer green
grasshoppers would be better camouflaged and so predated less, leading to a situation of balanced
polymorphisms within the population. The different selection pressures lead to different
phenotypes being favoured and in turn different genotypes.
It is important not to look at phenotypic variation and genotypic variation as completely separate
processes as they are highly related. Some of the most important arms races in nature may appear
to be phenotypic in nature but in fact are vital in maintaining genetic variation. For example,
predator-prey relationships drive the evolution of new traits in both the predator and prey, these
traits will have a genetic basis. With each organism under many different selection pressures, due to
their prey, their predators, their parasites and their environments, with not all of these favouring the
same traits, it is hard to see how a population could become genetically monomorphic. The anomaly
is not genetic variation but lack of it.