The importance of variation and diversity in organisms
One way in which variation is introduced is through mutations to the DNA base sequence of
individuals. Mutations arise when a base is deleted, added or substituted for a different base within
the nucleotide sequence. Equally, translocation, inversion and duplication mutations describe what
happens when whole sections of base sequence are changed on a chromosome. Mutations increase
the genetic diversity in a population by giving rise to new alleles, therefore it would code for a
different polypeptide with a different function. Although many mutations are maladaptive, they are
important because some give their possessor a selective advantage. For example, a mutation to the
base sequence of a bacteria, such as MRSA, can give rise to an allele coding for a protein that gives
rise to resistance to antibiotics. Exposure to treatment with antibiotics provides a selection pressure
that causes bacteria with the resistance allele to be selected for. They survive and reproduce,
passing on the allele to their offspring. Over time, the frequency of the allele for resistance increases
in the population, due to division of surviving bacteria by binary fission, but also horizontal gene
transfer. Plasmids can be replicated, then twisted up and pulled through a tube called a conjugation
pilus, allowing the genes that they contain to be exchanged between bacteria in a population. This
leads to the rapid spread of resistance to antibiotics, which is important for the bacteria as it allows
them to survive.
A second way in which variation is introduced between individuals of the same species is in meiosis.
During prophase I, bivalents form and non-sister chromatids of homologous chromosomes become
entangled, forming chiasmata. As sections break off, equal lengths of non-sister chromatids are
exchanged in a process called crossing over, resulting in the formation of new combinations of
alleles. Homologous chromosomes are then independently assorted (i.e. they line up at the
metaphase plate at random), which means that the combination of chromosomes of maternal and
paternal origin that end up in each daughter cell is also a matter of chance. Finally, variation is
introduced due to the fact that fertilisation is random, which means that any male gamete can fuse
with any female gamete to form the zygote. Meiosis is important because it increases genetic
variation in a population of sexually reproducing individuals as each of the daughter cells has a
unique combination of genetic material. This is crucial for evolution as it allows for natural selection
to act on a wider range of genetic variation within a population. This, in turn, decreases the
likelihood of extinction as it is likely that some individuals will possess an advantageous combination
of alleles which give them a survival advantage in changing environments.
A third way that variation is introduced between individuals is through epigenetics. Epigenetics
refers a heritable change to gene function without changing the base sequence of the DNA, given
rise to by environmental factors. Methylation of cytosine bases causes DNA to become more tightly
condensed as positively charged methyl groups attract the negatively charged phosphate groups of
DNA nucleotides. This makes the promoter regions of genes inaccessible, so transcription factors
cannot bind and transcription is inhibited. On the other hand, acetylation of histone proteins causes
the DNA to decondense as the negatively charged acetyl groups repel the negatively charged
phosphate groups on the DNA. Transcription factors can therefore bind to the promoter region and
transcription of the gene is stimulated. mRNA is produced and will be translated at the ribosome,
increasing production of the polypeptide coded for by the gene. The epigenome is important
because it alters gene expression depending on environmental factors (e.g. diet and stress), allowing
organisms to adapt to their surroundings quickly, rather than across multiple successive generations.
Crucially, epigenetic changes are sometimes heritable, which is important because it means that
organisms are born better adapted to their environment. The importance of epigenetic markers
being added to the DNA correctly is illustrated by what happens when it goes wrong.
Hypermethylation of a tumour suppressor gene (such as p53, BRCA1 or BRCA2) inhibits transcription.
