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Evolution in Action
Evolution occurs when there is a change in the environment or when there is mutation because the allele frequencies will change as a result. This change in allele frequency is called microevolution.
Usually mutations are harmful, occasionally they are neutral and infrequently they are beneficial. In this latter case, the mutation can increase an individual's chances of survival (fitness), reproduction and thus the frequency of that genotype.
The allele previously conveying fitness decreases in frequency in favour of the new allele.
Over many generations populations gradually change, becoming better adapted to their environment.
Antibiotics are chemicals usually produced by fungi.
They can kill bacteria by preventing cell wall formation and so are given to patients who have a bacterial infection. There may be however, one or more bacteria in the population that are able to inactivate the chemical making them resistant to the antibiotic.
These individuals have a massive advantage since they are the only ones that will survive when they come into contact with the antibiotic.
They are then able to reproduce and pass the gene for resistance on to the following generations. Since bacteria reproduce very quickly, the gene also will spread very quickly.
By using antibiotics we place a selection pressure on bacterial populations so that only resistant ones will survive. The more they are used, the more susceptible ones will be wiped out and the more the resistant ones have an advantage.
Haemoglobin is the pigment in red blood cells that carries oxygen around the body.
It is made up of 4 polypeptide chains, each one with an iron-containing haem group at the centre. 2 of these chains are called a chains. 2 of these chains are called b chains.
However, with the change of just one amino acid in the b chains (a mutation) the haemoglobin molecules stick to each other and form fibres inside the red blood cells. This makes the red blood cell change into a sickle shape and so they become very inefficient at transporting oxygen.
They become rigid and thus get stuck in capillaries, blocking them. Oxygen cannot get to the cells of the body and it is possible for the affected person to die.
People who are homozygous for the allele that causes the sickling of red blood cells have sickle cell anaemia. These individuals are less likely to survive and reproduce.
It would be assumed therefore that the frequency of this allele would be extremely low. It is not!! In some parts of East Africa, almost 50% of babies are born being carriers for this disease (they are heterozygous) and 14% are homozygous for the disease.
It so happens that in areas where there is a high incidence of the sickle cell anaemia allele, there is also a high incidence of malaria. The parasite that causes malaria infects the red blood cells and multiplies within them.
People who are heterozygous for sickle cell anaemia are much less likely to get the potentially lethal malaria than those who have 2 copies of the normal allele.
There is a strong selection pressure against homozygous sicklers because they can become seriously anaemic.
There is a strong selection pressure against homozygous normal haemoglobin alleles in malarial areas because these individuals are more likely to contract malaria.
Therefore, in malarial areas, there is a strong advantage to be a carrier for the sickle cell disease.