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Natural Selection in the Human Body

By Oskar Swartling

 

After his voyage with the Beagle, Charles Darwin started to go through the massive amounts of notes he had written during the almost five year long trip around the world. With these notes, including a tree of life with the words ‘I think’ written above it, Darwin composed what would later be known as The Origin of Species by Means of Natural Selection or the Preservation of Favored Races in the Struggle of Life. During the authorship, Darwin received an essay from the field naturalist Alfred Russel Wallace. He presented almost the exact same theory, although lacking the extensive amount of data that Darwin had collected during his travels. Darwin’s papers and Wallace’s essay was published together in 1858.

The theory of evolution by natural selection is arguably the most important scientific concept of history. Since it was presented by Darwin and Wallace it has not only explained a lot of the wonders we are surrounded by, but has also been essential for the advancement of an array of scientific fields. Evolution never stops to amaze, best described by the famous evolutionary biologist Richard Dawkins: “We are surrounded by endless forms, most beautiful and most wonderful, and it is no accident, but the direct consequence of evolution by non-random natural selection – the only game in town, the greatest show on Earth”.

Darwin TreeThe process of natural selection spans further than just creating the endless forms we are surrounded by. It is a central mechanism in the human body, in sickness and health. Since the theory of evolution is questioned by some outside of the scientific community, it might be easier to grasp the idea of natural selection when it is connected to the human body, instead of ancient life forms evolving over millions of years.

Our immune system consists of a lot of different cells, commonly divided into the innate and adaptive immune system. One of these cell types are the B cells, part of the adaptive immune system and responsible for producing antibodies against specific antigens. Although the B cell-receptors are unique, binding only to one antigen, it does not necessarily mean that the interaction between the receptor, i.e. antibody, and the antigen is strong. To solve this, B cells undergo a process called affinity maturation, based on Darwin’s natural selection.

In short, affinity maturation is a process of increasing the affinity of produced antibodies to an antigen. In germinal centers, B cells activated by antigen and T-helper cells are dividing rapidly in order to produce antibodies to fight the infection. Since all these B cells stem from the same ancestor, all the antibodies have the same affinity to the antigen. During this process, the activated B cells allow a part of their genome, the Ig gene, to be extensively mutated, a process called somatic hypermutation. These point mutations in the antigen-binding part of the antibody randomly create slightly different antibodies, with slightly different affinity for the antigen. Due to somatic hypermutation, we now have randomly created antibodies with a varying degree of affinity to the antigen. The important part of the affinity maturation is the selection and preservation of antibodies with an increased affinity. Activated B cells that fail to bind to an antigen will die by apoptosis. An antibody with higher affinity is more likely to bind to an antigen that an antibody with less affinity. Therefore, the B cells producing more effective antibodies (i.e. with higher affinity) will survive. As this process is continued and the concentration of antigen is declining, the affinity of antibodies is increased during a prolonged or persistent infection. Affinity maturation, with random variation among antibodies and the selection of the most effective, is very much analogous to the natural selection of Darwin and Wallace.

The theory of evolution by natural selection is arguably the most important scientific concept of history.

 

Cancer is a genetic disorder, a result of mutations, either induced by an environmental insult or acquired spontaneously. These cells also express some degree of epigenetic variation. When these mutations result in a change in the expression of key genes that regulate cell growth, survival and other essential cellular processes, a cell can be transformed. One crucial feature of these mutations is that they are heritable, meaning two daughter cells will express the same genetic anomalies, making the mutated cells prone to Darwinian selection. Cells with mutations that provide them with advantages in cell growth and cell survival will be more favoured during the selective pressure that exists, therefore outcompeting and dominating the population. Since all the cancer cells originate from a common ancestor, and all the cells express the same genetic alterations, every new mutation that arise and give a cell a further advantage will be selected and able to pass these new alterations on. It is important to remember that all the mutations that arise are not beneficial, but that random variation is followed by non-random selection. During this process, there is an accumulation of mutations in the cancer cell population, giving rise to a set of characteristics called hallmarks of cancer, including properties such as independence of growth signals, evasion of apoptosis and ability to invade local tissues.

Tumour progression means that over time, tumours become more aggressive and more malignant. This process is probably due to different subclones acquiring different mutations over time. This means that although all the cells in a tumour are monoclonal in the beginning, a tumour may later be very heterogeneous. Heterogeneity is an advantage for the tumour, since individual cancer cells may be differently adapted for various selective pressures. As the tumour grows, the population is enriched for cells with a higher malignant potential. This Darwinian selection and “genetic evolution” explains two properties of cancer with immense consequences: over time, cancer tends to become more aggressive and less sensitive to therapy.

Antibiotic resistance is an increasing global health problem and may leave the world without any efficient antibiotic in the future. Fortunately, the issue of antibiotic resistance is getting more and more attention outside the medical sciences and the recent findings of a potentially novel antibiotic substance brings hope.

The reason for the increase of various degrees of resistant bacteria is manifold, but includes overuse of antibiotics, uncontrolled sales and the use of broad-spectrum antibiotics when not needed. Not surprisingly, genes carrying antibiotic resistance occur naturally. Penicillin, for example, is a group of antibiotics derived from Penicillum fungi. Since bacteria and fungi has been in an arms race long before humans came into the picture, it naturally follows that some organisms have evolved properties to harm, like Penicillum fungi, and to defend. An interesting example of the latter is when scientists analysed dried soil from the British Museum that had been stored since 1689, they discovered dormant bacteria expressing penicillinase. Although antibiotic resistance occurs naturally, the misuse of antibiotics changes the microenvironment and the joint selective pressures, permitting resistant bacteria to be favoured, survive and spread.

The modern use of antibiotics has allowed an unprecedented evolutionary pressure, allowing multi-resistant bacteria to become a serious health issue. The international scene is now taking action, but one third of the public still believes that antibiotics will work against coughs and colds, and some sources states that the common cold is the most common reason antibiotics are prescribed.

Evolution by the means of natural selection is clearly not only for the history books. It does not only describe how we, or all the other vast types of living organisms, came to be. The principles of evolution are very much something that affects us today – everyday. Richard Dawkins wrote: “Without the ever-escalating arms races between predators and prey, parasites and hosts, without Darwin’s ‘war of nature’, without his ‘famine and death’ there would be no nervous systems capable of seeing anything at all, let alone of appreciating and understanding it.”

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