# Chapter 14: The principle of evolution

The theory of evolution and the field of evolutionary biology are most closely associated with the work of the English biologist Charles Darwin. However, ancient philosophers were already dealing with questions about the origin of life. These philosophers believed that living beings arose from the offspring, or individual parts, of organs that randomly joined together. In the 18th Century, Carl von Linné, one of the most important systematists, argued that individual species differ in many important characteristics, while individuals within a species differ in only a few important characteristics. However, at that time, various ideas about the origin of life persisted, including for example, that organisms arise from non-living matter and the idea of creationism, which says that all organisms were created as we see them today, and only some of them have already become extinct, was prevalent. The possibility of evolution, the gradual development of a species, was not accepted then.

The first biologist to develop the theory of evolution was Jean-Baptiste Lamarck. In 1809 he published his most important work "La philosophie zoologique". Lamarck assumed that organisms are created repeatedly and gradually improve (transform). He suggested that by using a certain organ its function improves, and if a certain organ is not used, then stunting occurs. An important thesis of his theory was that these acquired characteristics are passed on to offspring. Although Lamarck's reasoning about the mode of change was shown to be incorrect, he was the first major scientist to conclude that evolution occurs at all.

The foundations of the theory of evolution, the principles of which persist to this day, were laid by the famous English scientist Charles Darwin (Figure 14.1). Darwin was a very perceptive pedant, and despite the fact that he did not have a biological education, and we consider him one of the most important biologists of all time. Darwin made his most important records and observations during a voyage on the ship Beagle in 1831-1836, the purpose of which was to map the coast of South America. He was fascinated by the various forms of ocean life and on land he explored many exotic areas (Patagonia, the Andes, the Galapagos islands and Australia). Darwin collected a large amount of material, which he continuously sent to London. After returning from the voyage, he began to write down his observations and formulate the ideas of evolutionary theory. The acceleration of the publication of his main work was the responsibility of Alfred Russel Wallace, who also developed a theory based on his observations, which did not differ in principle from Darwin's ideas. The manuscripts of both scientists were published at a meeting of the Linné Society in London in 1858. However, Darwin accelerated the preparation of the publication of his main work "On the Origin of Species", which was published in 1859.

In this work, he explained the basic principles of the theory of evolution, which we can summarise as follows:

• Existence of species evolution – species change over time, that is, evolution takes place.

• There is a common origin of all species – species separated (diverged) from a common ancestor in the course of evolution.

• Gradualism – species change and diverge gradually, by the slow accumulation of small changes.

• Natural selection – this is the main mechanism driving evolution, selecting the fittest individuals from populations.

Of course, Darwin's theory of evolution met with many supporters, but also with great critics. The genetic knowledge of heritability, introduced by experiments of Gregor Johann Mendel, helped at the beginning of the 20th Century to understand some parts of the theory of evolution, which until then were considered as its shortcomings (e.g. that alleles are inherited from parents to offspring, but do not mix).

Although in the first half of the 20th Century while heredity and the existence of mutations were known, many scientists still thought that, for example, bacteria behave differently and can create hereditary properties depending on the environment. An important experiment that contributed to solving this question was the so-called fluctuation test carried out by Max Delbrück and Salvador Luria in 1943 (see also Chapter 2 - How does a scientist works). These scientists worked with cultures of bacteria sensitive to bacteriophage T1. Delbrück and Luria took a part of the bacterial culture, cultivated it for a certain time without the presence of bacteriophage, and then spread the bacteria on media with phages. They found that resistant colonies of bacteria grew on the dishes, and their number was approximately the same as on individual dishes. In a parallel experiment, the culture of bacteria was divided into independent subcultures, which were cultured for the same amount of time without the presence of phage. After that, each subculture was grown separately on media in the presence of phage, and it was found that the number of resistant bacteria was very different on different plates (Figure 14.2). Based on the cultivation of independent subpopulations of bacteria in the presence of selection pressure (the phage), Delbrück and Luria confirmed that the emergence of bacteria resistant to bacteriophage T1 is the result of a random mutation that "won" under selection conditions. Therefore, the hypothesis that the emergence of resistant bacteria is an active response to environmental conditions, or that mutations arise "by order" of the environment is false.

Evolution is not a process that must necessarily be connected by a huge period, but we can follow it in real time. An excellent example is the population of peppered moths (Biston betularia) in Great Britain in the 19thand 20th Centuries and the phenomenon known as industrial melanism. Before the year 1800, this moth existed in its natural light-coloured form. The melanic form (with dark colouring), which is caused by a mutation in one gene, occurred only very rarely. The industrial revolution brought with it the pollution of the environment by large factories, which resulted in the bark of the trees becoming darkened with soot. Since this moth spends most of the day resting on the bark of trees, the natural form with light-colouring were easily visible on the darker bark of trees, and individuals were more often eaten by predators. In contrast, moths with melanic colouring were less visible and were less likely to be eaten. Although their frequency in the population was initially low, under such conditions the melanic form of the moths began to prevail in the population. Since the 1950s, with more emphasis placed on environmental pollution control, conditions have greatly improved in many industrial areas. The bark of the trees have become paler again and the natural forms of the moth are beginning to dominate again with melanic forms becoming rarer in these areas.

One of the basic evolutionary factors is selection. Due to the influence of selection, there is a difference in survival and successful reproduction in harsh environmental conditions. Abiotic factors (climate, altitude, salinity etc.) but also biotic factors (availability of food, predators, parasites, pathogens) can act as selection factors. The intensity of selection in evolutionary biology is determined by the so-called selection coefficient, which we express in the range 0-1. If the selection is maximal, i.e., each bearer of the given character is excluded from reproduction under the given conditions, the selection coefficient has a value of 1.

We also recognise different types of selection with variation in how selection affects the preservation of the frequency of individual phenotypes, and thus genotypes. One type of selection is selection in favour of heterozygotes, an example of which is the maintenance of the mutated Hbs allele for β-haemoglobin, whose carriers in the homozygous state suffer from sickle cell anaemia (see also Chapter 5 – Mutations: how they arise and what to do about them). In the heterozygous state, the Hbs allele provides carriers with an advantage in the form of resistance to malaria. It is therefore important to note that this allele can be considered advantageous only under severe selection conditions, which in this case is a geographical area with a high prevalence of the Anopheles mosquito, the carrier of malaria. If a human intervenes in the selection process with this activity, we call it artificial selection. A typical example of artificial selection is breeding.

There are also different views on the level at which selection operates. The classic "Darwinian" view says that selection acts at the population level, but it can also act at the level of individuals or groups of individuals. The British ethologist and evolutionary biologist Richard Dawkins brought a different perspective on this question. Dawkins claims that selection acts at the level of genes and he published his ideas in 1976 in the well-known work "The Selfish Gene". In 1973, John Smith and George Price developed the theory of evolutionarily stable strategy, which asserts that individuals with a trait that conditions the highest possible fitness (reproductive fitness, the ability to reproduce in a given environment) will not prevail in the population, but such a behavioural phenotype will be established under the given conditions, which cannot be replaced by a better one by natural selection. Environmental conditions and relationships between individual members of the population thus help to keep the fitness of each individual in check.

A prerequisite for the action of selection is the existence of variability between organisms. Therefore, mutations are another important factor in evolution. We divide mutations in connection to selection as follows:

- negative mutations – they disadvantage the carrier of the mutation and reduce its fitness. As a result, the frequency of the given mutation in the population will gradually decrease. There are many examples of negative mutations that cause human diseases (e.g., cystic fibrosis, phenylketonuria).
- positive mutations – carriers of such a mutation are favoured. Such positive mutations result in a higher fitness than other individuals in the population, so the frequency of such an allele will gradually increase. At a certain time, the so-called fixation arises, which means that each individual will have a given mutation (e.g. butterflies that have a longer proboscis will more easily get nectar from a flower, and after time, all butterflies of a given species will have a long proboscis).
- neutral mutations – selection is not directly affected as these mutations do not change the coding information in DNA. We can also consider as neutral mutations those that are reflected in the phenotype, but different phenotypes in this case do not affect fitness. However, populations do not have an infinite size, they cannot increase indefinitely, although individuals produce a large number of gametes, only a certain part of them is used and gives rise to new individuals. Thus, the frequency of alleles that carry given mutations can change due to a random process known as genetic drift (Figure 14.3). We could describe the difference between selection and genetic drift by saying that selection is a controlled process while genetic drift is random, but both can lead to the disappearance of a certain genotype.

With the advent of molecular biology methods, it has become clear that neutral mutations and genetic drift are not marginal factors in evolution. In 1968, the Japanese scientist Motoo Kimura came up with the theory of neutral evolution, which says that neutral mutations are responsible for most evolutionary changes at the molecular level, and changes within species and between species occur mainly through genetic drift. This theory is also supported by the fact that mutations in sites that do not affect protein functions or non-coding regions are the most common.