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A Simple Guide to Evolution

This is a brief guide through the maze of information on evolution, written as plainly and simply as possible, given the complex nature of the subject. The first section of the guide deals with the classification of living things by early naturalists on which Darwin and Wallace based their studies,  the central section explains Darwin and Wallace’s theory of evolution by natural selection and the final section indicates how further scientific findings in geology, genetics and experimental biology have expanded the understanding of evolution.


The Study of Species from Ancient Greece to Revolutionary France

 The Greek philosopher Aristotle (384 to322 BC) developed a classification system which divided animals from plants and then divided animals into those with red blood and those without, and then divided the red blooded creatures into mammals, lizards, birds and fish. He believed the lowest organisms originated from mud and slime but he did not believe that more complex creatures evolved. He did think though, that there was a hierarchy in nature, and that humans were the highest animals.

The system of classification we still use today was introduced by a Swedish botanist Carl Linnaeus (1707 -1778). He used a Latin to give each species a family name and a species name. Linnaeus’s original conviction that species never changed was later modified when he observed that the mating of different species could produce hybrids. The idea of evolution did not occur to him however; his stance was one of awe at nature allied to a belief in a God who had created each living species separately. It was not until the eighteenth century that the idea that species changed over time and that everything that lived, men, mosquitoes or mushrooms, had the same origin was thoroughly investigated.

The French botanist Jean Baptiste Lamarck (1744 -1829), a Professor at the Jardin de Plantes in Paris who worked on worms and insects, was responsible for considerable advances in the classification of animals. His studies led him to believe that species evolved over time. Lamark suggested that the use or disuse of a part of the body would cause it to increase or decrease in size, and this change could be inherited: for instance, each giraffe that reached for the leaves hear the top of the trees would make its own neck a bit longer and this longer neck would be passed on to its offspring.

 Charles Darwin’s grandfather, Erasmus Darwin (1731 -1802), physician, scientist and poet, had similar ideas to Lamarck, and speculated in his work Zoonomia that all life had arisen from single living filament. Erasmus Darwin was an adherent of the views of the Enlightenment, a philosophical movement which believed that human reasoning was to be preferred to unquestioning faith. This made it easier for him to see the Biblical account of creation as a metaphor rather than the literal truth, and later it was also easier for Lamarck, who developed his ideas independently of Erasmus Darwin, to put forward his ideas in the climate of the French Revolution whose leaders opposed the power of the Catholic Church.

 Thomas Malthus (1766 – 1834) a clergyman and political economist, had a profound influence on evolutionary thought. His book, An Essay on the Principles of Population, was written in 1798 in response to his father’s attempt to convert him to the idea that humans would naturally progress towards a utopian society in which equality, wealth, happiness and virtue flourished. Malthus thought this idea was incompatible with the facts of human existence, which he pointed out was regulated by the same laws as the existence of other animals. He noted that humans had the potential to reproduce so as to double their numbers every 25 years, as had happened in the newly colonised America. He thought that it was unlikely that any land could provide food for this potential increase in population indefinitely, and that in fact war, famine and pestilence normally limited the population increase.


Darwin and Wallace

Charles Darwin (1809 -1882) and Alfred Russel Wallace (1823 –1913) were both influenced by Malthus’s views and began to investigate how populations of plants and animals were limited, and both developed the same theory of evolution independently.

 Darwin studied classics at Cambridge, and was influenced by William Paley (1743 -1805), whose book: ‘Natural Theology: or, Evidences of the Existence and Attributes of the Deity, Collected from the Appearances of Nature’ (1802) introduced the idea that the existence of God could be inferred from nature: just as anyone would infer from finding a watch that it had been made by a watch-maker, so anyone examining the wonderful array of plants and animals on earth could safely infer the existence of a creator. When Darwin set off on his voyage around the world on HMS Beagle after he graduated he was a reasonably orthodox Christian.

On the voyage Charles read ‘Principles of Geology’ by Charles Lyell (1797 -1875). Lyell revolutionised ideas on geology; it was believed that the surface of the earth had been moulded by a few past catastrophes like Noah’s flood, but that this period of upheaval was over, but Lyell found evidence that change was continuous over vast periods of time. Darwin demonstrated that the earth was still changing when he measured an eight foot uplift in the land after an earthquake in South America.

In South America Darwin found the fossils of extinct giant sloths and saw how similar their bones were to the skeletons of the smaller sloths which were alive in his time. He also saw that species of birds and animals found on the Galapagos Islands were similar to those found on the mainland and he was told that the giant tortoises were different on each of the thirteen islands. He began to wonder if species changed and if so how this happened. Malthus had pointed out that humans, who produced young very slowly compared with many other animals, could multiply at a much faster rate than was needed to replace the population, and Darwin knew that other animals and plants could produce far more offspring. Each moth produces many eggs, each oak tree many acorns and rabbits breed, well, like rabbits, and yet populations do not usually change much because many beetles, acorns and young rabbits are either eaten by animals, die of diseases, lose out in the competition for food and space, or fail to reproduce.

As every individual oak, rabbit or moth grows to be very slightly different from all others Darwin realised that those that were best adapted to their environment would survive and pass on their characteristics to the next generation.  This process, which he called natural selection, would gradually change the form of the species and new varieties would arise, just as they do when humans breed varieties of dogs or cattle. If new varieties were separated from others, as were the tortoises on different Galapagos Islands, then the different varieties could become different species.

It is important to note that Darwin did not say that it is always the strongest or most aggressive which survive, it is the fittest in the sense that they fit in with the environment in which they find themselves, and the environment is always changing, becoming dryer, hotter, cooler, wetter, darker, lighter and so all life forms must change in response to this and to the presence or absence of trees, tigers, towns etc. Darwin also rejected the idea that evolution always led to progress from ‘lower’ to ‘higher’ organisms, evolution promotes change in no particular direction. Darwin thought everything was descended from a single primitive ancestor and other forms of life spread out from this beginning like the branches of a tree or bush. Individual species were like twigs that grew from extinct branches of the tree.

Alfred Russel Wallace was a collector of natural history specimens working in the Indonesian Islands when he also thought of the idea of natural selection. He sent his ideas to Darwin, who had already been working on evidence for the theory for years but had hesitated to publish it. Wallace’s views were very similar to Darwin’s but later in his life Wallace came to question whether natural selection could account for human intelligence and consciousness. He thought that a higher intelligence must have placed these mental attributes in humans. Though he had been an agnostic since his teens he took up spiritualism and tried to prove scientifically that communication with the spirit world was a fact and not a fraud or fantasy.  Darwin rejected this idea, instead, in his book The Expression of Emotions in Man and Animals he looked at animals for evidence of qualities thought to be unique to humans such as intelligence, moral sense and appreciation of beauty.

 Darwin thought of man, like all other species alive then, as situated at the end of a twig on the evolutionary tree of life, but the philosopher Herbert Spencer (1880-1903) believed that man was at the top of the tree: the culmination of the evolutionary process. Some people thought that society could assist the process of perfecting mankind by allowing the strongest to dominate the weak. The message of ‘survival of the fittest’, a term coined by Herbert Spencer, seemed to fit the era of industrialisation and colonisation. Some people used this idea to justify eugenics and the neglect of the poor.

The theory of evolution also generated a great deal of controversy in religious circles as some of Darwin’s followers used evolution to try and discredit the beliefs of established church which, at that time, regarded the bible stories as facts rather than as metaphors to aid spiritual development. Many people disapproved of this anti-clerical stance because they looked to religion to provide stability at a time of great social change. 

 Darwin and Wallace were both agreed that natural selection was the main engine of evolution, and that geographical isolation, as on islands, often favoured the evolution of new species. Wallace  noticed that in some southern Indonesian islands the mammals were marsupials, as in Australia, but above a line he mapped out, (now called Wallace’s Line) the animals were similar those found in Asia.


 The Development of Evolutionary Knowledge in the 20th and 21st century

The reason for Wallace’s Line was not understood until the ideas of Alfred Wegener (1880 -1930) were accepted.  He was a German meteorologist and climatologist who deduced from the evidence of fossils that the continents had all been joined in one land mass which he called Pangea, and that this mass had broken up and the continents we know today drifted over the surface of the planet. Wallace’s line marks the junction between the Asian and Australian continents, which had been further apart in earlier epochs. Wegener’s theory was controversial until researches in the 1960s showed that the solid continents did indeed move and the earth’s crust was a series of plates which slid over a fluid layer.

In Darwin’s time no one knew anything about the mechanisms that enabled species to pass on characteristics to the next generation. A German monk and mathematician, Gregor Mendel, (1822 -1884) researched this question by crossing wrinkled and smooth peas and noting how these characteristics were inherited. He came to the conclusion that units (we now call them genes) passed from parent to offspring and different combinations of these units produced either round or wrinkled peas. Some units or genes he called dominant, like the unit/gene for round peas which would result in round peas even if only one unit/gene was present; the gene for wrinkled-ness he called recessive because two genes, one from each parent, were need to produce a wrinkled pea.

 In the early 1900s scientists found microscopic bodies called chromosomes in the nucleus of living cells, and saw that they split when the cell divided. They found that each sperm or egg cell contains a split chromosome, and the two halves unite to make a new nucleus.  Chromosomes are made of a chemical called DNA; genes are sections of DNA and, as Mendel foretold, some are dominant and some recessive. In 1953 Crick, Watson and Franklin discovered that genetic material or DNA had a spiral structure, a double helix, which unzipped when the cell divided.

Since then the structure of many genes has been worked out. It has proved more complex than was originally thought to know exactly how genes will affect the organism. Some, like Mendel’s genes in peas, have easily recognisable effects that are inherited in predictable patterns, but some will only show their effects in certain environmental conditions, or if other genes are present. The study of how the environment interacts with genes is called epigenetics.

Mistakes may occur when genes are copied in the reproductive process; these are known as mutations and have an effect, good or bad, on the development of the new organism. Mutations sometimes enable organisms to change in response to changes in the environment, for instance a random mutation may arise which protects insects against currently used insecticides; the offspring of insects with this mutation will multiply while those without it will die out. 

 In Darwin’s time competition between individuals and species appeared to be the main driver of evolution, but in 1970 Lynn Margulis uncovered a different aspect of the process in her book The Origin of Eukaryotic Cells.  Apart from viruses and bacteria all living cells are eukaryotic, that is, they have a nucleus. In addition to chromosomes the nucleus contains mitochondria, which generate energy for the cell, and plant nuclei also contain chloroplasts, which capture energy from the sun. Mitochondria and chloroplasts look like bacteria, and Margulis produced evidence that this was their origin. Her theory, now accepted, is that the first cells with a nucleus were combinations of different single celled organisms, that is, two or three cells in one envelope which lived in symbiosis, a union which benefits all partners. These new symbiotic cells had advantages that enabled them to develop into the vast array of multi-celled organisms we see today. Margulis demonstrated that co-operative processes enabled evolution to take a monumental leap forward.

There are many other instances of symbiotic behaviour, lichens, for instance, are composed of fungus and algae living together in one body. Many insects, bees and ants for instance, succeed by living co-operatively in colonies. The strategy of forming flocks or swarms also has benefits for birds and animals. Evolutionary understanding has been given another dimension by work on ‘niche construction.’ Living organisms not only fit into a niche, but also construct their own niches which have an effect on other organisms. Beavers, for instance, alter the environment by making dams, which changes the flora and fauna in the flooded land above the dam.

In the past people thought of evolution as a provisional theory because the process is often too slow to be observed, but patient field studies have shown examples of organisms evolving in the short term, for instance, mussels in the eastern US and  finches in the Galapagos. In the early 1900's scientists believed that the effect of natural selection had been overstated and that mutations, errors in the replication of DNA, were an important in generating new species, but natural selection has since been reinstated as the main driver of evolution.

Working out how evolution shaped all living things is a story of fervent human curiosity and the wish to understand the world as it really is. This quest has not been easy. The wish to believe what is desired has strained against the wish to know the truth, and even in science arguments which should be logical and evidence-based may be influenced by personal and emotional issues. Mistakes have been made by people who were too quick to rush to judgement, such as those who promoted eugenics, or who, like Wallace, decided that spiritualism could answer the question of how human intelligence and consciousness developed. The quest to understand the origin and development of the infinitely variable life on earth continues with work in many fields: genetics, epigenetics, genomics and other new and sometimes controversial areas of study. Like the dazzling array of living things that it constantly examines, the study of evolution changes over time and has split into new varieties of enquiry.

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Back to Strange Attractor homepage
Darwin biography, as impartial as I can make it, without  fundamentalist 'religious' disapproval or overzealous 'scientific' trumpeting of his work.
A Simple Guide to Evolution - a short history of evolutionary ideas
'Unsung by Singers'  considers the scarcity of poems on science.
Darwin, Wallace and pigeons - two poems
Tidal Haiku illustrated by Hilary Griffith