Theory of Evolution

Evolution through natural selection explains why species were so well adapted to their environment and how new species would form. The theory of evolution is considered to be fundamental not only to biology, but also to many aspects of contemporary science. Darwin’s ideas have also had a wide-reaching influence on theologians, historians, psychologists, philosophers, and sociologists.

Evolution Today

Evolution is not just relevant to the past, but it is also important in the present and the future. It is the foundation for all of modern biology, including research critical to human welfare, medicine and the fight against diseases. For example, an understanding of evolution is important in the fight against emerging diseases like HIV and pandemic flu, which have crossed the species barrier from other primates (HIV) or domestic livestock (flu) when they acquire the genes to do so, either from other microbes or by mutation. It explains the development of antibiotic resistance that has led to MRSA, multi-drug resistant TB and other drug-resistant bacteria becoming such problems today; and why the insecticides used for the control of the mosquitoes that transmit malaria are no longer as effective as they once were.

Darwin’s theory of evolution in a nutshell

Darwin noticed that even within a species there was variation, and that new varieties arose from time to time. Some of this variation was passed from parent to offspring, with the new generation closely resembling their parents. The mechanism through which this happened was not understood at the time, but was nonetheless widely recognised, and had been used for many years in the selective breeding of new varieties of domestic animals, and improved agricultural crops.

Darwin also recognised that organisms produce many more offspring than survive to reproduce. He postulated that this would inevitably result in a “struggle for existence” - a competition for food and resources - whereby those most suited to their environment would survive in favour of those with less favourable characteristics. Over many generations the differential survival of those with favourable characteristics would lead to the development of new forms, and ultimately of new species. He called this process natural selection: the survival and reproduction of organisms suited to their environment, at the expense of those less successful.

Darwin did not invent the idea of evolution. What made Darwin different from all earlier evolutionists was his discovery of how evolution works – through the process of natural selection. One hundred and fifty years after the publication of ‘On the Origin of Species’ Darwin's theory of evolution remains the only scientific explanation for the spectacular diversity of life on earth.

Controversy today

Darwin’s revolutionary ideas changed scientific thought forever, but it was the synthesis of his ideas with genetics, that explained how inherited characteristics are passed down through the generations (and also how new variation occurred through minute genetic mutations) that drove the widespread acceptance of his ideas.

Evolution remains something of a controversial subject because it challenges some of the fundamental beliefs about the origin of man and man’s place in the animal kingdom. Darwin’s concept of natural selection has also been applied to human society, most notably by Herbert Spencer, who coined the term “survival of the fittest”.

Is “survival of the fittest” at the expense of the weak?

Not necessarily. In an evolutionary context the “fittest” are those that are best adapted to their environment and are thus reproductively most successful. Being “fit” in this context is not necessarily the same as being the strongest or the most sophisticated, but more the best “fit” to the environment.

The evidence to support the theory

Darwin spent 20 years painstakingly collating detailed evidence to support his theories prior to publication of ‘On the Origin of Species’ in 1859. He laid out the enormous diversity of living things, and described the equally impressive variation within single species, brought about by people controlling the breeding of dogs, horses, pigeons or cattle. He also related the slow appearance – and disappearance – of species in the record left by fossils in the rocks.

The critical evidence came from close comparisons. Comparing fossils from different periods showed gradual change over time. Comparing body plans and bone structures of different living species showed how they were related to one another.

Particularly important to the evidence was the life found on the Galapagos Archipelago. Species found in the same kind of environments on the mainland that could have flourished on islands were often absent – this suggested that the species that live on islands were not created there, but had somehow, in the past, managed to colonise the islands from the mainland.

“Look closer at the details,” Darwin urged, “and see that the flora and fauna of any particular island are related to those of the nearest mainland, but are slightly different”. All this suggested that the variations in species depended on how long they had been separated. The longer they had lived apart, the greater the chance that they would have changed in ways that distinguished them from a common ancestor, but enabled them to survive and thrive in their new environment.

From these types of idea Darwin began to postulate that all living things were related and could be descended from a set of common ancestors, with natural selection being the driving force behind this evolutionary change.

The early shortcomings of the theory

Despite providing extensive evidence to support his theories, Darwin was aware that there were gaps that needed filling in order to fully establish his theory. He even devoted a Chapter in ‘On the Origins of Species’ setting out shortcomings which he believed would be removed by later research. “Anyone whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of certain number of facts will certainly reject my theory…I look with confidence to the future, to young and rising naturalists, who will be able to view both sides of the question with impartiality”.

Chief amongst these problems was a lack of understanding as to how inheritance worked. Darwin had no knowledge of the mechanism of genetics. At the time some thought that over time, blending would lead to increasingly homogenous populations, yet his theory of evolution through natural selection required exactly the opposite; it requires specific favourable variations to be passed down through generations to increase survival rate. It was not until Mendel’s research on genetics was re-discovered, that the genetic basis of variation and inheritance was understood. This shows that there is no blending of characteristics, and is confirmed by current knowledge of genetics. As such, offspring inherit a discrete set of genes from each parent and is in fact a composite of many different traits passed down by its parents and grandparents before them.

What can modern genetics and DNA tells us about evolution?

DNA not only confirms the scientific basis of evolution but it shows how it varies an organism’s characteristics. For example, by analysing the close similarity of their genetic code, scientists have today confirmed Darwin’s educated guess that the different species of Galapagos Finch did indeed evolve from a single common ancestor. The differences in beak shape that Darwin noted, which made the different species so well adapted to the varied environments found on the Galapagos Islands, are attributed to a single common gene. This gene expresses different proteins in the growing jaw of the finch embryo, resulting in the different beak types.

Looking closely at DNA reveals evidence as to how different species are related, because many important genes are shared across the animal kingdom. The longer the time since two species had a common ancestor, the more numerous the small differences in their genes are. So the human version of a gene is more like the chimpanzee version than that of a mouse or a fish; the mouse version is more like that of a rat.

Evolution occurring today

By virtue of the fact that evolution involves the progressive accumulation of favourable adaptations over many generations, it is often difficult to see in real time. However, some practical demonstrations of it occurring are seen in bacteria. The phenomenon of antibiotic resistance in bacteria (where bacteria are no-longer killed by antibiotic drugs) is as a direct result of evolution.

Antibiotics are toxic to the bacteria, and will kill them over a period of time (which is why antibiotics are given as a course over several days). If at the end of a five day course not all the bacteria are killed, the survivors will have a certain tolerance to that antibiotic. When these bacteria multiply, all the offspring will carry the ability to survive the antibiotic for five days - when these bacteria are exposed to the antibiotic, they will not even start to die until five days. Bacteria with this ability are better able to survive and reproduce, and therefore become the prevalent type of bacteria. The overall effect is that the bacteria have evolved from being killed by the antibiotic, to be resistant.

How new species form

The changes that eventually lead to the formation of a new species accumulate slowly over successive generations, but the results can be dramatic. A good example of this can be seen in the different varieties of domestic dog, albeit that these have resulted from human intervention and “artificial selection” rather than natural selection. The Chihuahua, Pekinese, Great Dane and St Bernard are all members of the same species, with its origin in the grey wolf, which was first domesticated around 15,000 years ago. However, the huge diversity in body shape and form in these breeds has been achieved through selective breeding over the last few centuries. If such profound shapes in shape and form could be achieved in such a relatively short period of time, what diversity could be achieved in far greater timescales? We now know that the earth is around 4.6 billion years old and that there has been life on earth for around 3.5 billion years. The divergence of humans and chimps from a common ancestor is believed to have occurred around 6 million years ago, with our direct ancestors evolving in Africa some 200,000 years ago.

Source : Darwin's Origin of Species ( British Council )