“Nothing in biology makes sense except in the light of evolution.”

This was famously said by Theodosius Dobzhansky, one of the most eminent biologists of the 20^th century, and practically every biologist in the world agrees with him. But what does such a sweeping claim mean? What is it in particular about the living world that the theory of evolution explains? The answer is why the huge number of different kinds of animals, plants and microorganism that are alive today exist at all, and what they are like. No one knows for sure how many species there really are, maybe 5 million but some think as many as 30 million. Each one of these teeming species is unique. They range from bacteria that are a mere 100^th of a millimetre long to the100 ton blue whale; some species dwell 6,000 feet below the surface of the sea in trenches of near-boiling water. Some burrow in the sand of the hottest deserts, while others can fly 25,000 feet above the surface of the Earth, There are dwellers in high mountains, tundra and polar regions. Some microorganisms even live deep inside solid rock. The tropical rain forests on land, and the coral reefs in the sea are home to a seemingly endless variety.

The theory of evolution tells us that this amazing diversity is the result of gradual changes in organisms, generation by generation, from remote common ancestors to modern descendants, like a sort of family tree.

The theory of evolution explains the classification of the living world

As more and more species were discovered and studied, a very interesting thing about this vast diversity of organisms stood out. Species sharing a number of similar characteristics with one another can be put into the same group. For example, the approximately 5,500 species we call mammals, the dogs, horses, bats, mice, whales, monkeys and so on, are put into the group Mammalia because they all have hair, mammary glands producing milk to feed their young, warm blood, large forebrains and many more similarities. Other species, such as frogs and newts, all have moist skin, broad heads, aquatic larvae and so on. They form a different group, called Amphibia. The birds, distinguished by their feathers, wings and beaks, are the group Aves. But it does not end there, because these groups can themselves be arranged into larger groups: amphibians, mammals, birds and also the reptiles all have four legs (sometimes reduced) and breathe with lungs. They form the group called the Tetrapoda.

Tetrapods in turn are distinct from, say, the Insecta which have six-legs and fine breathing tubes, or the Mollusca with shell and gills. Going the other way, we find that groups can always be divided into subgroups. The mammals for instance are made up of 20 subgroups, such as the Carnivora for the dogs, cats, bears, etc with cutting teeth, the Chiroptera for the bats with their wings, and the Primates for the monkeys, apes and humans. Each of these is then divided into even lower level groups all the way to the individual species. In fact the entire living world is classified as groups within groups within groups within groups, each defined by a unique sets of characters, an arrangement called a hierarchical classification.

The theory of evolution provides the answer to why the classification of life is hierarchical in this way, as you can see from the next diagram. A species may divide into two, and each one then changes a little bit by evolving new characters. Some of the new species go on to divide again and again, while others become extinct.  As this process continues over time, the groups of descendants come to differ more and more from one another in some of their characters. But they always still share some characters that they inherited from their common ancestor. Species that diverged more recently have less differences from each other and make up the lower level groups.

These lower level groups separated further back in time from each other and therefore have evolved more differences from each other. Put together they make up the next higher groups, and so on.

The theory of evolution explains adaptation

Every organism is adapted for its way of life. What this means is that the structure, the physiology and the habits of each one are well suited to perform the activities and processes of its particular life, however specialised that life is. Amongst the millions of examples we could mention are the wonderfully efficient design of the bird’s wings for flying; the waterproof cuticle of an insect facing the dangers of desiccation; the protective spines of an acacia tree; the high crowns and ridges of a horse’s tooth for grinding up tough vegetation; the dull green camouflage colour of a toad. The theory of evolution by natural selection, the great insight of two 19^th century naturalists, Charles Darwin and Alfred Wallace, explains how such adaptations come about. The offspring of organisms always differ very slightly from one another due to accidental minor changes in their genes. If an individual is born whose difference happens to slightly improve how well it copes with the rigours of its existence, it is more likely to survive competition with others of its species and go on to successfully produce offspring of its own. Maybe it can find a little more food, run a little faster to escape predators, or survive a colder winter. And if this new favourable characteristic is inherited through the genes by its own offspring, it will be passed on to successive descendants. In time, the improvement will spread throughout the breeding population, and the whole species will have become better adapted.  Given enough time, and we know from the fossil record that there have been many millions and tens of millions of years of evolution involving many, many successive generations, a species can continue to become better and better adapted to its environment.

The diagram is a very simple model of how, in principle, evolution by natural selection comes about from random heritable variation, plus competition because parents produce many offspring.

In the one hundred and sixty or so years since the world was first introduced to this idea of evolution by natural selection, everything we have learned about the genetics, ecology and palaeontology of organisms has added to our fundamental understanding of it. In this programme you will meet these different biological aspects of the theory.

The theory of evolution has wider implications than just for biology

The theory of evolution by natural selection is associated in most people’s minds with the diversity of the natural world, and the adaptations of living organisms. But it is actually a more general principle of how all sorts of things can improve over time. New variations can arise by chance, or by deliberate experimental modification. If such a variant proves to be better for its purpose it may be kept, and the older, less effective versions discarded. Millenia before Darwin, farmers knew how to improve the fat content of their pigs by only breeding from the fattest ones, or to increase the yield of their wheat by sowing seed from the ones with the heaviest ears. In fact this artificial selection, as it is called, was Darwin’s main insight for his theory of natural selection. In more recent times, medical science has used the idea to help improve the treatment of diseases. Engineers often deliberately change a structure to see if it works better, and to adopt the new version if it does. The same principle applies to developing artificial intelligence by allowing a computer programme to improve its performance by trial and error, be it playing chess or diagnosing disease.

Challenge yourself…

If you’re interested in exploring this subject further, you might want to take a look at Dr Kemp’s virtual lecture on the same topic:

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Dr Tom Kemp, Emeritus Research Fellow in Biology

I spent many years at Oxford University’s famous Natural History Museum, researching fossils of vertebrate animals, and thinking and writing about evolution. At the same time, I was lucky enough to be Biology Tutor at St John’s College, where I grew to love teaching these subjects. Even though I am supposed to be retired now, I still write and teach, and it is a privilege to be involved in helping you come to understand how important evolution is to understanding much about the world we live in.