The Theory of Evolution

Evolution explained clearly

Charles Darwin, late 1830s

The theory of evolution says that if some members of a group of organisms - plants or animals - are by chance better equipped to cope with a particular hardship, and then go on to reproduce; then their offspring will on average also be better at surviving that hardship, and the following generations even more so. The population as a whole becomes better and better adapted to thrive in a given situation or niche. Nearby populations may find themselves facing different pressures, and will adapt in a different direction, eventually leading to the formation of two or more species where once there was one.

Charles Darwin

In November 1859 Charles Darwin published his famous book, which is usually known as 'The Origin of Species' - its full title was 'On the Origin of Species by means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.'

Darwin had been employed in the 1830's as a geologist and naturalist on board the HMS Beagle, on a five-year expedition from Plymouth, England across the Atlantic, around South America, across the Pacific to Australasia, around the southern tip of Africa, and back to England.

The book was written some time after the voyage, and describes Darwin's thoughts about how the many different types of animals and plants he had seen on his travels could have come into existence. A few other naturalists at the time had similar ideas, in particular Alfred Russel Wallace, but Darwin was the first to publish, and his book was highly successful.

On his journeys, Darwin had found many intriguing fossils of creatures which were clearly related to living animals, but different from anything that currently walked the earth. He had also spent a lot of time in the Galapagos Islands, off the coast of Ecuador, where he had seen how each island had its own unique character, and that each local population of birds and animals - in particular tortoises and finches - was perfectly adapted to take advantage of its own island's qualities. At the time he did not fully realise the significance of these observations but on his return home, whilst writing up his journal of the voyage, Darwin began to pull together the strands of evidence for what was to become the Theory of Evolution by Natural Selection.

What is evolution?

Evolution really just means 'changing over time', but in the Darwinian sense it refers to the gradual change in a population of living things, occurring over hundreds to millions of years and many generations. For this to happen, a number of conditions have to be true:

• There must be some natural variation in the population. For example, some individuals are taller than average, some shorter.
• Those differences must tend to be inherited by offspring - so taller parents will probably produce taller children.
• There must be a certain level of competition for some resource which is needed for survival and producing offspring - perhaps food is scarce, or shelter is hard to find.
• There must be a relationship between inherited characteristics and success in competing for these resources - maybe taller individuals can reach more food and so better survive into adulthood, or maybe shorter individuals can find more shelter from predators.

With these conditions in place, it is not hard to see how the average characteristics of a group of organisms will change over time. Let's imagine that our creatures are ground-living plant-eaters, who feed on the leaves growing on a particular kind of tree. Individuals with longer necks will be able to reach more leaves, and so if times are hard and food is scarce, the longer-necked are more likely to survive and reproduce, passing on to their offspring a tendency to have longer necks.

So the next generation will have a higher proportion of  long-necked individuals. Some will have necks even longer than their parents' necks, and these individuals will be even more likely to reproduce the next time there is a food shortage - and so over time and many generations, necks will on average grow longer. This process will stop once a counter-balancing disadvantage to long necks appears - perhaps the difficulty of circulating blood to the brain, or the fact that a larger animal simply takes longer to grow and so reproduces more slowly.

What is the evidence?

1. Fossils

Fossils occur when an animal or plant dies and its remains are covered with sand or some similar mineral. Over millennia, the sand hardens, retaining the shape of the enclosed organism (which may be replaced by other minerals dissolved in water). In principle, the deeper underground a fossil is found the older it is, since this kind of rock forms by adding to it from above. In practice, the layers of rock can become tilted and even folded by geological processes - but careful matching of rock type and characteristics can show the sequence of formation of the various layers. When this is done, the fossils found in older rock often look quite different from modern creatures, but in intermediate layers fossils can be found which are closer to modern forms whilst still being similar to the older ones. It is possible to arrange fossils in a sequence from old to modern showing a gradual change in form over time. This can be used to show how humans and chimpanzees, for example, share a common ancestor about 5 million years ago - this creature was neither human nor chimp. Different sub-populations of its species diverged, specialising in different skills and features, leading on the one hand to humans with their upright stance, large brains, and complex speech - and on the other hand to chimpanzees with their superior strength and adaptation to life in the forest.

2. Physiology

Evolution can only make small changes from one generation to the next. It is exceedingly unlikely for a creature to be born with, say, an extra heart, since there would need to be a massive number of simultaneous changes to ensure that the new heart was connected to the blood supply, received correct signals from the brain, didn't get in the way of other organs, etc. So changes have to evolve gradually.

In modern fish, and in the four legged common ancestors of fish and mammals, there is a nerve (the recurrent laryngeal nerve) which connects the brain to the larynx in the throat. In the ancestral creatures, the direct route for this nerve took it past the heart to the gills. As body shape changed over millennia, the gills moved up the neck (and disappeared in mammals), whilst the neck became longer, and the heart moved lower down into the chest cavity. However the nerve still passes around the heart, since each tiny change in body shape only requires the relatively simple process of the nerve becoming a little longer rather than completely repositioning it. The result is that in animals such as the giraffe, this nerve starts in the brain, loops all the way down the neck to the heart, then travels all the way back up to the larynx - it is nearly twice as long as it need be.

3. Observation

When a population of creatures is stressed in some way, those individuals which are best able to cope with the stress will reproduce more successfully, and pass on the coping strategy to their offspring. If a colony of bacteria is exposed to low levels of antibiotic then many of the bacteria will die. Those that survive are the ones best able to tolerate the antibiotic, and this tolerance is passed on to the next generation. These will also be partly killed by the antibiotic, although in smaller numbers, and the strongest of this generation will contribute more to the next - until after many generations a population emerges which is relatively unaffected by the antibiotic. This has been observed to happen around the world, with antibiotic resistance evolving in many different bacteria, frequently with serious consequences for human health. Incidentally this is why antibiotics should not be prescribed unless necessary, to avoid prompting evolution of resistance - and it is why a course of antibiotics should always be completed even if the patient feels better, to ensure that all the bacteria are killed including (especially) those that have survived the initial part of the treatment.

Why is it called a 'theory'?

A theory in the scientific sense is a much stronger concept than the word might imply. In everyday language, 'having a theory about something' often just means having a hunch that an idea is correct. To a scientist this is closer to a 'hypothesis'. A scientific theory on the other hand is a complete framework of thoughts, ideas and explanations which make sense of a particular phenomenon, and allow predictions to be made and experiments to be performed. Each time a prediction is made and found to agree with observed reality, the theory gains strength. If an experiment disagrees with a prediction of the theory then the theory needs to be revised, extended, or if necessary abandoned. The theory of evolution is a robust theory which has explained many puzzling features of the natural world.

What are the alternatives?

Prior to Darwin, many people believed that each life form had been somehow placed in whichever environment best suited it, and that nothing much changed. Fossil evidence disproved this, showing that life had been very different in the past even though the environment had often been quite similar. Darwin also observed that modern environments that were similar but widely separated geographically often had very different species present.

Some naturalists had suggested that characteristics gained over the lifetime of an individual could be passed to its offspring - for example, they believed that if an individual animal made a lot of use of, say, its left arm, so that the muscles grew particularly strong, then its children would also be stronger in their left arms. This idea was proposed as an explanation for the necks of giraffes, which were supposedly elongated when they stretched to reach high-up leaves. The theory, known as Lamarckism, was largely discredited and replaced by genetic theory, although recently some aspects of Lamarckism have been revived in epigenetic theory.

Perhaps the most popular alternative explanation for the variety of modern lifeforms is 'Intelligent Design.' This idea suggests that life is simply too complex to have evolved through random changes, and proposes that some external designer directed the formation of living things. Supporters of Intelligent Design ignore the facts: a) that evolution has had nearly 4 billion years to work, b) that the overall process is not random but guided by natural selection, and c) that each time an organism reproduces it is an opportunity for evolution, so countless 'evolutionary experiments' are happening simultaneously all the time. Consider that the chance of any individual winning the lottery is tiny, but there is nearly always a winner somewhere. It is a numbers game.

The idea of Intelligent Design requires an unknown external entity to do the designing - it is not a disprovable idea and so does not qualify as a scientific theory. In fact many aspects of life seem rather unintelligently designed because of the slow step-by-step nature of evolution.

Left: mammal eye   Right: squid eye
1:light sensors 2:nerves 3:optic nerve 4:blind spot

The recurrent laryngeal nerve is one example, but there are many others - for example the human eye has a network of blood vessels supplying the light-sensing retina, but the blood vessels are actually in front of the retina and the brain has to perform all sorts of tricks to prevent us seeing this network as a kind of shadow. There is also a 'blind spot' where the retina's nerve signals (which also pass in front of the light sensing cells) collect together and pass through the back of the eye. In the eyes of squid, this position is reversed - the blood vessels and nerves are all behind the light-sensing cells, so there is no shadow cast, and no blind-spot where the optic nerve emerges. If a designer worked this out for squid, why not do the same thing for mammals?

How did life start?

The theory of Evolution does not address the question of how life started, only how it changed once it had gained a foothold. Most scientists believe that a self-reproducing chemical reaction occurred by chance in a particular primitive environment, and once that reaction could occur it could also change and evolve, kick-starting the whole tree of life. Some believe that primitive organisms arrived on the ancient Earth from space, having been formed elsewhere in the cosmos - but this only poses further questions about the origin of those organisms. Others still might believe that a supernatural entity created the first life forms and the rules by which they could change and evolve. That is outside the realm of science.

However life started, it seems clear that since that time it has been and still is constantly evolving into new forms, driven by competition for resources, random variation, and the inheritability of characteristics from one generation to the next