The Evidence for the Theory of Evolution by Natural Selection

Human Evolution

It's Only a Theory

"But it's only a theory"

This is a frequent rejoinder made by people who do not accept or approve of the Theory of Evolution.

It is also often heard about the Big Bang Theory.

Both of these Theories evoke fear and hatred in people who have a belief system that precludes the concept of living things evolving or the idea of a very old Universe. The phrase "it's only a theory" is not usually heard where belief systems are not threatened. I have never heard anyone say "gravity, it's only a theory" before jumping out of a window.

In common usage a theory is something vague like a guess, a stab at an explanation.

This is not the scientific definition of what a Theory is. Saying that something is "only a theory" shows that person not to understand how science works and what a scientist means by the word Theory.

So, what is a Theory in science?

Science begins by making Observations and Measurements. Without these there is no science. Here are three example observations.

Observations require explanations. Scientists will make a guess or a number of guesses at explaining observations. A guess is called a Hypothesis.

A scientific Hypothesis is an explanation for an observation that can be tested. If it cannot be tested it is not a scientific Hypothesis.

Here are some Hypotheses to explain the above observations:

Each of the above is a Hypothesis as it provides an explanation and it can be tested. A Hypothesis does not have to be correct but it must be testable. Indeed two of the above Hypotheses are incorrect.

The scientific method requires that all Hypotheses must be tested. If the Hypothesis fails it must be discarded or amended. This is the most important aspect of the scientific method. Because a Hypothesis is an untested guess, it is quite proper and correct to say "yes, but it's only a hypothesis".

If a Hypothesis passes a test, it remains a Hypothesis and is tested again. If it fails any test it is discarded, or at least amended.

Only when a Hypothesis passes several tests and resists all efforts to break it, does it then become elevated to the status of a Theory.

A scientific Theory is therefore an explanation for a series of observations that has passed every test thrown at it and has not failed any.

For scientists the Theories of Gravity, Evolution, Relativity, Plate Tectonics and Quantum Mechanics are all accepted because the evidence in support of them is overwhelming and no observation or measurements or discovery has been made that counts against them.

This point is extremely important. People who deny evolution, or who do not understand how science works, will often use the "only a theory" phrase as a smear against the Theory. The correct scientific way to debate the merits of the Theory of Evolution is to present evidence and examples either for or against.

A good Theory should do three things:

All of the above Theories do these three things and do them well.


Evolution by Natural Selection

The Theory of Evolution states that all of the millions of species of life that exist on the Earth have a common ancestor. In the long period that life has existed on this planet, life has undergone changes. These changes have caused single species to slowly evolve into new and different species. If two populations of the same species are separated, the changes they undergo may go in different directions. This will cause them to split into two or more species. Over the course of millions of generations, this has given us the diversity of life now found on Earth.

This process of change is called Evolution.

The theory states that all life on Earth has common descent.

Some species evolve rapidly. Others evolve very slowly. Most species that have ever existed on Earth have died off, or become extinct.

Life reproduces itself by many routes, some sexual, most asexual. In both cases, genetic material within the organism has to replicate itself. The genetic material consists of long and complex molecules with hundreds of thousands of atoms arranged in a very specific way. This genetic material is responsible for the manufacture of the chemicals of life and for the series of instructions that determine how the organism will develop.

A group of organisms of the same type (species) will have natural variations among themselves. Some of them will have traits that make them better adapted to survive and reproduce. In a multi-celled organism, it might be a quicker response to danger, being a better hunter or being better able to extract nutrients from soil.

The organisms that do manage to survive and reproduce will pass on their advantageous genes to their offspring.

During the process of replication, changes can occur to the genetic material. These changes are random and unpredictable. They have many causes including errors in the replication process, toxins or natural radioactivity. These genetic changes are called mutations.

Most of the time these mutations do not affect the organism or its ability to reproduce. Some mutations are detrimental to the organism's chances of survival. These "bad" mutations are weeded out of the gene pool because they hamper the organism's chances of surviving and therefore reproducing. If an organism does not survive, it cannot reproduce.

A few mutations give the organism an advantage over its fellows. This advantage allows the organism to survive and to reproduce. These "good" mutations are likely to spread in the gene pool because they are more likely to survive.

Although the mutations themselves are random, the process driving survival is not random.

Any advantage that an organism has will cause it to out-complete its fellows in the struggle to reproduce. This process is commonly known as Survival of the Fittest. The scientific name is Natural Selection.

The full name of the theory is Evolution by Natural Selection.

Evolution is the changes to organisms - Natural Selection is the process that causes the changes to be passed on - OR NOT - to the next generation.

Evolutionary changes are very slow. In larger organisms, it takes long periods of time, many generations, for changes to become apparent.

It is worth noting here that there are two other types of selection that can operate on life.

Sexual Selection is gender based and produces traits like display plumages. Artificial Selection is controlled by humans and has produced domesticated animals and plants throughout history.

Alternatives to the Theory of Evolution

Opposition to the Theory of Evolution is often not based on science but on ideological reasons. Two alternatives to the Theory of Evolution have been postulated.

The first is Creationism, or Creation.

This states that all species were created in one go in the forms that we find them. Furthermore species do not change or evolve. The creation took place in the recent past, often in the past few thousand years. The inspiration for Creation is usually an interpretation of a religious text. Supporters of Creation are called Creationists.

The second alternative to Evolution is the more subtle Intelligent Design.

This accepts evolution but states that some structures (like the human brain and the mammalian eye) are too complicated to have evolved without the help and intervention of an intelligent designer.

This discussion is not about God. It simply looks at whether the diversity of life on Earth can be explained using scientific principles.

The Evidence for Evolution

The evidence for the Theory of Evolution by Natural Selection is overwhelming. It consists of two types of evidence.

The first type is evidence that Evolution (change) has occurred and / or is occurring. The second type of evidence is of Natural Selection driving the changes.

In this essay, we will look at information obtained from the inter-relationships of organisms. We will study the DNA and genetic record, a field that has grown spectacularly in recent years but was unknown during the time of Charles Darwin. We will look at evidence from the development of embryos.

Of course, we will examine the fossil record. This is another area where recent spectacular finds have filled in many gaps and produced fascinating evolutionary series.

We will see how evolution explains the geographical distribution of life on Earth. The study of vestigial structures tells us about ancestor species. We will look at some atavisms, throwback features that sometimes appear. Another interesting area we will study is imperfections in life caused by the fact that creatures have evolved rather than having been designed from scratch.

Finally we will look at some incredible recent experiments in the laboratory and even see examples of evolution in the wild.

Supporters of Creation and Intelligent Design often concentrate on refuting the fossil evidence for Evolution and attempt to imply this is all the evidence there is and it is full of gaps and uncertainty. However This essay presents nine different strands of evidence, fossils being just one of them. Even without the fossil record, the evidence for evolution would still be overwhelming.

The Theory of Evolution explains the many and varied observations of nature in a consistent way. In addition, like the good scientific theory that it is, it makes predictions that can be tested. Many of its past predictions have been verified. It also makes negative predictions; in other words, what you should not observe.

How form and features between animals indicate a family tree relationship. This is backed up by studies of genes (including dead or inactive genes shared between different species). The development of animal embryos indicates a shared past and also confirms the family tree relationships between animals.

A look at undated rock layers and shows that fossils occur in a distinct sequence in the rock, a sequence that makes evolutionary sense and confirms the family tree relationships in form, genetics and embryology. There are examples of transitional forms and also evolutionary sequences involving the horse and the whale. There is a section on dating artifacts and rocks.

The distribution of life around the world. Convergent evolution between continents - how the same environment produces unrelated plants and animals with the same features and shapes. The peculiar and unique life forms found on islands and isolated continents. Plate tectonics and evolution.

Life is imperfect as a result of the compromises that evolution has to make. Animals have vestige structures that are no longer used but are present as a result of having evolved from creatures that used them. Examples are wings and eyes. Atavisms are throwbacks like human children born with tails when genes for making a tail are accidently switched back on. Many structures appear "badly designed" because they are not designed but have evolved from pre-existing structures.

Evolution produces immunity to antibiotics and new strains of diseases. Laboratory experiments and mathematical models show evolution in action and capable of forming complex structures. Two examples of evolution in the present day: a lizard in the Mediterranean and gulls living around the north polar regions.

Summary and Conclusions

In the past humans required supernatural explanations for (among other things) the motions of the planets, lightning storms and disease. Slowly as knowledge has grown, natural explanations were developed, verified and accepted.

The Theory of Evolution explains the existence, distribution and features of life on this planet. It does so in a logical, clear and concise way. It explains a number of diverse observations ranging from the distribution of species around the Earth to the genetic record found in all organisms.

It is a theory that makes testable predictions.

As a proper scientific theory it can be proved wrong. There exist a number of potential observations and discoveries that could disprove evolution. As J B S Haldene quipped, evolution could be disproved by the discovery of "fossil rabbits in the Pre-Cambrian". In fact any fossil out of evolutionary sequence would seriously damage the Theory of Evolution.

The discovery of a feature or trait that could not be explained by evolution would also be an argument against the Theory.

In fact, every new discovery and experiment strengthens the Theory.

The Theory of Evolution explains the family tree of life. It explains why mammals have similar features to each other. It explains why the genetic code in all living creatures uses the same coding sequences. It explains why dead genes appear in different species even though they are inactive.

The Theory of Evolution explains why embryos go through a mini-evolution cycle before developing into a foetus. It explains the fossil record which, itself, only makes sense if evolution has actually occurred. The geographical distribution of life can only be understood from an evolutionary standpoint. Vestiges and atavisms confirm evolution by showing common descent. The evidence of sub-optimal (bad) design is perfectly explained by the Theory of Evolution but makes no sense to a hypothesis based on intelligent design.

Finally, evolution can be observed in the laboratory and in the wild. It occurs in the present and has been documented in the recent and historical past. Evolution may even be a threat to humanity as microbes become even more resistant to the most powerful antibiotics we can design.

Some evolution-deniers also deny the Universe is more than a few thousand years old.

To hold this view means ignoring or being ignorant of most of modern science.

If the Universe is really only a few thousand years old and evolution never happened and does not occur, it means humans are completely mistaken about large parts of the following sciences.

Science is consistent - many different strands of science point to similar conclusions. Evolution and Plate Tectonics combine perfectly to explain the presence of marsupial fossils in Antarctica, a prediction that was eventually verified.

In science not everything is known but that very lack of knowledge prompts further research. In contrast Creationists point to gaps in scientific knowledge and attempt to use that lack of understanding itself as evidence for Creation or Intelligent Design. This is similar to people who believe UFOs are extra-terrestrial. They will point to an unexplained observation and use the lack of explanation as proof of UFOs. This is not science but classic pseudo-science.

Science involves debate. Debate is part of science and shows a lively and thriving field of study with new discoveries being made all the time. Creationists use the normal debate in science as "evidence" against evolution.

Science makes testable predictions. When Creationists cannot counter powerful evidence for evolution, they will deny the science, ignore the evidence, or put it down to the whim of a creator. The latter is not a testable prediction, making Creationism unscientific.

The Theory of Evolution has the status of a fact in the scientific community. Many details are still being worked out but no reputable scientist or biologist denies the fact of evolution by natural selection. In fact, most of biology, genetics and medicine makes no sense without an understanding and acceptance of evolution.

Books From Amazon.co.uk and Amazon.com

External Evolution Links

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Evidence of Evolution
Wikipedia page listing the evidence of evolution.

Time Line of Life
A historical time line of life on Earth.