Years ago (and still now in the developing world) people with genetic problems did not survive to reproduce, so those genes died out. As medicine improves we are able to treat more and more of these conditions and the genes responsible for them survive. This section explains what evolution is and how scientists investigate the evolutionary process.

What is evolution?
What is natural selection?
How does natural selection occur?
Role of sexual reproduction in natural selection
Evolution and religion

What is evolution?

Biological evolution is the theory that groups of organisms change with the passage of time, causing their descendants to differ morphologically and physiologically from their ancestors. This process is dependent on the need of organisms to compete for the things which are essential to them individually to survive and reproduce. It is a continuing process that has been going on for millions of years, rather than a finished event, although there is some debate over the degree to which human beings are evolving now.

What is natural selection?

Natural selection is the mechanism that drives evolutionary change. Put simply, it is a process whereby organisms best suited to their environment become the ones most likely to survive and leave descendants. It is also often referred to as "survival of the fittest", which conjures up images of the biggest, strongest individuals being the most successful, but in a biological sense, evolutionary fitness refers to the ability to reproduce in a particular environment as well as survive. Therefore organisms may be the "fittest" because they co-operate with other organisms, rather than competing with them. Charles Darwin, often known as the father of evolution, was the first person to document the operation of natural selection.

Darwin formulated the central argument of this theory from his observations, the existence of variations among animals. He reasoned that variations must occur in nature that are favourable or useful in some way to the organism in its struggle for existence. Favourable variations are ones that increase chances for survival and procreation. Those advantageous variations are preserved and multiplied from generation to generation at the expense of less advantageous ones. The outcome of the process is an organism that is well adapted to its environment.

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How does natural selection occur?

The variations that occur are as a result of changes that are produced at the genetic level. They occur as an organism's genes mutate and/or recombine in different ways during reproduction and are passed on to future generations. Genes are the portions of an organism's DNA that carry the code responsible for building that organism in a very specific way. From generation to generation, molecular mechanisms reshuffle, duplicate, and alter genes in a way that produces genetic variation. This variation is the raw material for evolution. Mutations are 'mistakes' introduced into the genetic material used for reproduction, which can occur naturally, or as a result of something like exposure to radiation. Positive mutations that give some benefit to the organism provide the new material for natural selection to operate on. This could lead to individuals inheriting new characteristics that give them a survival and reproductive advantage in their local environments; these characteristics tend to increase in frequency in the population, while those that are disadvantageous decrease in frequency. Any non-genetic changes that occur during an organism's life span, for example an increase in muscle mass, cannot be passed on to the next generation and are not examples of evolution.

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Role of sexual reproduction in natural selection

Sexual reproduction allows an organism to combine half of its genes with half of another individual's genes, which means new combinations of genes are produced every generation. When eggs and sperm are produced, genetic material is shuffled and recombined in ways that produce new combinations of genes. Sexual reproduction therefore increases genetic variation, which increases the raw material on which natural selection operates. Genetic variation within a species, also known as genetic diversity, increases a species' opportunity for change over successive generations.

Evolution and religion

Like in the early days when evolutionary concepts were first developed, some creationists still strongly disagree with evolutionary theory, believing that it denies the presence of God. However, these days many people from evolutionary biologists to religious leaders, including Pope John Paul II, believe that evolutionary theory does not refute the presence of God. They argue that evolution is the description of a process that governs the development of life on Earth, dealing only with objects, events and processes in the material world. Therefore, it does not say anything one way or another about the creation of the world.

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