Where are we now along the evolutionary path? Have we stopped evolving? And what does it mean if we have? The Next Big Thing explores how the evolution theories have changed and developed over the years.

Evolutionary concepts first appeared in early Greek writings, for example, in the work of Anaximander and Empedocles. Anaximander proposed that animals could be transformed from one kind to another, and Empedocles speculated that they could be made up of various combinations of pre-existing parts. However, evolutionary theories were subsequently prevented from developing and challenging the belief of special creation for some fifteen centuries, due to the restraining influence of the Church.

The growth of scientific observation and experimentation led to some theories of evolution beginning to emerge from the mid 16th Century. Despite the idea of human progress being central to the Enlightenment of the 18th Century, this did not lead to the theory of evolution. Pierre-Louis Moreau de Maupertuis proposed the spontaneous generation and extinction of organisms as part of his theory of origins. However, he advanced no theory of evolution. Georges-Louis Leclerc, a prominent naturalist of the time, explicitly considered, and rejected, the possible descent of several species from a common ancestor. The physician Erasmus Darwin, grandfather of Charles Darwin, offered some evolutionary speculations in his book Zoonomia, but they had no real influence on subsequent theories. The Swedish Botanist, Carolus Linnaeus, who devised the hierarchical system of plant and animal classification that is still in use today in a modernised form, showed an inclination towards the ability of species to mutate, as a result of his observations of variations among species.

In the early 19th Century, the French naturalist Jean-Baptiste Lamarck presented a clearly stated evolutionary theory. This theory, later known as 'The Inheritance of Acquired Characteristics' was ridiculed at the time and thoroughly disproved in the 20th Century, although it did contribute to the gradual acceptance of biological evolution as well as stimulating later studies.

The founder of the modern theory of evolution was Charles Darwin. In 1859 the first edition of his On The Origin of The Species was published and it sold out in one day. The naturalist Alfred Wallace had also hit upon the idea of natural selection independently, however, his view differed from Darwin’s, most notably in that he did not think that natural selection was sufficient to account for the origin of man, but divine intervention had been required.

Herbert Spencer, a philosopher, was also an influence in evolutionary theory during the latter part of the 19th and early 20th Centuries. He popularised a number of slogans, including "survival of the fittest" which was taken up by Darwin in later editions of Origin of Species. Spencer’s ideas damaged proper understandings of natural selection and Darwin often attacked him for his "fundamental generalisations".

The main weakness of Darwin’s evolutionary theory lay in gaps in the explanations of the mechanism of evolution and the origin of species. The concept of natural selection is that inheritable variations among the individuals arise in nature and that some variations prove advantageous under prevailing conditions, in that they enable the organism to leave more surviving offspring. Darwin did not understand how these variations initially arise or are transmitted to offspring and subsequent generations.

The missing link in Darwin’s argument was provided by Mendelian genetics. Gregor Mendel was an Augustinian monk who carried out a long series of experiments with peas in the garden of his monastery. His paper, published in 1866, formulated the fundamental principles of the theory of heredity, and it is still current today. Darwin did not know about these discoveries and they did not become generally known until 1900 when natural selection and Mendelian genetics were simultaneously rediscovered by scientists.

In the 1880s, German biologist August Weismann published his germ-plasm theory defending natural selection. His ideas became known as neo-Darwinism. The rediscovery of Mendel’s theory of heredity in 1900 led to an emphasis on the role of heredity on evolution. Hugo de Vries proposed a new theory of evolution known as mutationism, which discounted natural selection, and was heavily opposed by naturalists, especially biometricians. Biometricians, led by Karl Pearson, defended natural selection.

In the 1920s and 30s, theoretical geneticists including R.A. Fisher and J.B.S. Haldane in Britain and Sewall Wright in the United States, used mathematical arguments to support natural selection and contributed to the downfall of mutationism. They also provided a theoretical framework for the integration of genetics into Darwin's theory of natural selection.

In 1937 Theodosius Dobzhansky published Genetics and the Origin of Species, which combined Darwinian natural selection and Mendelian genetics. This led naturalists and experimental biologists to understand the process as one of genetic change in populations. This stimulated interest in evolutionary studies and scientists from a variety of biological fields contributed to the theories, most notably the zoologists Ernst Mayr and Sir Julian Huxley, the palaeontologist George G. Simpson and the botanist George Ledyard Stebbins.

By 1950 acceptance of Darwin's theory of evolution by natural selection was universal among biologists and it is now widely accepted in society and taught within schools. Some creationists still oppose it.

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.

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.

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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