Throughout the history of evolutionary science there has existed debate as to if, and how it occurs. Even today controversy reins and new theories of the possible mechanisms driving evolution are emerging, alongside the older religious beliefs. This article provides a concise review of some of the major view points of past and present evolutionary hypotheses.
Lamarck and Evolution
Before Darwin, one of the pioneers of evolution was the French botanist Jean-Baptiste Lamarck. He observed that evolution occurred, but was unable to back up his hypothesis with evidence of a mechanism. Lamarck suggested that evolution occurred by transformation. The most common example of transformation is that of a giraffe’s neck. Lamarck believed that giraffes originally had short necks, and were unable to reach the fruit high up in trees, and therefore within one generation grew their neck and passed this on to offspring in their genes. However the mechanism he used to explain evolution was not adequate and allowed for other schools of thought to cast doubt on his theory, which is no longer regarded as correct in modern science.
Darwin
Charles Darwin, the father of modern evolutionary theory was most famous for his theory of natural selection. Natural selection is believed to be one of the main mechanisms by which evolution takes place.
The term ‘survival of the fittest’ was later coined, and describes how the ‘fittest’ individual is the most likely to survive long enough to reproduce, because the genetic characteristics it possesses are desirable for the particular environment. Fitness is a very difficult factor to measure, so most commonly it is defined by how many offspring an individual produces in its life time.
The famous voyage of The Beagle allowed Darwin to travel to isolated islands where natural selection was occurring. One such area was the islands of The Galapagos from which Darwin later drew the example of finch beak morphology. The finches of the Galapagos had been separated by geography and so had evolved separately by natural selection under different selection pressures, such as food type. This separation caused reproductive isolation of the finch groups, meaning they were no longer successfully able to reproduce, and so are now considered to be different species.
Darwin’s renowned book, The Origin of Species by Means of Natural Selection was received with varied response by the socially conservative and religious society in which it was written; even so the work was generally accepted. Darwin explained evolution as ‘descent with modification’ along vertical gene transfer, illustrated by The Tree of Life.
Neo-Darwinism
Neo-Darwinism is a school of scientific thought that follows the theory of modern synthesis. Modern synthesis is a development in modern evolutionary studies that originated in the mid-19th century. It links different areas of genetic and evolutionary study carried out by prominent scientists previously unrecognised as contributing in this area, for example Mendel, and R.A.Fisher. Some of the areas that now help to support the theory of natural selection as a mechanism for evolution include botany, mimicry, and sexual selection studies.
Horizontal-gene transfer
Current evolutionary science is now suggesting an alternative theory to Darwin’s ‘descent with modification’. This theory is still in the early stages of investigation and there are many assumptions which accompany it.
The theory supplants Darwin’s tree and replaces it with a web like structure, where genes not only travel from parent to offspring within a species, but also across species barriers. The most convincing example of this is the evolution of the eukaryotic cell, called endosymbiosis. Endosymbiosis is a theory that chloroplasts in plant cells and mitochondria in animal cells were once free living, prokaryotic organisms. As seen today these organelles have a double membranous structure, supporting the theory that these structures were engulfed by another prokaryotic cell.
Horizontal gene transfer was initially believed to mostly occur only in the passing of antibiotic resistance in viruses, but it is now believed that high levels of exchange take place across diverse species boundaries.
Creationism
Creationists do not believe in the occurrence of evolution, but that a powerful being created the earth and its different species. Often the chapter of Genesis in the bible is taken in a literal sense, that a god created the world in seven days. Creationism is not based on scientific principles but is another view point which is held by some.
Intelligent Design
Linked to Creationism is the theory of Intelligent Design, illustrated by the watchmaker analogy, it is a belief that all living things are designed by an intelligent being, and design is so perfect and specialised that it cannot have occurred by chance mutations. This was publicly critiqued by Richard Dawkins in his work The Blind Watchmaker. Like Creationism there is no science behind the belief of intelligent design: its roots are based religious faith rather than fact.
References
Lawton, Graham. 29 Jan 2009 "Uprooting Darwin's Tree" New Scientist
Pollock, J. W. 1984. Evolutionary Theory Paths into the Future. John Wiley and Sons Ltd.
Tattersall, Ian. 2002. The Monkey in the Mirror. Oxford University Press
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