Der Fluss des Lebens

What is Evolution?..........

Evolution is a general sense means cumulative change. so we can consider the evolution of galaxies, languages, motor cars and many other categories. Biological evolution (sometimes referred to as organic evoltuion), however, means change in the characteristics of descendent populations of organisms. Theories of biological evolution set out to explain diversity amongst organisms, the origin and history of that diversity, and the natural processes by which it ha developed and by which it is sustained.

Central to our current theory of biological evolution is the idea that species of organisms originate as modified descendants of other species. The scientists that study evolution today, perceive all living organisms on Earth to ultimately share a common ancestry in the simple organisms that arose thousands of millions of years ago.

From simple beginnings, evolution has produced some extremely complicated organisms.  However, there are many simple organisms living today. Therefore, biological evolution cannot be viewed just as a process in which new, more sophisticated organisms replace older, simpler ones.

With regard to evolution, the most important feature of living organisms,is their capacity to reproduce. It is stated that life's continuity is preserved by descent. Some forms of biological reproduction can almost produce exact copies, however, most organisms reproduce sexually. This is a process that generates offspring and so allows the descent of one generation from another, but with modification. Darwin actually defined evolution as 'descent with modification'.  

What is Evolution?.............

The process by which leads to the origin of new species and how in turn evolution of new species has given rise to the sheer amount of diversity and complexity of living organisms today, are questions that can be answered by further investigation.

For instance the foundations of modern evolutionary thinking were laid by Charles Darwin and Alfred Wallace (for more information on these two important biologists and others, please got to the 'Biologists of Importance' pages that are within this section). Their joint communications on the theory of evolution by natural selection presented in 1858 and published subsequently in the Journal of the Proceedings of the Linnean Society.

In November 1859, Darwin published the famous book The Origin of Species. Darwin succinctly suimmarised the theory of evolution in his Introduction to The Origin of Species. The argument he gave, has four basic premises:

• More individuals are produced than can survive
• There is a struggle for existence, because of the disparity between the number of individuals produced through repoduction and the actual number that ca survive
• Individuals sho variation. No two indivduals are exactly the same. Those with advantageous features have a greater chance of survival in the natural struggle (natural selection)
• As selected will tend to produce offspring similar to themselves (the principle of inheritance), these varieties will become more abundant through later generations

What is Evolution?..............

Selected varieties may therefore increase in frequency within a population, altering its character over time. Darwin went onto argue that these favoured varieties, could go on to eventually become separate species. Varieties that were able to exploit new aspects of the environment would therefore, not be constrained to the same limitations as other members of the population (such as food). Natural selection would favour the increasing divergence of such varieties. These varieties would eventually supplant the intermediate forms and would become a distinct species. As these new species would arise, others in turn would become extinct, through competition. This (in its basic sense) is the central argument of The Origin of Species.

This theory is so important, due to the fact it was the first evolutionary model that clearly separated the existenceof variation from the direction of evolutionary change. There were evolutionary theories previous to this one, however they tended to assume that variation occurred due to an organism's needs or perhaps because of some innate tendency towards complexity. The Darwin-Wallace theory was never satisfactorily explained, but it could be thought of as due to perfectly natural processes. The way in which natural selection could be moulded into the course of evolution, was supported by the argument that has been given on the is page, and the principle could be tested.

As well as suggesting what the mechanism of evolution could be, The Origin of Species also gave many examples as evidence that evolution had occurred. The book swayed consensus of biologists and palaeontologists, in favour of evolution, and modern ideas on evolutionary mechanisms stem from it.

Text: Skelton, P. 1993. Evolution: A biological and palaeontological approach, The Open University, pp1-4