THE ORIGIN OF SPECIES:
The basic tenets of biological evolution.
When Darwin returned to England in 1936 from the voyage around the world, he had the opportunity to carefully study his notes as well as the large collection of specimens he brought back with him. He turned over his collections to various experts in Cambridge and London for identification. The results were fascinating and provided clear evidence about the geographical variation of species. The finches of the Galapagos were particularly interesting. not variation of the same species but different species of good standing and yet somehow they were related to some of the birds of the main land. The conclusion was unavoidable: species are not for ever fixed entities but they change in time according to the needs dictated by their geographical distribution. At this point, the most important question was about the mechanism that is able to produce and direct such changes.
In October 1838, Darwin read Thomas Malthus An Essay on the Principle of Population. Malthus argued that populations grow geometrically, while the food supply can only increase arithmetically. Consequently, population growth is limited by the availability of food. Darwin noted in his Autobiography that under such circumstances of constant competition favourable variations would tend to be preserved, and unfavourable ones to be destroyed . . . The result of this would be the formation of new species. Here, then, I had at last got a theory by which to work. This was the first time in 1838 that darwin had written down his insight about evolution of species by means of natural selection.
By 1842 Darwin wrote a short sketch about his understanding of biological evolution, and in 1844 he wrote a longer version about the same ideas. This 1844 version, which he shared with his old friend, the botanist Hooker, essentially contained all the relevant information about evolution by means of natural selection, and yet, he was hesitant and did not officially publish his work. On June 18, 1858 he received a paper from Alfred Russel Wallace, a naturalist who was working in the tropics of Malay. In this paper Wallace proposed the same ideas as those proposed by Darwin. At this point, his friends Lyell, Hooker, and T.H. Huxley arranged for a joint paper by Darwin and Wallace to be read before the Linnean Society of London on July 1, 1858. The finally the colossal work came to light on November 24, 1859 with the title: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.
A practical way to sum up the basic tenets of biological evolution proposed
by Darwin in the Origin is from the headings of the first 4 chapters.
Chapter 1. Variation under domestication.
Chapter 2. Variation under nature.
Chapter 3. Struggle for existence.
Chapter 4. Natural selection, or the survival of the fittest.
Chapter 15. Recapitulation and conclusion.
Then from chapters 5 to 14 Darwin presents such issues as geological succession and geographical distribution of organic beings together with some corroborative studies from comparative anatomy and embryology. He realized that his conception of the evolutionary process is not without difficulties. These he sums up in chapters 6 and 7. Most of these difficulties and objections are related to the question: What are the sources of variation? Most people around Darwin believed in a system of blending inheritance where the features of the offspring are a mixture of the features of the parents. It is obvious that such system of blending would eventually erode all variation.
Contrary to this view, Darwin found a rich natural variation in all forms of
life, maintained somehow at a high level. It is ironic that one of the answers
to his question on sources of variation was in his library: the monograph of
Gregor Mendel, Experiments in Plant-Hybridization. This monograph
has been published in 1865, years before the final edition of the Origin in
1872. Mendel showed that the factors of inheritance do not mix but
remain distinct and interact with each other in each new generation, and they
are shuffled through independent assortment into ever new combinations by the
mechanism of sexual reproduction. And yet, Darwin, as his many contemporaries,
did not recognize the tremendous significance of Mendels discoveries.
Darwins understanding of the evolutionary process survived the test of time. His basic ideas stand firm and may be summed up as follows:
1. Variation is a universal characteristic of all living things. The extent of this variation is such that no two individuals in a sexually reproducing species are the same. More recently we were able to add some important details to these statements. We know that there are three major sources of variation, the genetic endowment, the constantly changing environmental conditions, and a number of random factors. Their contribution to the observable form of life, to its anatomy, physiology, behavior, and molecular organization is quite complex because the various factors interact constantly all through development and life, and often they are not independent of one another.
2. In their concreteness, living things are to be understood as they live in their social and physical environments. The relationship between living organisms and their environment contains elements of stress due to competition for the same and limited resources.
3. The result of this competitive stress is natural selection, that is, the survival of the fittest. Today this statement is a somewhat toned down from the original yes or no, live of die concept by the expression of differential survival of the variants. This means that natural selection operates on a relative scale in terms of degrees of fitness. Those more suited to life in a given situation will leave more descendants than others, and this will render the whole species more adaptive.
4. We also know today, which was only hinted at by Darwin, that the environment is constantly changing and so does the practical meaning of fitness. In this dynamic relationship appropriate genetic variation is a condition of survival. Without genetic variation there is no response to selection. The sources of genetic variation are primarily the gene and chromosome mutations, but in more practical terms, it is the mechanism of recombination or the reshuffling of gene combinations in each individual event of sexual reproduction, which is the more immediate source of variation.
5. The overall results of these relationships in space and time is the dynamic
process of the evolution of species.
All aspects of the above described evolutionary process, such as variation, competition, and natural selection resulting in gradual evolutionary changes within and between species are open to scientific investigation. They can be verified, measured and compared. The assumption that what we observe as functional today was also functional in the past is a reasonable one. In addition to the gradual process proposed by Darwin and verified by observation today, a number of other explanations have been proposed, including the sudden major changes postulated by some investigators. Unfortunately, such proposals are mostly based on the fossil record which eludes the rigorous demands of the scientific method, and thus, must remain in the world of theory. For that reason, we cannot use the fossil record as evidence of evolution. What we can do is to interpret the fossil record in evolutionary terms, and the most reasonable interpretation is that of gradual changes since that is what we observe today. The same applies to any statement on the Origin of life.