On the voyage of the Beagle (1831-1836) Darwin collected and described thousands of animals and plants. In South America he observed the adaptations of organisms to a variety of habitat from jungle to grassland to mountain habitats. In the temperate regions the species resembled more closely the species of the tropical regions of South America rather than the corresponding species of the temperate regions of Europe. For example, in the grasslands of Argentina there are no rabbits, however, there are rodents that resemble rabbits; these rodents are unrelated to European rabbits but are similar to other rodents in South America. Moreover, the fossils in South America are dissimilar to European fossils but have similarities with extant (i.e. currently living) plants and animals in South America.

Darwin was particularly intrigued by the finches on the islands of Galapagos which are located approximately 500 miles from the mainland of South America. These finches, although unique to these islands, were clearly related to mainland species. There were 14 different species or genera of Galapagos Finch and their bills were adapted for particular diets. Darwin amassed these and other data including observations on variability in domestic animals (for example, dogs) which had been brought about by generations of selective breeding.

As well as drawing on his own observations, Darwin drew from the work of Linnaeus, Cuvier, Hutton, Lyell, Malthus and Lamarck. In the hierarchial classificatory system of Linnaeus there is a tacit acknowledgement of relatedness, for example, species belonging to one genus have more in common with each other than they do with species belonging to another genus. Linnaeus was a creationist — as evidenced by his egotistical proclamation "God creates, Linnaeus arranges". Cuvier, also a creationist, was a comparative morphologist (he described the similarity/dissimilarity in anatomy of diverse animals). Cuvier founded the science of paleontology and described the differences between the fossil flora and fauna in different strata of rock: he observed that the more recent strata had fossils that more closely resembled extant organisms. Cuvier believed that the discontinuities between fossils in different strata were brought about by catastrophes such as floods which caused the extinction of many species living at a particular time. This interpretation of earth's history is termed catastrophism and was also held by many contemporary geologists. By contrast, Hutton and subsequently Lyell held that geological processes are slow and subtle but that over prolonged periods of time (millions of years) these can lead to major changes; implicit in this viewpoint is an age for the earth radically different from the 6,000 years of the biblical creationists.

Other key influences on Darwin were Malthus who had concluded that war and famine were inevitable as the human population grew more rapidly than available resources, and Lamarck who had proposed a theory of evolution based on a continuous process of gradual modification due to acquired characteristics.

Both Darwin and Wallace brought together a multitude of facts including the geographical distribution of organisms, comparative morphology of living organisms and their fossil precursors. They postulated that long-term environmental changes including movement of land masses and changes in climate could have served in the process of natural selection over many generations with the result that diverse species arose from ancestral types. Darwin termed this "descent with modification" (the term "evolution" was introduced later, as was the tautology "survival of the fittest"). Darwin's ideas can be summarized in his own words from The Origin of Species:

As many more individuals of each species are born than can possibly survive, and as consequently there is a frequently recurring struggle for existence, it follows that any being, if it vary in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of survival and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form. [Introduction, p. 5]

In assessing the contribution of Darwin and Wallace it should be noted that the key role in heredity of the nucleus, chromosomes and DNA were not demonstrated until 1892, 1903, 1943, respectively. Also, the Darwin-Wallace theory of evolution was developed without an appreciation of Mendel's work on inheritance (ironically, Mendel had written to Darwin with his ideas but Darwin overlooked their significance).

Subsequent studies have drawn closely together the fields of evolution, genetics and molecular biology. For example, the inherited disease of sickle-cell anemia is caused by a minor change in DNA which causes a minor change in the hemoglobin protein of red blood cells. Individuals with two sickle-cell genes suffer sickness and may die, but those individuals with one sickle-cell gene have a greater resistance to malaria. Thus it can be seen that this gene may have a survival advantage for Africans in malarial areas. Not all changes in DNA produce such dramatic effects in proteins and in the individuals who possess those proteins. Some changes in DNA cause only minor effects or may have no tangible effects on the organism. By systematically comparing the DNA of different organisms it is possible to determine the degree of similarity/dissimilarity between organisms and thus determine phylogenetic (i.e., evolutionary) relationships between them. For example, in the case of human phylogeny both skeletal structure (comparative morphology) and gene structure (molecular biology) indicate that humans are more closely related to chimpanzees than to New World monkeys.

References

Charles Darwin. On the Origin of Species. A Facsimile of the First Edition. New York, Atheneum, 1967.


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