Darwin founded his Theory of Descent with Modification through Natural Selection by likening Natural Selection to artificial selection by Man of domesticated animals.
Charles Darwin knew a lot about artificial selection from his pigeon fancying hobby –which was more of a home-based experiment than a simple hobby. Man has produced many varieties of domesticated animals and cultivated plants, often with quite stunning traits. In Chapter 1 of The Origin of Species Darwin describes artificial selection as nature providing variations in traits and Man adding them up in certain directions useful to him to produce an accumulative selection. The importance consists in the great effect produced by the accumulation in one direction, during successive generations of slight differences (Origin, page 38).
The domestic breeds of pigeon – pouters, fantails, runts, barbs, dragons, carriers and tumblers are all descended from one wild species –the rock pigeon. Darwin thought that the different breeds differed so extraordinarily in length and form of beak that they would have been ranked as distinct genera had they been found in the wild.
The breeder of domesticated animals produces the desired trait over many generations of breeding by selecting offspring that carry the desired trait to some degree and allowing these favoured individuals to survive and breed, while disposing of the remaining offspring.
In the wild, in order to survive, organisms must be adapted to their environment. Darwin showed by careful analysis of many facts that natural populations have many variations in physical traits and individuals carrying advantageous traits that afford them greater adaptation survive and leave more offspring than less adapted individuals. Darwin theorized that the continuing natural selection of slight variations in traits carried by individuals brings about the modification of these traits, and the continuing process leads to speciation.
Species and varieties
In the first two chapters of The Origin of Species Darwin discusses the difficulties that naturalists had of deciding whether closely related forms were distinct species or varieties of the same species. He shows that each naturalist made a different classification with some naturalists ranking forms as distinct species and others ranking the same forms as varieties or geographical races of the same species.
Darwin concluded that no clear line of demarcation can be drawn between species and sub-species. Species can be thought of as strongly marked and permanent varieties (Origin page 59). This leads to the conclusion that species could not have been separately created.
Darwin supports this conclusion by information on hybridism. The degrees of fertility between plants and animals of different varieties, species and genera do not show a definite demarcation.
From the amassing of observations Darwin concludes that allied species are co-descendants from common stocks; species belonging to a genus are descended from one species that has been modified through Natural Selection. He writes that the view that most naturalists at the time entertained and which he formerly entertained – that each species has been independently created – is erroneous.
“I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species.” (Origin page 15).
Later in The Origin of Species Darwin expands this view of common descent to families and orders, and finally to class level.
To summarize, Darwin defined Natural Selection as the preservation of favourable variations and the rejection of injurious variations. Darwin established his Theory of Common Descent mainly to class level. Thus, Natural Selection would bring about the modification of structures and other traits such that all the species belonging to one class would have evolved from a common ancestor. (Common descent at class level will be discussed further in Chapter 6).
The modification of traits is the first strand of evolutionary thought in The Origin of Species.