Every day this week I will be publishing excerpts from my distilled version of On the Origin of Species. If you never have, or never will read it, hopefully this will give you a sense of what an amazing book it is.
Chapter 1: Variation Under Domestication
Humans eat and use a vast array of domesticated animals and plants with little thought as to how we came to do so, assuming that domestic breeds are composed of uniform individuals. When you look closely, it is striking how much variability there is between individuals within a single breed.
Though we do not know the basic reasons why individuals vary, I would argue that the mechanisms involved affect the reproductive functions of adult organisms. There are numerous examples of apparently thriving species that are completely sterile in captivity, which suggests that the reproductive system is the most susceptible to changes in conditions. Despite identical growing conditions, puppies in the same litter or seedlings from the same fruit can differ greatly from one another.
Once varied characteristics arise, they are transmitted between generations. We do not know how this inheritance occurs, but it is undoubtedly central to how domestic breeds have come to diverge from one another. The question of whether domestic breeds are true species or just varieties divides opinion: those holding the former opinion argue that each breed must have had a wild ancestor, those holding the latter argue that domestic breeds are the result of crosses between wild species.
Neither of these explanations appears plausible. Domesticating a single species is very challenging, never mind domesticating many thousands of species to produce every extant domestic breed. The idea of our domestic sheep, cows dogs and other animals running wild in England in distant history seems hardly credible. Crosses tend to produce only intermediate forms, and most of the extreme characteristics of our domestic species are found nowhere in nature. No wild bird exists with as many tail feathers as a fantail pigeon, nor the enormously developed crop of a pouter pigeon.
The idea that our varieties have gradually diverged from a few ancestral species domesticated by early humans is more plausible. Humans may have caused these changes by carefully selecting the individuals with desirable characteristics to produce the next generation. For example a pigeon fancier might choose birds with unusually numerous tail feathers to breed from. Some of the offspring inherit the tendency towards feathery tails and are used to breed. Over many generations the pigeon’s tail feathers become increasingly numerous. Eventually, a new variety is defined and a strain of pigeons with thirty or forty tail feathers, rather than the usual twelve or fourteen, is named the ‘Fantail’.
The process usually takes more than a single breeder’s lifetime, the process of selection is usually a very gradual one. Over human history, livestock and plant breeders have generally tried to breed from only the ‘best’ animals and plants. Over long periods, this has changed the nature of those stocks in ways that are beneficial to humans.
‘The key to Man’s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. In this sense he may be said to make for himself useful breeds.’
Chapter 2: Variation Under Nature
The unimaginable variety of life that exists in nature is due to the gradual accumulation of small variations, just as in our domestic breeds. The process is directed by natural conditions just as a breeder directs the changes in domestic stock.
Naturalists place much importance on the terms ‘species’ and ‘varieties’, that are widely used to define groups of similar animals and plants. Yet, these words are so ill-defined as to be almost irrelevant. Even between some widely accepted species, uncertainty exists. For example, the primrose Primula vulgaris and cowslip Primula veris, two species in different habitats, have different flowering times, different appearances and don’t cross-breed. Yet they are linked by intermediate forms that could either be interpreted as two species’ hybrid offspring, or as yet another variety within a single, variable species.
There is a great deal of variation between individuals even within a clearly-defined defined species, which provides raw material for Natural Selection to act upon. In the same way in which human-imposed selection causes changes in domestic breeds through selective breeding, natural conditions cause beneficial inherited variations to accumulate in wild species. These small differences are the first steps towards the divergence of varieties, subspecies, species and even the larger groups of genera, families and classes.
The relationships between species within larger groups of closely related species, genera, give some clue as to how small variations lead to bigger divergences. The genera with the highest number of species also contain the most most numerous and widely dispersed species, and those with more varieties per species. Because the divergence of species is more frequent in larger genera, divergences have also occurred more recently, so there has been less time for difference to develop between sister species, and they are more similar.
Were each species an individual act of divine creation, the relationship between species dominance, variability and size of genera would not exist. Each species emerged from a variety, each variety emerged from smaller variations. New types of life continually proliferate, multiple new forms radiating out from the forms of their ancestors.
‘Where many large trees grow, we expect to find saplings. Where many species of a genus have been formed through variation, circumstances have been favourable for variation; and hence we might expect that the circumstances would generally be still favourable to variation.’
Chapter 3: The Struggle for Existence
Natural Selection has infinitely more power over the forms of organic beings than humans ever could do. It relies on three things: the fact that individuals are varied, that these variations can be inherited between generations, and that organisms must struggle to survive. This struggle results from the fact that more individuals are produced than can possibly be supported by their environment.
Any variation which might give an individual some advantage will make it more likely to survive and reproduce. The individual’s offspring may inherit this beneficial variation, and will be more likely to survive and reproduce themselves. Advantageous variations depend on the environment and surrounding organisms. In this way, natural conditions cause gradual changes to the character of species.
By the term ‘struggle for existence’, I include the struggle that occurs between individuals of the same species, between different species and between individuals and their environment. This struggle is a result of the fact that offspring of every species are over-produced. Every species would fill the Earth within a short space of time if allowed to reproduce indefinitely.
This over-production of new offspring means that competition exists in every aspect of life. Limitations to the increase of species occur at different stages of life and in different ways for each species. A common factor seems to be that early vulnerable stages suffer a heavy death-toll. Individuals compete over food, predators and prey compete to eat or not to be eaten. When a species increases, its parasites become more prevalent.
Climate and physical conditions contribute to this struggle to a lesser degree. We can see that the organisation of the natural world, even the apparently random distribution of plants in a hedgerow, is in fact the result of bitter struggle. Each centimeter of ground, each ray of light, drop of water and nutrient from the soil is contested over. Only the best competitors attain what they need and survive. The struggle between individuals of the same species, with the same requirements of life, appears to be the most fierce.
The result of this struggle is that every aspect of an individual’s anatomy and habits is tailored closely to those of the other organisms it and its ancestors have come into contact with, not just its environment. The feathery fronds on some seedheads serve to carry them away from adult plants against whom they cannot compete for light; the tough skulls of woodpeckers allow them to drill through thick tree bark to access hiding invertebrates. The wonderful forms we see in nature have not been not placed there by a divine hand to please us, they serve animals and plants struggling for existence in the midst of the organic and inorganic world.
‘the face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.’
Darwin’s Origin: Introduction