Reading Origin of Species – 2

(3) The theory: natural selection (pages 55-58)

(4) The tree of life (pages 59-65)

(3) The theory: natural selection (pages 55-58)

Up to this point Darwin has been laying the groundwork for his theory about how species originate. In Chapter 4, Origin now pulls these key elements together in a sustained presentation of Darwin’s major insight: the driving force that works on variations is ‘natural selection’, which produces modified organisms ever more beautifully adapted to their life’s situation. Natural selection works through divergence and extinction.

The principle of divergence: One species doesn’t simply morph into another, but splits into new varieties, and eventually into new species. If a region is filled to capacity with a species of carnivorous animal, for example, its numerous descendants will thrive only if they diverge to feed on new kinds of prey, climb trees, take to the water, or become less carnivorous. Divergence likewise explains the pressure for organisms to occupy living places farther away, bringing the species into new conditions that may trigger yet more variations.

• The principle of extinction also looms large in the working of natural selection. As selected and favored forms increase in number, filling niches and consuming resources, so the less favored forms will decrease and become rare. Extinction comes about through a species’ failure to adapt to changing conditions.

• Sexual selection: Along the main line of evolution, divergence drives lineages apart, and extinction erases evidence of the transition. As these forces shape species, Darwin sees another dynamic – which he calls sexual selection – which is not a struggle for existence but a struggle between males for possession of the females; the female will choose the healthiest, most attractive partner.

Since natural selection can act only by the accumulation of infinitesimally small modifications, Darwin saw it as a slow, intermittent process. What makes the process work are vast periods of time, large spaces, and great numbers of varying individuals and species.

(4) The tree of life (pages 59-65)

Beasts 3-2
Darwin’s diagram tracks the course of life up to the 14,000th generation (Beasts, p. 59)

To help the reader understand how natural selection works to bring about the world we see today, Darwin drew this diagram, using letters and numbers to show how the process of evolution leads to a divergence of forms over millions of years. The spaces between the horizontal lines in the diagram may represent a thousand generations – or a million or even a hundred million generation.

In a way more graphic than words, this diagram charts the cumulative outcome of the births, lives, and deaths of a few species over an unimaginable run of times past. The process has gone on for as long as life has existed on Earth, back to what is today called a universal common ancestor.

Darwin devotes ten pages to discussion of this diagram in Chapter 4, and 25 pages more in Chapter 13. The only drawing in Origin, it is a profoundly influential interpretive device that allows scientists and general readers alike to get an imaginative grasp of the working of evolution. In Darwin’s fertile imagination this diagram converts, finally, into the metaphor of the tree of life. Everything alive today has come forth from this synthesis of birth, change, and death. This is an audacious account of the origin of species. It opens up new imagination with regard to the natural world.

During this whole process of descent with modification, extinction plays an essential role. Better adapted to the natural environment, the species at the top of the diagram have supplanted all but one of the starting species. But note that from the original 11 species, the diagram ends up with 15: life has diversified. Eight of the original 11 species have no descendants at all; they have gone extinct. All the other new species come from two of the original 11.

Recall the standard idea in of ‘special creation’ – that all species are immutable, each exquisitely designed and located in its time and place by a special act of the Creator. Operating with that assumption, Linneaus’ work of classification identified organic beings based on visual similarities and differences. The theory of evolution argues that the true basis of classification is not external appearance, but genealogical. Darwin’s theory uncovers the inner affinity of all organic beings to one another, rather than their merely external relations.


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One thought on “Reading Origin of Species – 2

  1. Darwin’s careful reasoning depends on his observations. While the general public can follow the development of variation in pigeons, opportunities to spend time in nature are now limited and many don’t develop a critical eye or the ability to generalize and draw conclusions. At the elephant seal bluff, the seals have several adaptations that are inconsistent: although the seals spend most of the year at sea, never coming on land, the pups must be born on land, as they cannot swim when they are born. To me, it’s clearly a remnant from a land-based ancestor. But I’ve had visitors comment that the seals clearly demonstrate that God must have specially created such a strange animal.

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