The Origin of Species: Chapter 4 Summary & Analysis

Darwin asked rhetorically how the struggle for existence could lead to variation. He suggested that because humans had so much evidence of how it was possible to intentionally breed animals for human use, it should be no surprise that similarly helpful variations might arise in a wild population over many generations. Similarly, unhelpful variations would quickly die out. This is what Darwin called natural selection or survival of the fittest.

Some writers misunderstood the concept of natural selection. They suggested that natural selection creates variations, but in fact, it just preserves variations, which is an important distinction. Darwin admitted that he found it difficult to avoid personifying the word “Nature” and treating it like an entity, but in fact, he believed that nature was simply an aggregation of laws and events.

Darwin argued that if humans could produce such amazing results by breeding domesticated plants and animals, natural selection would produce even more stunning variations, since unlike humans, nature does not have to work with external, visible characteristics. Similarly, humans are limited in the way they create variations in plants and livestock, often staying within the same regions and climates and breeding them for specific purposes, but the struggle to exist in nature is more open-ended.

The key feature of natural selection is that helpful modifications to organisms must not only arise, but also have a way of being preserved and passed on to offspring. Even matters that seem insignificant to humans may prove significant in the natural selection process, such as how leaf-eating insects are green, giving them camouflage.

Though natural selection works to the advantage of individual species, it can have advantages that go beyond an individual organism: for example, natural selection can affect a whole community of social animals. What natural selection can’t do, however, is preserve a modification in one species that gives an advantage to a second species—at least not unless there is also some benefit for the first species.

Natural selection is not a perfectly orderly process, and sometimes organisms that would have had advantages are destroyed through some accidental means. In such cases, natural selection might seem powerless, but these exceptions don’t negate the fact that natural selection is very powerful under other circumstances.

Sexual Selection. Darwin wrote how, as part of natural selection, the sexes of the species may be modified in relation to each other. Darwin called this sexual selection, by which he meant the struggle of “one sex, generally the males, for the possession of the other sex.” Some species of males, particularly carnivores, will literally fight each other for females. Others, like birds, have more peaceful competitions, but they nevertheless compete by performing strange ceremonies to show off.

Darwin concluded that if males and females of a species have the same habits but differ in form or external appearance, the main cause of the difference is usually sexual selection. Sexual selection can involve giving organisms ways to attack, ways to defend, or even simply ways to charm.

Illustrations of the Action of Natural Selection, or the Survival of the Fittest. Darwin suggested illustrating his points with some imaginary examples. He imagined wolves, where some of the wolves have to be slim enough to chase after deer but still muscular enough to overpower their prey, while other wolves have to be stockier but don’t need to be as fast to hunt sheep. Over time, wolves with one of these advantages would tend to out-survive others and through chance would pass some of their traits on to offspring.

In the case of wolves or other animals, the different regions where the different varieties live are often what keep them from intermixing with others of their species (which might result in the varieties not being passed on).

Darwin put forward another example of plants that put out nectar, which attracts insects. While these visits from insects don’t benefit the individual plant, they help spread pollen, crossing different individual flowers of the same species. Because the plants with the most nectar would be visited the most frequently and therefore spread their pollen most widely, these plants tended to survive in populations.

From the insects’ perspective, some individuals may also have variations that help them get more nectar. For example, some bees might have a proboscis shaped to give it a slight advantage in getting nectar, even if the change is too small for humans to perceive.

Darwin ended the section by admitting that some might object to his ideas, but that his work was like that of the renowned geologist Charles Lyell, whose ideas inspired Darwin and fundamentally changed the way that people in the field of geology thought.

On the Intercrossing of Individuals. According to Darwin, with some exceptions (like parthenogenesis), animals and plants have different sexes, and an individual of each sex is required to produce offspring. Even in the case of hermaphrodites, Darwin noted that reproduction in animals usually involved two individuals that pair (although there remain some exceptions that do not pair). Among plants, the majority are hermaphrodites.

Darwin found that with plants and animals, interbreeding helped produce more vigorous and fertile offspring, except for very close interbreeding, which had the opposite effect. With plants, certain varieties have pollen that are more potent than other varieties, but the pollen of a plant’s native species takes precedence over pollen from other species.

Some animal species, like earthworms, are hermaphrodites that pair to reproduce. However, Darwin wasn’t aware of any terrestrial animals that could fertilize themselves. Many aquatic animals self-fertilize, although this may be because the currents of the water create the opportunity for crossings.

Circumstances favourable for the production of new forms through Natural Selection. Darwin noted that variability in a population was always favorable, since it offers increased probability for helpful variations to arise and eventually be preserved. It might seem that free crossing between animals would get rid of the effects of natural selection, since it might cause traits to move toward the average instead of toward new varieties. In fact, however, there are many factors like geographic isolation or even different niches within the same environment that keep varieties of animals distinct from each other.

Isolation is an important condition for allowing new varieties to arise through natural selection. The passage of time itself is not enough to create natural selection; it is only important because natural selection can be a slow process, and more time gives more opportunity for variations to arise and be preserved.

Extinction caused by Natural Selection. As better adapted organisms grow in number, less well-adapted organisms become rarer, and this leads to the possibility of extinction. New varieties and species tend to put the most pressure on their closest relatives, often driving them to extinction.

Divergence of Character. Darwin believed that varieties were “incipient species” and that they were often in the process of becoming different species. In small areas with a lot of competition for resources, there is often a lot of diversity, coming from different genera and orders. This advantage of diversity is why foreign plants sometimes thrive compared to indigenous plants when they are introduced in a new country.

The Probable Effects of the Action of Natural Selection through Divergence of Character and Extinction, on the Descendants of a Common Ancestor. The descendants of a species will be most adapted to survive if they become more diverse. In this way, the descendants of a species can be plotted as a sort of “tree” with lines branching out, some of which end (in extinction) and others of which continue branching off.

Darwin admitted that in the real world, natural selection probably doesn’t proceed as methodically as in his tree diagram. Nevertheless, the diagram shows important ideas, like how extinction plays a role in natural selection. It also shows how, over generations, one species tends to become many different descendant species, and sometimes branches become so separated that they become different genera or even families.

Though it might seem that this means the future will have more diversity and the past had less diversity, because of the role extinction plays, it’s possible that diversity in both periods would be similar to the present.

On the Degree to which Organisation tends to advance. Because natural selection preserves positive traits, organisms tend to become better adapted to their conditions. This means natural selection tends to lead toward organization, such as the development of specialized, interlocking systems of organs within a creature.

As Darwin wrote, however, this seems to present a problem: if natural selection leads to organization, why do simple organisms still exist? He argued that this wasn’t actually a problem: if there is no advantage for an earthworm to have a more complex system of organs, then it will not be naturally selected. All life would have originally started out as simple unicellular organisms before new variations arose.

Convergence of Character. One of Darwin’s critics believed that he overestimated the importance of divergence between species. This critic believed that Darwin didn’t put enough emphasis on convergence, and that creatures of different species might actually come to resemble each other over time and go from two genera to one genus. Darwin believed this view was unlikely—that if species did converge like this, they would be vulnerable to extinction.

Summary of Chapter. Darwin repeated some of the main ideas from the chapter: how helpful variations arise in nature and are preserved; how sexual selection plays a role; how extinction acts as part of natural selection; how widely diffused species show more variation; and how the process of new species descending from ancestors can be diagrammed as a tree.