Chapter 4 – Natural Selection

Chapter 4 marks the centerpiece of Darwin’s argument in the Origin. It is the longest chapter of the book, and it has drawn more commentary than any other. Here, Darwin lays out the mechanism that underpins evolutionary change: natural selection. But this is not the only idea he weaves into this chapter. He also introduces the principles of divergence, sexual selection, common descent, and species multiplication, all of which are integral to how evolutionary change unfolds in the natural world. These ideas, combined, form what Ernst Mayr famously identified as Darwin’s five theories. For the modern reader, this is where the theoretical core of Darwin’s work takes shape.

Darwin begins by revisiting the success of artificial selection. In Chapter 1, he had shown how breeders could generate remarkable forms by selecting for even minor variations. The question now becomes whether this same principle can be extended to nature. Why, Darwin asks, should we not believe that useful variations occasionally arise in wild populations? Why should we doubt that individuals with even slight advantages have a better chance of survival and reproduction?

Darwin constructs an ecological framework in which natural selection becomes not only plausible but inevitable. Though the concept of an ecosystem did not yet exist in 1859, Darwin describes with surprising modernity how species interact with each other and their environment. He envisions scenarios in which climate shifts, migration, and local changes in biotic composition lead to new ecological opportunities and pressures. Species are constantly being shaped and reshaped by these shifting conditions. Crucially, variation must be present for natural selection to act, but wherever profitable variation arises, it is scrutinized by the daily and hourly work of nature. Darwin famously writes: 

This is not an intelligent force. Selection is blind. It does not work toward a goal, and it does not act for the good of other species. Darwin is emphatic: natural selection only works through and for the benefit of the individual organism. There is no foresight and no benevolence. Selection cannot shape one species to benefit another, and any traits that persist do so because they confer an advantage to the individuals that carry them.

Still, Darwin recognized the limits of natural selection as he initially defined it. He saw that survival-based selection could not explain certain extravagant features, like the peacock’s tail or the ornate plumes of birds-of-paradise. These features did not improve survival but were clearly tied to reproductive success. To account for them, Darwin introduces the concept of sexual selection. This is not a struggle for existence, but a struggle for mates. The outcome is not death but differential reproductive output. In many species, males compete with each other for access to females. In others, female choice plays a central role, with females selecting mates based on traits such as coloration, ornamentation, or vocal displays. Though Darwin only touches on the topic briefly in this chapter, he would expand it later in The Descent of Man. He lays the foundation for a second mode of selection beyond the purely ecological.

Darwin then illustrates how natural selection might operate by constructing imaginary but realistic scenarios. In one, he describes how wolves in different regions might come to specialize on different prey. Mountain wolves might hunt deer; lowland wolves, sheep. Over generations, natural selection would preserve those individuals best suited to their environment, gradually leading to divergence between populations. In time, these ecological specializations could lead to distinct varieties, or even species.

One of Darwin’s most remarkable insights in this chapter is his early grasp of coevolution. In 1861, while writing a book about orchids, he received a specimen of Angraecum sesquipedale, a flower with a nectary nearly 30 centimeters long. He immediately predicted that there must exist an insect with a proboscis long enough to reach the nectar. At the time, this was met with skepticism, even mockery. But decades later, his prediction was confirmed with the discovery of a hawkmoth in Madagascar that matched the orchid perfectly. The insight demonstrated how mutual adaptations could arise over time between interacting species.

Darwin also tackles a potential problem for his theory: sexual reproduction and the blending of traits. If traits mix in offspring, wouldn’t natural selection simply erase the distinct features it favors? He addresses this with a short digression on the benefits of crossbreeding. In hermaphroditic species and others, he observed mechanisms that reduce self-fertilization and increase crossing between individuals. In experiments, such as one involving the planting of different cabbage varieties together, he found that the majority of offspring resulted from cross-pollination. The conclusion he draws is that sexual reproduction increases variation rather than diluting it. Moreover, nature favors crossing between distinct individuals of the same species, ensuring a continual supply of diverse traits for selection to act on.

Next, Darwin reflects on the circumstances favorable to natural selection. Large populations, he argues, are especially conducive to evolutionary change. They produce more variation, and with more individuals, there are more opportunities for advantageous traits to arise and spread. Geographic separation also plays a crucial role. In mobile animals like birds, new varieties are most likely to arise in isolated regions. In contrast, in organisms that reproduce rapidly and don’t move much, such as certain invertebrates. New and improved varieties can form locally and persist without being blended away. Wherever isolation and variation meet, the conditions for evolution are ripe.

Although the detailed treatment of extinction is reserved for later chapters, Darwin briefly emphasizes that extinction is a necessary consequence of natural selection. If some forms improve and spread, others must decline and disappear. The number of ecological niches is limited. As improved forms arise, they displace older ones. Extinction, therefore, is not a separate phenomenon but a byproduct of evolutionary success. Nature is not infinitely accommodating. Each species that flourishes does so by outcompeting another.

Darwin now introduces what he calls one of the two most important principles in the book: the divergence of character. In a letter to Joseph Hooker, he described this principle, along with natural selection, as the keystone of his theory. Divergence explains how varieties become more distinct over time. It also explains why species diversify into multiple forms. If individuals exploit different ecological niches, selection will favor those that specialize. This divergence creates new varieties, which in time can become separate species.

To support this, Darwin points to the first ecological experiment ever conducted: George Sinclair’s 1824 test showing that plots sown with diverse grasses produced more biomass than monocultures. Darwin uses this as evidence that diversity confers ecological advantage. In nature, as in agriculture, a variety of forms is more productive than uniformity. The logic is simple: diversity allows for more complete use of available resources, which gives natural selection a push toward divergence.

To make his case visually, Darwin introduces the only diagram in the entire Origin. It depicts 11 species diverging over time into 15 descendant forms, branching upward like limbs from a common trunk. This diagram represents the principle of branching evolution, grounded in common descent. Some lineages flourish and diversify. Others go extinct. The figure illustrates not only how species change, but how lineages split and multiply. It captures in a single image the dynamic, branching, and selective nature of evolutionary history.

Darwin closes the chapter by reinforcing his central point: the mechanism of natural selection explains both the preservation of advantageous traits and the disappearance of unfit forms. It leads to the rise of new species, the extinction of others, and the endless branching of lineages. In his final paragraph, he turns to metaphor, describing evolution as a great tree of life: 

This image remains one of the most enduring legacies of Darwin’s work. From a single origin, life has diverged, spread, and adapted to fill every corner of the planet.