Chapter 2 – Variation under Nature

In the second chapter of the Origin, Darwin shifts from domesticated animals to variation in wild populations. His goal is to show that the kind of variation we see under human influence also occurs in nature and that species, far from being rigidly defined categories, are fluid and variable. This move is key to Darwin’s argument: if species show variation in nature similar to what we observe in domestic animals, then the conditions for evolution are not confined to artificial selection but are present in the wild as well. One of the first issues Darwin tackles is the difficulty of defining what a species even is. He openly admits that he will not attempt to provide a new definition of the term, pointing out that:

The ambiguity is revealing. If even the experts can’t agree on what makes one species distinct from another, then perhaps species are not as fixed as traditionally assumed. This point becomes more striking when Darwin contrasts species with varieties. A species, he notes, is usually understood to be the product of a distinct act of creation, at least within the creationist framework. A variety, meanwhile, is harder to define but generally assumed to share a common ancestry with its parent species. The distinction, however, is tenuous. Both species and varieties are marked by degrees of difference, and the difference between the two often comes down to arbitrary judgment rather than clear biological criteria.

Geographical isolation plays an important role in Darwin’s thinking. He recalls comparing the birds of the Galápagos Islands, especially the mockingbirds, with one another and with birds from the South American mainland. The lines between species and varieties in these cases were so vague that Darwin began to question the reality of species as separate creations. In the Galápagos mockingbirds, each island’s population looked slightly different, but not enough to warrant the label of a distinct species. These intermediate forms, distributed across space, forced Darwin to consider whether species were truly separate entities or just points along a continuum of change.

This leads him to the concept of doubtful species, which he later describes using terms like subspecies or geographic races. These are populations that differ consistently from one another, often due to geographic separation, but not to the extent that they’re universally recognized as separate species. Darwin emphasizes that no clear line exists between species and subspecies, or between subspecies and varieties, or even between varieties and individual differences. All of this blurriness is central to his argument: if the natural world doesn’t present us with clear species boundaries, then maybe those boundaries are not real in any absolute sense.

Darwin turns to individual differences, which he considers the raw material for evolution. While systematists might ignore minor differences between individuals as trivial, Darwin sees them as the first step toward forming distinct varieties. If some of these differences are heritable and beneficial, natural selection can act on them. Over time, they can accumulate into recognizable varieties or even full species. The fact that variation exists at all levels, from minor individual traits to well-marked subspecies, shows that evolutionary change is not just possible but already underway.

Yet Darwin does not reject the biological reality of species. He believes that species are real, but their reality is not that of fixed or immutable types. Instead, they are branches along the lines of descent, temporary groupings of organisms sharing a common history. The species boundary may be vague and shifting, but it still reflects a real process of divergence over time. Taxonomists, Darwin implies, are right to identify species, but wrong to assume that these groupings reflect eternal, unchanging categories.

Much of the remainder of the chapter is devoted to what Darwin calls his botanical arithmetic, a statistical approach to the relationship between species, varieties, and genera. He observes that genera containing many species tend also to contain more varieties. Where many trees grow, he says, we expect to find saplings. In other words, when a group has already produced many species, it is likely in a phase of active diversification and therefore also produces more varieties. If species were created individually by divine acts, this pattern would make no sense. But if species arise through natural processes like variation and selection, then the pattern is exactly what we should expect.

Darwin reflects on why some species are highly variable while others remain very constant. He suspects that widespread species in large genera are especially prone to producing varieties. These species exist in diverse environments and large populations, both of which increase the chances for variation and adaptation. He recognizes that his data are incomplete, especially given the limited knowledge of species distributions in his time, but he still finds the pattern compelling. In short, variation is not randomly distributed: it is associated with ecological opportunity and evolutionary potential.

In closing the chapter, Darwin circles back to the taxonomic challenge of distinguishing species from varieties. He notes that varieties resemble species in most characteristics. The only things separating them are the presence or absence of intermediate forms and the degree of difference. And even then, the amount of difference required to classify two organisms as separate species is subjective. For Darwin, this is not a problem for his theory, it is part of the evidence for it. If species were separately created, we would expect them to be clearly distinct. Instead, we find a messy continuum.

Darwin concludes that good species do exist in nature, they are the products of divergence and selection. But the fact that these species vary so much in how sharply they are defined, and how many varieties they contain, suggests that species are not static entities. They are instead stages in a dynamic process of change. This insight allows Darwin to challenge the fixity of species and set the stage for his own vera causa. The real cause of evolution, which he will begin presenting in the next chapter through the concepts of the struggle for existence and natural selection.