The Origin of Species

Many naturalists believed that the crossed offspring of different species were sterile in order to stop the chaos that might ensue. Darwin, however, argued that this sterility was just an accident. One of the key differences between species and varieties seems to be how fertile the offspring of crosses between species are vs. how fertile the offspring of crosses between varieties are.

Degrees of sterility. Building off the work of other naturalists, particularly those working with plants, Darwin noted that sterility is not a black-and-white condition, and that different crossings of parents can lead to different degrees of sterility in offspring. He observed that close interbreeding was often the culprit. Animals had been studied less than plants, but Darwin noted that breeders found that hybrid offspring became more sterile after being bred with siblings over successive generations—although none of these rules were necessarily universal.

Laws governing the Sterility of first Crosses and of Hybrids. Darwin noted that hybrids between two species that are difficult to cross (since the crossing rarely produces offspring) are usually very sterile. Still, there were exceptions, and some specific crossings of plants yielded extremely fertile offspring.Even among hybrid offspring created under similar conditions, fertility can vary widely.

It is difficult to figure out which factors govern fertility of offspring. Logically, it would seem that similar species would produce more fertile offspring, but in fact, in some genera of plants, very similar species produce infertile offspring, while in other genera of plants, all species within it can be crossbred freely—even when the genera are in the same family. Rules vary by species: some species cross well with others, and some species are better at creating hybrid offspring that look like them (as opposed to looking like the other species in the hybrid crossing).

Ultimately, Darwin believed that these complex systems of laws did not support the idea that hybrids were infertile simply to limit confusion in nature. He believed that such a complex system suggested that sterility was either incidental or that it depended on differences in reproductive systems that were difficult for naturalists like him to observe.

Origin and Causes of the Sterility of first Crosses and of Hybrids. Darwin used to believe that sterility in hybrids arose gradually through natural selection and that preventing species from blending was an advantage. After studying the issue more, particularly in plants, he reached the conclusion that it must be due to some other cause and that similar rules probably held true in animals as well.

Darwin believed that the real cause behind sterility in hybrids involved changes to the reproductive system. Hybrids generally arise from unnatural conditions, and these unnatural crossings are reflected by disturbances in the reproductive system. Darwin likened this reproductive disturbance to the behavior of elephants, which don’t breed in confinement, even in their native countries.

Reciprocal Dimorphism and Trimorphism. Some plants look the same on the outside but have different reproductive organs, such as the length of the pistil and stamen. There can be two or even three such varieties (“dimorphic” and “trimorphic”), and some crossings between these different forms produce fertile offspring and others don’t. These dimorphic and trimorphic plants help demonstrate that infertility of offspring is solely related to reproductive features and that external features of a plant or animal don’t necessarily help predict whether its offspring with a similar-looking variety will be sterile.

Fertility of Varieties when Crossed, and of their Mongrel Offspring, not universal. While Darwin argued that there was some truth to the popular idea that varieties produced fertile offspring when crossed but species didn’t, he also argued that the rule was not universal. Some varieties of dogs generally don’t produce fertile offspring with others. Gourds and maize provide even stronger evidence of the difficulty of creating fertile offspring from crossing varieties in some cases, proving that the rule isn’t universal.

Mongrels compared, independently of their fertility. Darwin decided to compare the offspring of crossed species and crossed varieties on traits other than simply fertility. He drew a distinction between “mongrels” and “hybrids,” arguing that in the first generation of breeding, mongrels were more variable, but in species that have been domesticated for a while, the opposite might be true. Ultimately, he concluded that if you left fertility out of the equation, the offspring of varieties and the offspring of species were, in general, more or less the same.

Summary of Chapter. Darwin reiterated that the sterility of first-generation crossed offspring was not due to natural selection. He admitted that he didn’t know exactly why this sterility arose, but believed that this didn’t fundamentally change his ideas about how species and varieties lived in nature.