Darwin Still Informs the Origin of Species

Among the natural sciences, biology has always occupied a central but at the same time uncomfortable position. Central, because observations of the living world have traditionally always been a starting point for “natural philosophers” to further expand into palaeontology, and then geology, physics, meteorology, astronomy, and chemistry. Uncomfortable, because whereas those other natural sciences are constantly searching (and finding) generally applicable laws of nature, biology often tends to get bogged down in the details of particular slivers of the diversity of life. Diversity is biology’s source of inspiration, of course, as the sheer endless variation in appearances and ways of life of the many millions of life forms on earth has been the core fascination for almost all biologists. Diversity is also the main obstacle in biology to look for and find those same universal laws that come so much more naturally to physics and chemistry. Many biologists can lead fruitful and meaningful careers while studying the particular biology of one particular species or group of species, without ever even thinking of generally applicable theories.

It is therefore both rare and ground-breaking when some biologists assume a sufficiently broad scope to cut across the fragmented organism-specific subfields in biology and look for patterns that apply to nearly every living thing. In recent decades, we have seen several such persons. Stephen Hubbell of the Unified Neutral Theory of Biodiversity comes to mind¹ as well as William Eberhard and his book Sexual Selection and Animal Genitalia² . A few decades ago, similarly, Lynn Margulis revealed the endosymbiotic origin of organelles in all eukaryotic organisms³ , and Robert MacArthur and E.O. Wilson found the general theory of island biogeography⁴ . But it is Charles Darwin who, a century earlier, was the first genius to craft an encompassing and foundational biological theory by amassing data and insights from all imaginable corners of biology. This is the reason why, unlike the 19th-century giants in many other fields, Darwin is still actively cited in biological scientific literature today. He has a posthumous h-factor of 98 and in 2025 alone his works received around 6,500 citations, thereby dwarfing today’s citation scores of Isaac Newton, for example.

To me, the reason that Darwin is still such an important touch-stone today, is that he thought and wrote about many of the same questions that biologists today are grappling with, and he did so in a disarmingly honest and equitable manner. Whenever I get interested in a particular biological problem, it is both sobering and heartening to look up a passage in one of Darwin’s many works and see him struggling with the same problems two centuries ago, and often coming up with the same reasoning and ideas for experimentation. This is true in sexual selection, community ecology, palaeontology, phylogenetics, evolutionary genetics, evolutionary ecology, and many other fields, but here I would like to focus on something that has always formed a core of evolutionary biology and of Darwin’s most famous book, namely: the species concept and the process of speciation (the evolution of new species).

Defining the undefinable

It is often said that the title of Darwin’s most famous book, On the Origin of Species by Means of Natural Selection, is misleading, since it is about a lot of important aspects of evolution, but not about the splitting of species into two or more new species, i.e., the central process of speciation. In line with this, Darwin has been criticised for not being clear about his species definition. Indeed, in On the Origin of Species, we see him taking what many have felt to be a defeatist stance, writing, after many years of studying species and their characteristics, that species are no more than “well-marked varieties”, and that the term was “arbitrarily given for the sake of convenience to a set of individuals closely resembling each other”. He also said that the “search for the undiscovered and undiscoverable essence of the term species” was in vain, as it was trying to “define the undefinable.”

This is a very different approach to species than the proponents of the Modern Synthesis (the mid-20th century amalgamation of genetics, evolutionary biology, palaeontology, and morphology) would prefer. Central in the Modern Synthesis was Ernst Mayr’s biological species concept (BSC), which defines species as “groups of actually or potentially inter-breeding natural populations, which are reproductively isolated from other such groups”. Consequently speciation, the process by which new species arise, was defined as “that stage of the evolutionary process at which the once actually or potentially interbreeding array of forms becomes segregated into two or more separate arrays which are physiologically incapable of breeding”⁵ .

This view of species and speciation was strongly influenced by Mayr’s field observation as an ornithologist. He documented endemism and patterns of variation in birds in Melanesia and found as a general rule that separate species were often present on geographically isolated pieces of land: islands, but sometimes also mountaintops or across large rivers. This convinced him that the “crucial process in speciation is not selection [...], but isolation”⁶ . This meant that the processes responsible for speciation would break down if there was a possibility for genetic exchange (gene flow). So, two sister species would only be able to occur together once reproductive isolation had evolved, otherwise they would fuse again. Therefore, in the context of the Modern Synthesis, reproductive isolation needed to be the decisive criterion for species, and the evolution of reproductive isolation would define the point where speciation has been completed.

Mayr proved that, under the right circumstances and in small, isolated populations, the combined effects of neutral genetic drift (that is, random changes, over time, in allele frequencies), natural selection, and epistasis (i.e., genes influencing each other’s actions) could produce new, internally co-adapted genomes with reconstituted reproductive systems. In other words, new species under the BSC⁷ . So, the theory of species and speciation that developed during the Modern Synthesis relies on the evolution of reproductive isolation for explaining both the origin and the maintenance of species. To many biologists at the time, this theory was a great improvement on Darwin, who, they felt, lacked a clear theory on speciation, and only seriously concerned himself with evolutionary change within species.

Divergence of character

These claims about Darwin, however, are not accurate. Darwin was well aware of reproductive isolation between species. Indeed, he opens chapter 8 of On the Origin of Species by noting: “The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms.” At the same time, Darwin also knew that hybridisation is common in many groups of animals and plants, often without compromising the distinctness of species. This recognition led him to downplay the importance of reproductive isolation, arguing that “neither sterility nor fertility affords any clear distinction between species or varieties.” For Darwin, then, speciation (which he called “divergence of character”) did not arise through the evolution of reproductive barriers, but through a process that could drive a single species in two different directions, whether reproductively isolated or not. That process was natural selection, which he saw as capable not only of modifying a single species through adaptation, but also of splitting one species into two.

This insight was precipitated by, among other things, Darwin’s observation that a group of species exploits a given habitat more efficiently than a single species does. In On the Origin of Species, he writes: "It has been experimentally proven that if a plot of ground be sown with one species of grass, and a similar plot be sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbage can thus be raised." In other words, being specialised on a different aspect of the available resources can help a species, and also an individual, in competing with others. By analogy, he reasoned that under intense competition natural selection would favour individuals with more extreme characteristics, as these would compete less directly with their relatives. As he put it: “Consequently, I cannot doubt that in the course of many thousands of generations, the most distinct varieties of any one species [...] would always have the best chance of succeeding and of increasing in numbers, and thus of supplanting the less distinct varieties; and varieties, when rendered very distinct from each other, take the rank of species.”

The key difference between Darwin’s view and that later developed by Mayr and Dobzhansky therefore lies in the role assigned to natural selection. For Darwin, natural selection could either shape a population to fit a changing environment (adaptation) or split a population into two in order to exploit available niches more effectively (speciation). Mayr and Dobzhansky, by contrast, separated adaptation from speciation, treating environmental selection as a driving force only for adaptation. Today, it is Darwin’s view that is more and more supplanting the ideas popular during the Modern Synthesis.

Darwin’s revival

Superficially, when we survey text books and popular accounts, the BSC and speciation by geographic isolation may seem to reign supreme. Indeed, the Biological Species Concept is still taught in schools and universities. However, its popularity seems to be more and more based on its simplicity than its reality.⁸ Similarly, in popular accounts geographic isolation is still mentioned as a chief, if not crucial mode for speciation to be initiated, often citing Darwin’s finches on the Galápagos Islands as a famous (though inappropriate) example – inappropriate because most divergences probably took place within populations due to competition⁹ .

In the evolutionary biology subdiscipline of speciation research, however, Darwin’s views have certainly seen a revival. Partly because of very detailed field and experimental studies of recent speciation, partly also because of fine-grained, genomic dissection of species differences, we now understand much better that reproductive isolation and natural selection can evolve hand-in-hand, without the need for populations to be geographically separated first¹⁰ . By and large, the feeling is that speciation-with-gene-flow is the norm, and that this process will, in incremental ways, eventually result in species that answer to the BSC. It is heartening that, once again, Darwin’s evolutionary views were visionary, even in an age when he did not have the genetic tools that we have today.