At its heart, Darwin’s theory of evolution is founded on the hypothesis that variations within the same species have resulted in the variations between different species. The different variations that we see throughout the natural world—colors, shapes, sizes, bodily organs, physical structures, wings, feet, fur, feathers—are apparent because natural selection has caused these traits to be perpetuated in the species that exist today. These variations do not exist randomly, but because they have proven advantageous to the survival of the species. Wings, for example, allow birds to migrate easily while also giving them the means of escaping environmental disasters and harsh climates. The variations that are perpetuated provide the advantages that allow certain species to survive over others.
Darwin argues that variations are hereditary, which is key to their role in natural selection. As traits are passed from parents to offspring, the advantageous traits that are exhibited in parents will also be exhibited in their children. Natural selection ensures that those with advantageous traits will win the struggle for existence, surviving and bearing more offspring that carry their advantageous trait. If traits were not passed on from parents to offspring, there would be no mechanism through which advantageous traits could reappear in subsequent generations and therefore no way for species to diverge from one another through their unique traits. While scientists in Darwin’s era had not yet discovered the mechanisms by which traits were passed on through heredity, evidence existed that traits in parents often appeared in their offspring, which led scientists to hypothesize that characteristics were somehow inherited. The heritability of variation, then, was a key component of Darwin’s theory.
At the time of Darwin’s writings, most scientists defined species as groups of organisms that were unable to breed successfully with one another, as opposed to varieties within a species that could interbreed. Darwin points to flaws in this reasoning and argues throughout the text that there is no real distinction between varieties of the same species and separate species. The variations that create varieties may eventually lead to the emergence of different species—in other words, variations create the descent with modification that leads to the development of new species. Darwin’s theory does not explain how the divergence of varieties into species creates infertility between these new species. Instead of defining species according to their fertility, he argues that the line between varieties and species is arbitrary. While varieties of the same species are generally more alike than are varieties of different species, only degrees of difference separate species from varieties, and Darwin argues that these distinctions are scientifically undefined. This argument, while contested, allows Darwin to explain how varieties can easily diverge into species through natural selection without having to explain in detail how barriers to fertility might develop.
Borrowing from Thomas Malthus’s principle of population growth, Darwin argues that species must engage in a struggle for existence that limits their possibilities for survival. Malthus shows that while population growth occurs exponentially (doubling every generation until it reaches infinite numbers), geography and limited natural resources (food, water, shelter) restrict the number of organisms that are able to survive in nature. Since the earth cannot accommodate an infinite number of inhabitants, species must compete with one another to survive. This competition creates what Darwin calls the “struggle for existence,” in which species must fight to survive to avoid extinction. Any small advantage that an organism might have—the ability to run away quickly from predators, coloring that allows it to hide from predators, physical adaptations that allow it to gather food more easily—will give it an edge over other organisms, bettering its odds of winning the struggle over others and surviving.
Darwin identifies two ways in which species can win the struggle for existence. First, they can simply survive and live longer than other organisms. Second, they can leave more offspring than other species do, ensuring that their traits will be passed on to subsequent generations more frequently than will the traits of organisms that reproduce less. These methods of perpetuation are interrelated: An organism that survives over others is also more likely to produce more offspring in its longer span of life. The struggle for existence, then, is not just about individual survival; it is about the survival of groups of related organisms over other groups. Organisms are related through the variations they share, and it is the advantages these variations give to the group that allow them to survive over other organisms.
The struggle for existence is the filter through which natural selection flows. Since competition for survival between species is fierce, any advantageous variation that a species possesses—wings that allow it to fly, special coloring that allows it to hide from predators, and so on—will give it an edge in the struggle for existence. Organisms with the most advantageous variations, therefore, are most likely to survive in nature. Moreover, their survival over others makes it more likely that they will produce more offspring that carry the same heritable advantageous variations. Since that parent was able to survive over and birth more offspring than others in its species, subsequent generations of that species will see this advantageous trait appear more often. Eventually, Darwin argues, the perpetuation of this trait will result in an entirely different species from the original, an outcome he calls “descent with modification.” Darwin uses the phrase “natural selection” to describe nature’s process of selecting organisms with advantageous variations for survival and allowing those traits to be perpetuated in subsequent generations.
Darwin does admit that there are limits to the extent to which natural selection can bring about variation in a species. Since the origins of variations are unknown, natural selection can only work with variations that randomly appear. Over time, new variations will appear that are better or more complex than old ones. These new variations will come to replace the old ones, slowly leading to the seemingly perfect adaptive states that are apparent in species existing today. Moreover, natural selection cannot perpetuate variations that will harm one species solely for the benefit of others. A trait that is harmful to one species but helpful to another—for example, the slow movement of an organism that allows its predators to hunt it easily—cannot be naturally selected solely due to its helpfulness to the latter species. If a species does exhibit a trait that proves beneficial to another, there must also exist some advantageous trait in the first species that allows it to procreate successfully. Otherwise, the first species will eventually become extinct as a result of its harmful variation.
Finally, natural selection leading to descent with modification is understood by Darwin to be slow, gradual processes. Advantageous traits are not selected overnight; it takes many generations before a population slowly begins to diverge from its parent species and many more generations until one trait becomes dominant and another one becomes extinct. Over time, natural selection has led to the successful adaptations of today’s existing species, all of which exhibit outstanding adaptations to their natural environments, and will continue to influence descent with modification as future variations arise and environments change.
Discussing geographical isolation is a helpful way to understand how natural selection produces new species. The voyage of the H.M.S. Beagle took Darwin to the Galapagos Islands, each of which housed its own distinct species of birds, mammals, and plants. This phenomenon alerted Darwin to the possibility that isolation of species from one another results in divergent development. Continuing his research after the voyage, Darwin noticed the wider distribution of species on continents where migration occurs easily and the much smaller distribution of species found only in geographic isolation. Moreover, the unique species found on islands were most similar to related species found on the nearest mainland continent. All of these factors provided Darwin with evidence that geographical distribution and isolation greatly influence the processes of natural selection and the development of new species.
Darwin argues that all of the existing species in the world have descended from one or a few primary forms that originated in one part of the world. As these original organisms multiplied in subsequent generations, their descendents migrated throughout the world. The surface of the earth changed, due to the Ice Age and the subsequent period of rewarming, which raised water levels and cut off previously connected lands—and populations—from one another. The geographical isolation of species from one another allowed different variations to be perpetuated in different species, depending on which variations proved advantageous in the environment in question. Moreover, the ease with which birds migrated to and from isolated lands also haphazardly spread species to isolated islands, as birds could carry from place to place seeds, plants, eggs, and food in their beaks, feet, and stomachs. The migration of various species to new, geographically isolated locales allowed for their proliferation in new places, but also their divergence from other species, as whatever traits were apparent in the few species that migrated would be perpetuated in the new population.