Charles Darwin III: _{Descent with modification}

"Evolution" vs. "Descent with modification"

At that time, scientists were commonly using the term evolution in discussing physical growth, the changes an individual goes through as one matures. The other meaning of evolution referred to structural change in a species that took place over time, which some, including Darwin, also called transmutation. So, there was a reason for his preference for "descent with modification" over "evolution". First, he wanted to make clear that his discussion of evolution dealt with transmutation (modification), not growth and development. Second, Darwin meant to emphasize that the big picture of biological evolution was far more complicated than the image of the fur of a fox transmuting from reddish brown to white as an adaptation to life in the Arctic. It also involved the production of a pedigree that linked species because they are genetically related, through the process of descent.

The concept of phylogenetic descent was a new idea that made Darwin's theories of evolution more sensible than previous proposals that tried to explain certain observations. For many decades before Origin appeared, natural scientists had wrestled with a puzzling problem of biodiversity. While taxonomists who classified organisms never intended to find patterns, it was clear to all those who studied taxonomy that there was a "natural order" that grew out of the process of classifying animals and plants (see our Taxonomy I: What's in a name? module).

Scientists wondered why, when classified, groups of species seemed to form clusters, as if some sort of biodiversity magnet pulled them together and put them in one place. Within clusters, species tended to be similar to one another by different degrees. Surely it was no coincidence that all species of cats are alike, from the alley cat to the lion to the prehistoric saber tooth that roamed the western United States. Chance could not be the reason why dogs, wolves, and coyotes are all variations on the theme of "Dog." Similarly, chance could not explain the similarities and differences of the Galapagos finches that Darwin collected while he was with the Beagle expedition. What was behind the repeated pattern of species clusters that was so common in nature?

Biodiversity as a staircase: A "macro" view of life

Before Darwin, there was only one available model that naturalists used to explain species' similarities and differences, and that would not work to solve this problem. Scientists had thought that the most important pattern of biodiversity was what they called the Scale of Nature. This was the notion that the vast range of living organisms – say, from snail to ant to fish to mouse to monkey to man – was a feature of divine creation meant to highlight our own superiority. Life, they thought, was arranged like a set of stairs, with "lower" forms situated on the bottom and "higher" forms, humans, appearing at the top. This idea can be traced as far back as Aristotle, more than 2,000 years ago, and it was popularized in the 1800s by writers like Robert Chambers. Of course, it was all based on assumption. No evidence was provided to support the model, but it was generally accepted by tradition.

The Scale of Nature, which was actually more of a "macro" view of life, would not work for Darwin because it did not relate to the clusters of similar species that he had observed. Why would so many types of cats or finches exist? The Scale of Nature suggested an unchanging, linear quality to evolution, but that surely could not explain the explosive variety of adaptations that Darwin saw among the finches he found on the Galapagos Islands during the Beagle voyage. Darwin observed finches that were adapted to feeding on different things: birds with beaks that were specialized to eat seeds, leaves, insects, or nectar. The list of items could not be interpreted as a scale-like linear climb involving a worse-adapted food source to better-adapted food source, or from poor-food to rich-food.

Biodiversity as a tree: A "micro" view of life

Instead of the macro view offered by the Scale, Darwin was focused on a "micro" view of biodiversity: What could explain the small variations distinguishing species that actually resembled one another? He came to see that evolutionary changes on the micro level would add up to the differences that were obvious at the macro level. So, instead of a stairway or ladder as a metaphor for understanding the cluster pattern of biodiversity, Darwin pictured a tree.

This was a brilliant insight. Rather than being arrow-like and linear, a tree has many elements that spread out in different directions. Rather than being static, it is dynamic. It grows over time, just as evolution is embedded in time. It sprouts branches, as if it were generating new varieties and new species. Or, it may have branches that do not subdivide. Some branches grow straight up, parallel to the trunk, while most head off in different directions as they develop, resembling alternative adaptations. Some branches grow into stumps and die out, becoming extinct. Others may grow long and last for generations, thousands of years, tens of thousands of years, and even longer. None of the branches of a tree is judged to be any better than others; none is superior and none is inferior. They are all simply different.

Crucially important is the fact that all the branches of a tree are interconnected. You can trace their origins from their endpoints to the parent shoots from which they grew, just as you might trace the roots of dogs, or cats, or Galapagos finches to their original ancestral species.

The Origin tree diagram illustrates how a branching pattern of evolution can produce a greater number of species over time than what was there to begin with. It shows how some lines of species, or lineages, split more frequently than others. It shows that some lineages do not split at all but evolve almost like a column. It shows that extinction is a basic property of descent: Many populations are left behind and do not reach the top because they have died out.

Darwin's early model

Coming up with this tree-model of evolutionary process and pattern was not easy. In fact, Darwin's personal notebooks reveal how his own understanding grew over the years. In one notebook, which he began writing soon after returning from the Beagle expedition, he drew a crude stick-like diagram to show that many species could evolve from a single ancestral species by somehow splitting apart.

This early graphic, shown in Figure 4, is shaped like a cross between a tree and a starburst. It seems as if Darwin was trying to form an idea of how the great diversity of species could come about naturally from a single origin, rather than having each species being specially created. It is a flat image, as if he were drawing the diversity of Galapagos finches on a map of the islands. That, in fact, was key to his figuring out that evolution had occurred on the Galapagos, that is, how the birds had evolved across space. But his 1859 drawing, clearly for the first time, provided the blueprint of evolution though time. It illustrated his notion of descent with modification, how natural selection produces change and also a pedigree of connections between species that shows where they came from historically, meaning phylogenetically.

Darwin's finches

To better understand the one illustration that Darwin included in Origin, it is helpful to picture the finches that he studied on the Galapagos. What would this diagram look like if he had illustrated it with species of finch? At the bottom of the figure we would see the ancestral finches. As the lines diverge and branch out higher in the diagram, new species of finch would appear, leading to the array of modern birds at the top of the picture.

With the tree of life as a metaphor for evolution, Darwin changed the way both scientists and the public view the origin of species. There would no longer be a need to interpret the biodiversity of nature as a ladder or scale, with some species better or worse than others due to the details of their size, fur, and teeth or as measured by their intelligence or ancestry. All are adapted to their specific environments, even though some might not survive. And all, at some level, share a common source of origin.