1.8: Darwin and Common Descent
When on board H.M.S “Beagle,” as naturalist, I was much struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to past inhabitants of that continent. These facts, as will be seen in the latter chapters of this volume, seemed to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers.
—CHARLES DARWIN1
Making Sense of What Is Already Known
The story of Semmelweis is one of a scientist confronting a mystery and systematically going out and gathering new data that helped him solve the mystery. The story of Darwin is very different. On the Origin of Species , Darwin’s four-hundred-plus-page “abstract,” is one of the best-argued works in the history of science. He makes a powerful case for, what we shall see, are two monumental theories. Although he was an excellent field biologist and geologist, and although his experiences on the Beagle were clearly formative, the evidence he presented in On the Origin of Species was not original; nor did it contain cutting-edge discoveries. Darwin’s genius was seeing, I would say, explaining, known facts in original and insightful ways.
The history of science is full of examples where the revolutionary breakthrough comes, not in the laboratory, but in the theorist’s study. This is hardly surprising, since when you are trying to make sense of the myriad of facts and results that compose any of the natural sciences, the level of generality and abstraction required for the grand overarching theories will seldom, if ever, come from one nicely designed experiment. Darwin was able to articulate general principles that have unified biology for more than a century and a half. And most remarkably, fit perfectly with biological discoveries that Darwin himself could never have imagined.
FIGURE 2. Simple cladogram. Retrieved from https://www.geol.umd.edu/~jmerck/honr219d/notes/06.html .
The Two Theories
The “theory of evolution,” what Darwin called a “theory of descent with modification by natural selection,” is really three distinct theories, two of which owe a great deal to Darwin. We can translate the term evolution as simply meaning “biological change.” The biological world we see today, including the species now in existence, is different from the biological world at different historical periods. Many theorists before Darwin, including his own grandfather, Erasmus Darwin, had proposed theories of biological change. The problem was that, although (as we shall see directly) these theories nicely explained many known facts, no one before Darwin had any good ideas as to the causes of this change.
Descent with modification , or equivalently, common descent , is Darwin’s theory about the patterns in, and the history of, this biological change. Darwin cannot really be given credit for originating the theory of descent with modification, but one of the great achievements of On the Origin of Species is that he laid out the evidence for this theory so powerfully that within less than a generation almost every biologist accepted the truth of this view of biological history.
The above very simple graphic gives us the heart of descent with modification. The vertical axis represents time, and the horizontal axis represents the present or past picture of biological diversity. So in this little snippet, we learn about the history of three related species, A, B, and C. That they are related is indicated by the common ancestor at the bottom (i.e., earlier in history). We also learn that species B and C are more recent evolutionary arrivals than species A, since the common ancestor that begins their history is higher (later). Descent with modification says that current species are related to one another through a series of ever-narrowing common ancestors (thus common descent). The logical extension of this line of reasoning is, as Darwin saw, that all life can be traced back to a single common ancestor.
I cannot doubt that the theory of descent with modification embraces all the members of the same great class or kingdom. I believe that animals are descended from at most only four or five progenitors, and plants from an equal or lesser number.
Analogy would lead me one step further, namely, in the belief that all animals and plants are descended from some one prototype. 2
Darwin worried that “analogy may be a deceitful guide,” but contemporary facts about the molecular structure of DNA make his speculation look even stronger.
If the history of life is as descent with modification outlines, the obvious question is what brings about all this change? If new species arise from ancestors, what is the origin of species ? Darwin’s stroke of genius was an original answer to this question. Breeders “select” in order to improve the stock. Nature, by analogy, also selects, but not consciously, nor with a purpose. Natural selection is the engine that drives biological history. More individuals in every generation are born than will survive, so there is a “struggle for existence.” Some individuals are lucky enough to be born with slight advantages in this struggle, and these advantages improve their chances of surviving and passing along these advantages to their offspring. So over time, there is inevitable change within the species, and given enough time, these changes accumulate to result in the start of a new species.
Rival Explanations to Common Descent
In the rest of this chapter, we will spend most of our time focusing on the least scientifically controversial of Darwin’s two theories but the one that is clearly the most controversial in the popular culture—descent with modification. I want to try to convince you that evidence Darwin presented for this view of biological history is quite overwhelming. More recent additional evidence has only further strengthened his original argument.
We should candidly address the sources of this cultural controversy. The primary source, of course, is that Darwin is widely believed to be antireligious. The theory of descent is seen by many as not only denying the literal truth of the Old Testament but flat-out denying the existence of God. This latter interpretation is surprising in light of the fact that many contemporary scientists see themselves as conventional religious believers yet accept the scientific truth of common descent and natural selection. My students are constantly surprised to hear that Darwin nowhere denies the existence of God, nor does On the Origin of Species say anything about either the creation of the universe or the origins of life. Many sincere theists have seen complete consistency between what traditional religion teaches us and what our best natural sciences, including biology, teach.
The second source of controversy regards the place of our own species in Darwin’s picture. Although he tried to soft-pedal it in Origin , it was perfectly obvious to his contemporaries, both opponents and supporters, that humans were just as much a part of descent with modification and natural selection as every other part of the biological world. This is troubling to many, I realize. I personally think it explains a lot of what I know about myself, my friends, and the social world I live in.
Before On the Origin of Species , the most widely accepted rival account of biological history assumed that species were permanent, unchanging, and the individual products of divine creation. This view was not just a religious one but a scientific one as well. The theory of special creation should be interpreted in our context as a rival explanation for the facts that Darwin offered in support of descent with modification. It will be helpful to distinguish two versions of special creation. One accepted the book of Genesis as literal history—all the earth’s species, as well as everything else, were individually created by God over the course of six, twenty-four-hour days.
t 1 . Fundamentalist special creation
A second version of special creation that enjoyed more support among Darwin’s contemporaries in the scientific community might be termed relaxed special creation. It conceded some of the data about biological change and allowed that divine creation of individual species took place at different times in the history of the earth and life.
t 2 . Relaxed special creation
The Expanded Age of the Earth
The first part of Darwin’s case for descent with modification consisted of a review of, indeed a powerful subargument for, the relatively new estimates of the age of the earth.
e 1 . The earth is much older than had been previously believed—thousands of millions of years.
This was essential to his theory, for a great deal of time was required for the sort of biological change he was postulating. It’s doubtful that any theory such as common descent could have come before the discovery of “geologic time.”
We should pause here to notice something general about inference to the best explanation. You will remember that in chapter 4 we treated the premises in an argument as data and the theory being defended as an explanation of the data. That is actually a bit of an oversimplification. Oftentimes, important facts will be included in the premises that are not explained by the conclusion but are relevant to that conclusion being an adequate explanation of those data that are being accounted for. Charlie’s being a poor writer was not explained by his being a cheater but was still relevant for this explaining the identical exams. Descent with modification does not explain a much more ancient earth than previous generations had believed, but it is required for descent with modification to work.
The Fossil Record
Darwin spends a great deal of time discussing fossils, and well he might. Whatever else it tells us, it virtually screams out change.
Let us now see whether the several facts and rules relating to the geological succession of organic beings, better accord with the common view of the immutability of species, or with that of their slow and gradual modification, through descent and natural selection. 3
We see species that once thrived and are now extinct. We see progressions such as the changes in the American horse. His emphasis on fossils shows us that he saw this as a particularly strong bit of evidence.
e 2 . The fossil record
The fossil record, particularly in Darwin’s time, was a bit of a two-edged sword. Critics complained that if descent with modification was true, there should be a fossil record of these “transitional forms.” Darwin wisely conceded the force of this objection but also offered a very sophisticated explanation of how difficult it is for fossils to form and why gaps in the record were inevitable.
The Scala Naturae , or the Natural System
A widely held view in the century before Darwin postulated a very different kind of order to the biological world. According to the scala naturae (scale of nature), life was static but hierarchical. There was an observable and classifiable progression from the simplest and most primitive forms of life to the most complex and advanced. This view had been pretty thoroughly rejected by the time Darwin began his work. But a remnant of it remained at the heart of biology. It was now understood that though the structure wasn’t hierarchical, there was a structured order to life nonetheless.
From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups. This classification is not arbitrary like the grouping of stars in constellations. 4
Any intelligent four-year-old can go to the zoo and recognize that the different feline species in the cages are all cats, just like Boots at home. Feline species around the world are “related” to each other, and they are more “closely related” to each other than they are to canine species living in the same environment with them. Nature seems to sort itself out into one giant natural system .
The obvious question is why do we observe the following?
e 3 . The natural system
Darwin’s answer was unequivocal:
The real affinities of all organic beings are due to inheritance of community of descent. The natural system is a genealogical arrangement, in which we have to discover the lines of descent by the most permanent characters, however slight their vital importance may be. 5
Patterns of Geographical Distribution
Darwin was fascinated by the connection between life and where that life was found on this earth. Before Darwin, the only viable explanation of this connection was that God chose to put it there. Darwin is the founder of modern, causal biogeography.6 And biogeographical facts are, perhaps, the most widely used evidence in On the Origin of Species .
e 4 . Patterns of geographical distribution
Let’s start with a macro question. If an omniscient and all-loving God deliberately created each species to fit perfectly with its environment, why do we see such diversity in virtually identical climates between the Old and New Worlds?
If we travel over the vast American continent, from the central parts of the United States to its extreme southern point . . . There is hardly a climate or condition in the Old World which cannot be paralleled in the New—at least as closely as same species generally require. . . . Not withstanding this general parallelism in conditions of the Old and New Worlds, how widely different are their living productions! 7
From continents separated by whole oceans to islands separated by just a few miles of ocean, the microquestions of biogeographical distribution are just as puzzling on the theory of special creation.
The same law which has determined the relationship between the inhabitants of islands and the nearest mainland, is sometimes displayed on a small scale, but in a most interesting manner, within the limits of the same archipelago. Thus each separate island of the Galapagos Archipelago is tenanted, and the fact is a marvelous one, by many distinct species; but these species are related to each other in a much closer manner than to inhabitants of the American continent, or any other quarter of the world. 8
Descent with modification, of course, beautifully answers both questions. The flora and fauna in the Old and New Worlds are generally different because they spring from very different lines of descent. Species in the Galapagos (think of his famous finches) all descend from a common ancestor on the South American mainland but have different histories of descent on the individual islands.
FIGURE 3. Frog, lion, dinosaur, and hawk. Retrieved from https://www.flickr.com/x/t/0094009/photos/aspidoscelis/31098104412/ , https://www.flickr.com/x/t/0097009/photos/mathiasappel/26260010225/ , https://www.flickr.com/x/t/0093009/photos/internetarchivebookimages/14777663574/ , and www.flickr.com/x/t/0098009/p...s/29599225312/ (respectively).
Morphological Facts
Morphology is the science of shape and form, hence the computer notion of an image “morphing.” Consider the following four tetrapods. Why the common four-“leg” structure? The frog hops, the lion runs, this particular dinosaur swam, and the bird flies. If you were engineering a hopping machine, a running machine, a swimming machine, and a flying machine, would you automatically use the same overall design?
Darwin saw morphology as fundamental to his defense of descent with modification.
We have seen that the members of the same class, independently of their habits of life, resemble each other in the general plan of their organization. This resemblance is often expressed by the term “unity of type” . . . The whole subject is included under the general name of Morphology. This is the most interesting department of natural history, and may be said to be its very soul. 9
Immediately following this quote, Darwin articulates as a question the data regarding the forelimbs in mammals that is perhaps, for my students at least, the most convincing bit of evidence for common descent.
What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of a horse, the paddle of the porpoise, and the wing of a bat, should all be constructed on the same pattern, and should include the same bones in the same relative positions? 10
Pretty darn curious, wouldn’t you agree?
e 5 . Morphological commonalities
Embryological Facts
As Darwin remarks several times in his discussion of embryos, just as the remarkable similarity in the bones in the forelimbs of mammals require an explanation, curious features of embryos also must be accounted for.
How, then, can we explain these several facts in embryology,—namely the very general, but not universal difference in structure between the embryo and the adult;—of parts of the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at this early period of growth alike;—of embryos of different species within the same class, generally, but not universally, resembling each other;—of the structure of the embryo not being closely related to its conditions of existence, except when the embryo becomes at any period of life active and has to provide for itself;—of the embryo apparently having sometimes a higher organization than the mature animal, into which it is developed. I believe all these facts, as follows, on the view of descent with modification. 11
Give me a common ancestor, Darwin seems to say, and I can explain the circuitous route, with many detours, from egg to adult in animal development—why young baleen whales develop teeth, why land-living vertebrates (including ourselves) go through a gill-arch stage, and why higher vertebrates have a notochord.
There is no obvious reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all the parts in proper proportions, as soon as any structure became visible in the embryo. 12
e 6 . Embryological oddities
I must tell you here that were you to Google “Darwin, embryology,” you would find that some of the sites are highly critical of Darwin and argue that his appeals to embryology have been discredited in modern biology. This isn’t really true, but we can make sense of these, I believe sincere, criticisms. Part of the problem Darwin faced in his section on embryology was that centuries of thought had tied embryonic development to the static scala naturae that we discussed earlier. These scientists believed that each stage in embryonic development represented an earlier, nonchanging stage in the hierarchy of life. Darwin, of course, completely rejected this view, but it remained part of the common (mis)understanding within embryology.
Even more problematic, though, was that many scientists sympathetic to biological change believed that there was a very strong correlation between the different stages of the history of life—its phylogenetic structure and the different stages of an individual’s embryonic development. Thus one of Darwin’s contemporaries, Ernst Haeckel, claimed
ontogeny is a concise and compressed recapitulation of phylogeny, conditioned by the laws of heredity and adaptation. 13
We now know that the recapitulation theory is mistaken and that embryonic development is much more complicated than either Darwin or Haeckel could have ever imagined. Darwin conceived of descent with modification as applying to individuals exemplifying a species—that species’ phenotype (its appearance and behavior). Modern biology, though, also includes the descent with modification of its genotype (the genetic instructions for building the phenotype) and if that were not complicated enough, it also must include the descent with modification of the underlying biochemical processes that take the information in the genotype and physically develops the individual. We are really only just getting a handle on all this in the twenty-first century.14
Darwin’s Evidence for Descent with Modification
The evidence can now be schematized.
e 1 . The earth is much older than had been previously believed—thousands of millions of years.
e 2 . The fossil record
e 3 . The natural system
e 4 . Patterns of geographical distribution
e 5 . Morphological commonalities
e 6 . Embryological oddities
t 0 . Descent with modification
The central question in inference to the best explanation is always the same—is t 0 the best explanation? We’ve already discussed the two serious rival explanations in Darwin’s time.
t 1 . Fundamentalist special creation
t 2 . Relaxed special creation
Within ten years or so of the publication of On the Origin of Species , say 1870, up to this first decade of the twenty-first century, there has been clear, overwhelming consensus in the broad scientific community that descent with modification—evolution—does such a manifestly better job of explaining all this uncontroversial data and that the evidence is so strong that we can talk of common descent as a scientific fact . You, of course, must rank order the explanations for yourself. Some of you will insist on a different ranking, and I maintain that is your moral and intellectual right. My job as a philosopher and a teacher is accomplished if you can simply see why Darwin, his contemporaries, and his scientific descendants all thought the evidence was so powerful. I do want to remind you, however, that many traditional theists have seen complete consistency between mainstream religious doctrine and evolution. Consider the words of Richard Swinburne, for many years the Nolloth Professor of the Philosophy of the Christian Religion at the University of Oxford, at the beginning of his book The Evolution of the Soul :
Men evolved from apes, and apes from more primitive animals, and the primitive animals evolved from the soup of inanimate atoms which consolidated to form the Earth some four thousand million years ago. Although there is much uncertainty about the exact stages and mechanisms involved, the fact of evolution is evident. 15
Natural Selection
Why do we see such change in biological history? Why does this change so often seem exactly what is required for changing circumstances? What is the engine that drives descent with modification?
In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts, might come to the conclusion that each species had not been independently created, but had descended, like varieties from other species. Nevertheless, such a conclusion, even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified, so as to acquire that perfection of structure and coadaptation which most justly excites our admiration. 16
One of my teachers called natural selection an algorithm, and Ernst Mayr, who I will be borrowing heavily from in this section, calls the theory “very logical.”17 All this could be taken to mean that natural selection is automatic or that the inference is deductive. The argument, though, is explanatory, just like the argument for common descent. I will follow Mayr, indeed I will use his wording, and treat Darwin’s reasoning as “three inferences based on five facts.”18
Three of these facts provide evidence for what he called the “struggle for existence.”
A struggle for existence inevitable follows from the high rate at which all organic beings tend to increase. Every being which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must be in every case a struggle for existence, either one individual with another of the same species, or with distinct species, or with the physical conditions of life. 19
e 7 . All species have such great potential fertility that their population size would increase exponentially . . . if all that are born would again reproduce successfully.20
e 8 . Except for minor annual fluctuations and occasional major fluctuations, populations normally display stability.21
e 9 . Natural resources are limited. In a stable environment, they remain relatively constant.22
t 1 0 . There is a fierce struggle for existence among individuals of a population, resulting in the survival of only a part, often a very small part, of the progeny of each generation.23
The game of life is unfair—not all compete in this struggle for existence equally. Darwin, who was a pigeon breeder and recognized its relevance to his argument, saw that some individuals in any species would have slight advantages (and others would have slight disadvantages) in surviving long enough to reproduce. Nature, itself, would be selecting.
Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. 24
e 10 . No two individuals are exactly the same; rather, every population displays enormous variability.25
e 11 . Much of this variation is heritable.26
t 2 0 . Survival in the struggle for existence is not random but depends in part on the heredity of the surviving individuals. This unequal survival constitutes a process of natural selection.27
From t 2 0 , Darwin makes one more inference that gives him the title for his book and the explanation of descent with modification.
t 3 0 . Over generations, this process of natural selection will lead to a continuing gradual change of populations—that is, to evolution and to the production of new species.28
One Long Argument
Darwin called On the Origin of Species “one long argument.” I have stressed that he actually defended two, quite distinct theories, but at the same time, it’s easy to see the truth in the one long argument characterization. Having clearly presented the evidence for descent with modification, he then (actually, this is not the progression in the book) lays out the case for natural selection.
e 1 . The earth is much older than had been previously believed—thousands of millions of years.
e 2 . The fossil record
e 3 . The natural system
e 4 . Patterns of geographical distribution
e 5 . Morphological commonalities
e 6 . Embryological oddities
t 0 . Descent with modification
e 7 . All species have great potential fertility.
e 8 . Populations normally display stability.
e 9 . Natural resources are limited.
t 1 0 . Fierce struggle for existence—more are born than will reproduce
e 10 . No two individuals are exactly the same.
e 11 . Much of this variation is heritable.
t 2 0 . Survival in the struggle for existence is not completely random; it is the product of natural selection.
t 3 0 . Over generations, this process of natural selection will lead to evolution and to the production of new species.
As always, rival explanations are possible. Almost every biologist now accepts t 0 , t 1 0 , and t 2 0 as the best explanations. Some controversy remains, however, regarding t 3 0 . Most grant that natural selection does result in species change, but some question whether it is the primary cause. This kind of controversy, rather than calling evolution into question, is precisely what healthy science is all about. There do remain questions, not about the overall theory, but about the details. Darwin wrote On the Origin of Species in ignorance of genetics, population biology, and molecular biology. It is quite remarkable that these revolutions, rather than undercutting his theories, actually over time came to add further support.
EXERCISES
- 1. Darwin marshaled a lot of evidence in support of descent with modification—e 2 through e 6 . If you had to make his case using just one of these categories of data, which would you choose? Why?
- 2. Can you think of any rival explanations to t 1 0 , t 2 0 , or t 3 0 ?
QUIZ EIGHT
The quiz for this chapter is to write a short (no more than three pages) paper on Darwin’s two theories. Your paper should do three things. It should carefully explain Darwin’s theory of common descent (or descent with modification). It should then explain what natural selection is. Finally, it should use the tools of inference to the best explanation (the IBE recipe) to critically assess the quality of Darwin’s evidence for the first part of his “one long argument,” the theory of descent with modification .
Notes
1. Charles Darwin, On the Origin of Species (Cambridge, MA: Harvard University Press, 1964), 1.
2. Darwin, 484.
3. Darwin, 312.
4. Darwin, 411.
5. Darwin, 479.
6. Ernst Mayr, The Growth of Biological Thought (Cambridge, MA: Harvard University Press, 1982), 446.
7. Darwin, Origin of Species , 306–7.
8. Darwin, 338.
9. Darwin, 434.
10. Darwin, 434.
11. Darwin, 442–43.
12. Darwin, 442.
13. Quoted in Mayr, Growth of Biological Thought , 474.
14. See John Maynard Smith, Shaping Life: Genes, Embryos, and Evolution (New Haven, CT: Yale University Press, 1998).
15. Richard Swinburne, The Evolution of the Soul (Oxford: Oxford University Press, 1997), 1.
16. Darwin, Origin of Species , 3.
17. See Daniel C. Dennett, Darwin’s Dangerous Idea (New York: Simon & Schuster, 1995); and Mayr, Growth of Biological Thought .
18. Mayr, Growth of Biological Thought , 479.
19. Darwin, Origin of Species , 63.
20. Mayr, Growth of Biological Thought , 479.
21. Mayr, 480.
22. Mayr, 480.
23. Mayr, 480.
24. Darwin, Origin of Species , 61.
25. Mayr, Growth of Biological Thought , 480.
26. Mayr, 480.
27. Mayr, 480.
28. Mayr, 480.