Rereading Darwin's Origin of Species

The Hesitations of an Evolutionist

Richard G. Delisle, James Tierney

• Editorial: Bloomsbury Academic
• Año: 2022
• ISBN:
  9781350259591
• Edición: 1
• Categoría: Literature
• Categoría: Modern Literary Criticism

Widely seen as evolution's founding figure, Charles Darwin is taken by many evolutionists to be the first to propose a truly modern theory of evolution. Darwin's greatness, however, has obscured the man and his work, at times even to the point of distortion. Accessibly written, this book presents a more nuanced picture and invites us to discover some neglected ambiguities and contradictions in Darwin's masterwork. Delisle and Tierney show Darwin to be a man who struggled to reconcile the received wisdom of an unchanging natural world with his new ideas about evolution. Arguing that Darwin was unable to break free entirely from his contemporaries' more traditional outlook, they show his theory to be a fascinating compromise between old and new. Rediscovering this other Darwin – and this other side of On the Origin of Species – helps shed new light on the immensity of the task that lay before 19th century scholars, as well as their ultimate achievements.

Part One

The Charles Darwin We Think We All Know

1

A Primer of Evolution’s Complexities

Before taking up Charles Darwin’s intellectual achievements, it will be useful for us to review a few of the key elements of evolutionary theory in its contemporary form. This may serve to refresh the memories of readers who have been away from these issues for a while and to familiarize general readers not intimately acquainted with the details of the subject. Others more familiar may give this a cursory glance, if only to see where we are headed in later chapters, as this overview of these particular aspects will later form the central themes of the book. Our hope here is to leave readers with a vivid sense of the staggering complexity of evolution. Since one of our central contentions is that our modern understanding has saturated our assessment of the Origin of Species and of Darwin’s achievements therein, this chapter will be a first step toward elucidating the contrast between evolutionary theory then and now and will serve to lay out specific bones of contention to be taken up in subsequent chapters.

Unity versus Disunity

Life is characterized by a strange and fascinating paradox. Looked at from one perspective, one can’t help but notice its profound unity. The fundamental building blocks of all currently living forms are identical; all are built out of a basic, four-part molecular “code” consisting of four substances (proteins, to be precise): adenine, thymine, cytosine, and guanine, which combine in different ways as a kind of genetic alphabet, of which many organisms can be made. Better known as DNA, these form the biological blueprint for all living things: kingdoms as far apart as animals, plants, and fungi, for instance, share the same molecular code. At the same time, seen from another perspective, life is profoundly diverse. No fewer than several million species roam the Earth today, all distinct in appearance, each with its own “lifestyle.” Countless others have come and gone since life’s beginnings. How can this be explained? Reconciling these two sharply opposed truths is where evolution’s complexities begin, and making sense of it all has dominated the research agenda of scholars since evolutionism’s beginnings in the nineteenth century.

Living organisms are organized around biological levels. For our purposes, the cellular level makes for a good place to start, since cells are in some sense the basic building blocks of living things; an organism is, in effect, a collection of cells but is not merely so. Inside cells is housed the genetic material that provides instructions for the structure, maintenance, and reproduction of living organisms. These bits of material, commonly referred to as genes, are built out of the four-letter molecular code (DNA) mentioned above. Thus far we have described three biological levels: the four-letter molecular code constituting the basic material for the genes, the genes proper, and the cells housing the genes. Moving upward, we can further distinguish between two biological levels that are readily observable and more familiar: the body’s individual parts and the organism as a whole. While the genes of a single organism are the same throughout all the cells of that organism, the cells themselves come in a range of types serving specialized roles and functions in the body. For instance, skin cells, muscle cells, blood cells, bone cells, and liver cells are all different from one another. These distinct and specialized organs or structures combine to form a complete and functional organism, be it a starfish, an ant, or a human being.

This simple sketch of biological levels should suffice for present purposes, but it is worth saying a brief word about how these levels are linked to each other. Although all life shares the same four-letter molecular code, the vast, nearly infinite number of ways of combining the four letters means that each species possesses its own unique set. This is reflected in the sum of genes shared by all the members of the same species. This larger set of genes shared by a species is called a gene pool. Similarly, the number of possible gene combinations between members of a sexually reproducing species is so immense that only a miniscule portion of that variability can be expressed in a living form at any moment of its evolutionary history. Each living creature thus represents, in a sense, a single throw of the dice. The picture that emerges is one of organisms whose separate biological levels are interconnected in complex ways. Indeed, while the gene pool of a particular species is based on the four-letter molecular code, that code is open enough to allow for the rise of a myriad of distinct gene pools, which is reflected in the number of existing species. Similarly, while the gene pool of a particular species is responsible for the range of variations observable among its members, a significant part of that genetic variability is never expressed. Again, the astronomically large number of possible combinations far outstrips the capacity of any species to actualize them through reproduction in a world where resources are always limited.

The organization of species around biological levels is usefully captured in the fundamental distinction between genotype and phenotype. The genotype refers to those parts of an organism unobservable to the naked eye, such as the genes and their constituent molecules. These can be thought of as the “lower” levels of living matter. The phenotype designates the parts observable to us, the structures and organs that make up a whole organism, the “higher” levels of the biological organization of living matter. This distinction enables us to reframe our earlier question about the unity of life as, “how can we reconcile the evident unity (uniformity) of life at lower levels with its disunity (apparent diversity) at higher ones”? This leads us to wonder whether the aforementioned unity is always explained by genealogical links between living forms, and the disunity by their independent origins, a question that might appear readily answerable in the affirmative. Unfortunately, evolution’s complexities do not allow for such a straightforward reply. To see why, we must first look more closely at the lower biological levels and the several issues surrounding them.

Monophyletism and Polyphyletism

We know from the fossil record that around 3.5 billion years ago, simple, single-celled organisms inhabited the surface of Earth. These sprang out of a transformation process called “abiogenesis,” which gave rise to basic organic compounds, the building blocks upon which life rests. The gradual cooling of the Earth from its initial molten state created the physical conditions for the rise of more complex organic molecules and self-replicating units (such as DNA), which have since greatly multiplied and evolved into an impressive array of simple and complex life forms.

What is not entirely clear is whether primitive life arose only once or several times during these ancient geological times. Two possible scenarios are in play. According to the first (see Figure 1), life is thought to be polyphyletic (the corresponding view is known as polyphyletism), having several ancestral sources with life arising independently in different geographical locations and/or different geological horizons. Conversely, one might also suppose life is monophyletic in its origins, that it shares a single common ancestor. The fact that the wide diversity of life forms springs from the same DNA strongly favors monophyletism, the hypothesis that there is a single and unique ancestry to all life, at least on Earth (see Figure 2). Of course, this does not exclude the possibility that life on our planet went through a number of false starts in ancient times, with primitive life forms independently appearing here and there and maintaining themselves for various amounts of time, each finding different pathways and bridges across the nonliving/living threshold. Since only DNA-bearing organisms now remain, polyphyletism would imply that all but one genealogical strain of life—the one carrying DNA—has managed to survive and give rise to all the forms alive today. This explanation assumes the monophyletic origin of DNA carriers, while at the same time claiming that all other non-DNA carrying strains met with extinction.

Figure 1 Polyphyletism. Evolution is polyphyletic when a group of life forms has several ancestral sources. Two examples are provided here. In case 1, all A and B forms when taken together are said to be polyphyletic since they independently arose from two distinct sources, “y” and “z,” respectively. In case 2, although forms A, B, and C are ultimately bound in common ancestry (as seen in “w”), the fact remains that in a less distant past they arose from two separate and immediate ancestors, with B deriving from “y” and A and C stemming from “z.” With regard to these more recent ancestors, A, B, and C are said to be polyphyletic when taken together.

Figure 2 Monophyletism. Evolution is monophyletic when a group of life forms has a single ancestral source. In the two cases shown here, all forms share a single and unique ancestral root in the common ancestor “z.”

Despite appearing at first blush somewhat far-fetched, another possibility should be considered here, if only to illustrate the staggering complexities of biological evolution. Framing it as a question, we might ask whether several “alternative” DNA strains might have arisen independently of each other on ancient Earth, hitting upon precisely the same “solution.” While this is surely possible, it also runs against a deeply held assumption that there is but one sustainable way for living creatures to arise out of inorganic materials. If this were the case, it would mean that what we take today to be the unity of life bound together by monophyletism under a single DNA code is, in reality, hiding the polyphyletic origin of DNA and life in general. While this theoretical possibility should be contemplated in the name of a fully open and investigative science, it seems unlikely on its face, since it would directly imply that a complex macromolecule like DNA—whose function, again, is to encode the instructions for the structure, maintenance, and reproduction of living organisms—had been invented more than once on Earth. If one accepts that the more complex something is, the less likely it is to be replicated, this hypothesis would appear to have little going for it.

However, while polyphyletic DNA seems a longshot, it would be a mistake to reject polyphyletism as we climb to the level of the genotype, since we cannot underestimate the possibility of independent original sources for phenotypes. Of course, we are all familiar with look-alikes among people who appear so similar to one another they might almost be twins. Were they twins, however, this phenotypic similarity would be easy to account for: since they share the same parents, they also share the same genetic material. Look-alikes, who do not share parents, are less-than-perfect copies of each other. Nonetheless, the fact that humans share a common gene pool means that even remotely related individuals can sometimes look quite similar. Close similarities between unrelated creatures at the phenotypic level also testify to the vast arsenal of possibilities at nature’s disposal, its dynamic power to create what seem like “carbon copies” by different routes, highlighting, the contingency of any existing order.

Tackling the issue of polyphyletism at the phenotypic levels requires moving beyond the confines of a single species to a consideration of how life in general interacts with the outer world. Since its formation some 4.5 billion years ago, Earth has experienced continual change. We can think of these changes as falling into two types. Some emerge from the intermittent but perpetual geological events on our ever-changing Earth, which can subtly or radically change the playing field on which organisms must compete. Examples of such events include the gradually cooling of the Earth from its original molten state, the migration of continental lands on its surface, and episodes of glaciation or ice ages and their subsequent retreat. The list is long. We are now confronted with an all-too-apparent example in the form of global climate change: the Earth’s warming—albeit due to human factors—has made previously hospitable environments hostile to some organisms and vice versa: one can think here of changing growing seasons or the highly sensationalized northward march of fire ants, killer bees and, more recently, so-called Murder Wasps. These environmental changes are called abiotic factors. A second type of change is brought on by the encounters between life forms. A life form rarely has free reign to exercise its capacities to exploit nature’s resources. Earth has been inhabited by a myriad of forms for a very long time, all of which need resources to live. During the first few billion years of Earth’s history, single-celled organisms were the dominant form of life, with much more complex forms first appearing 500 or 600 million years ago. Earth has been pretty crowded ever since, posing many challenges to each species, who must share whatever food and accommodations a given habitat provides. Transformations brought about by the interaction between life forms are known as biotic factors.

The combination of biotic and abiotic factors exposes life to permanent challenges. If evolutionary change itself is a central reality of life, it is precisely because evolution is often the only way to respond to these challenges: to put it bluntly, the imperative is often as simple as “change or die.” This response is called adaptation and is most conspicuously reflected in the phenotypes of species, each differently adapted to make their living. Ever faced with the risk of complete and definitive extinction, life forms try to find ways to adapt to new conditions. Evolution has quite a bag of tricks at its disposal to help life forms respond to these constant and ever-changing challenges. In order to see why polyphyletism at higher biological levels is still a live option in explanatory terms, we will review some of the most prominent tactics nature employs, which form well-worn and recognizable pathways as it rolls out through time.

Anagenesis or Continuous Evolution

Just as you look (more or less) like your parents, you also differ from them somewhat, of course. When they made you, your parents transmitted the genes that made them what they are, which explains many of your shared physical similarities. At the same time, you are no carbon copy: differences were also introduced, a factor inherent in the very nature of sexual reproduction. The combination of your parents’ genes during your conception introduced some novelties reflected in, and contributing to, your uniqueness, your individuality as a physical being. Your sisters and brothers, should you have any, are not exactly like you, since your parents handed down different genes during each separate act of conception. It just so happens that sexually reproducing organisms contain a significant reservoir of genetic variations that can only express themselves at conception time. The only exceptions to this are identical twins. In this case, exactly the same genetic material is used for the conception of two individuals or more. Of course, twins are fairly rare.

Applying our reasoning to the entire human species, we can see how even in the simple process of reproduction the dice are cast and so automatically introduce, with each generation, novelties or new variations. The introduction of randomness in sexually reproducing species also endows a protective mechanism; continually generating new sets of variations helps such species cope with new external conditions through adaptation. This periodic injection of randomness acts as a mechanism for generating new evolutionary solutions, raw material for tomorrow’s life-saving adaptations.

One important piece is still missing from our understanding of the multiple and independent (polyphyletic) rise of phenotypic features. The existence of biological variations is merely a precondition for adaptation. Something more is needed to set the process of adaptation in motion: an encounter between the potential variations available to a specific species and the external conditions (biotic and abiotic) at a particular place and time in evolutionary history. If the climate becomes colder over a given stretch of time, for instance, organisms belonging the same species with resistance to the cold are more likely to survive than their conspecifics not so equipped. Through the grace of genetic variation, they may have wound up with, for example, a more efficient thermoregulatory system for internal heat, a thicker skin, or more body hair. The features that give such organisms an edge in a cold climate are precisely those that help them live long enough to reproduce and perpetuate the species in the next generation. Conversely, organisms unable to cope successfully with the colder climate will die out, no longer contributing to the set of variations transmitted to future generations, in effect withdrawing their variation...