Functional Integration in Philosophy of Biology: Mechanisms, Individuality, and Cancer Research

This article contributes to debates in philosophy of biology concerning biological individuality, mechanistic explanation, and functional integration, with implications for cancer research and evolutionary theory.

Introduction: Functional integration, a familiar claim with hidden facets

When we open a biology or medical textbook, it seems obvious that an organism (for example, a plant or an animal) is a system composed of anatomical parts that work in a coordinated and integrated way.

It also seems self-evident that the heart works in integration with the lungs and the circulatory system to allow oxygen to reach all tissues. But is this sufficient to define functional integration?

Consider, for example, a car: its parts are functionally integrated to achieve a systemic goal (namely, movement). Yet a car is not considered an organism. Likewise, if we again consider the integration between heart, lungs, and circulation, is this sufficient to define an animal? What about its capacities to search for food and to interact with other animals?

It therefore appears that understanding functional integration as mere causal coordination among functions is not sufficient to define an organism.

Functional integration in medicine

Physiology and pathophysiology routinely employ the concept of functional integration. Medical physiology textbooks explain the functioning of the whole organism in terms of functional integration within individual organ systems and among different organ systems.

This physiological integration can be disrupted in pathological states. For example, heart failure is characterized by a loss of normal integration between cardiac contraction, cardiac output, and their regulation by the endocrine and autonomic nervous systems.

Pathological states may also give rise to a new form of functional integration in the form of physiological compensation. In heart failure, this involves compensatory responses by the heart, kidneys, endocrine system, and autonomic nervous system. This new pathological integration is what physicians usually call “disease”.

Functional integration in oncology

A disease in which the notion of functional integration plays a particularly important role is cancer. But what exactly is cancer? In simple terms, cancer is the abnormal proliferation of cells (a process that physicians and biologists call carcinogenesis). These cells can also detach from the original tumor, travel through the bloodstream or lymphatic system, and colonize other tissues, a process known as metastasis.

In cancer, we observe a twofold phenomenon. On the one hand, cancer cells establish a strong functional integration among themselves and with the surrounding cells in their local environment. On the other hand, this new level of cellular integration comes into conflict with the broader functional integration of the organism as a whole, ultimately contributing to its disruption. Cancer can therefore be understood as a disease in which a new level of functional integration emerges, but at the expense of the functional integration of the entire organism.

Functional integration in biology

Molecular biology, genetics, and biochemistry are fundamental parts of cell biology, which studies cellular processes such as metabolism, intracellular and intercellular communication, and the cell cycle. The latter includes cell growth, DNA replication, transcription and translation, and reproduction.

These processes are often defined integrated for three reasons: first, metabolism provides the cell with the energy for all its activities, thus sustaining each phase of the cell cycle; second, both metabolism and the cell cycle hinge on numerous intra- and inter-cellular signals that collectively constitute cellular communication; finally, each cell produces chemical signals that responds to the cell’s internal conditions (e.g., metabolic and developmental), so each cell-cell communication is related to the metabolic and developmental conditions of each cell. Despite its frequent use, the concept of functional integration is not conceptualized as such in cell biology and do not explain which cellular processes make a cell a functionally integrated whole.

In evolutionary biology, functional integration does not seem to play an explanatory role. However, evolutionary transitions can be interpreted as global modifications in the functional integration of a certain biological organization. Indeed, what evolves are only genes and phenotypic traits but also the functions performed by these traits and the way they are integrated to perform systemic properties.

What is a biological function?

To grasp the meaning of “functional integration,” it is necessary to go back to the roots by asking: what is a biological function? In other words, how should we answer the question, “What is the function of the heart?”

Philosophers have answered this question in different ways. Some (etiological accounts) argue that the function of a biological trait consists in its evolutionary history. Thus, the function of the heart consists in pumping blood, a process selected by natural selection. In a slightly different way, dispositional accounts hold that a function is the propensity of a trait to increase an organism’s fitness. Accordingly, the blood-pumping function has evolutionarily provided animals with a fitness advantage. Others (systemic accounts) maintain that functions are what contribute to the physiology of an organism. On this view, the function of the heart is its contribution to the overall physiology of the animal. Finally, organizational accounts attempt to synthesize etiological and systemic views by arguing that a function is both the result of an evolutionary history and a current contribution to systemic physiology.

Within these philosophical frameworks, the notion of functional integration plays no role in etiological and dispositional accounts and is only minimally examined in systemic and organizational accounts. While these approaches acknowledge the foundational role of functional interdependence, they do not explicitly explore or define functional integration within the broader context of biological systems.

Integration of mechanisms

Contemporary biology and medicine cannot be understood without a fundamental concept: biological mechanism. As emphasized by neo-mechanistic philosophy, mechanisms are entities and activities organized in such a way that they produce regular changes in a biological system. Thus, for example, when a drug is administered to a patient, we speak of a “mechanism of action”, referring to the way the drug interacts with cellular receptors to induce a systemic change.

Biological functions (e.g., feeding, breathing, acting) can therefore be explained in terms of specific biochemical and biophysical mechanisms. Mechanisms thus play a key role in how biologists, physicians, and pharmaceutical industries explain the functioning of cells, organisms, and drugs.

What, then, is the relation between functional integration and mechanisms? Quite simply, we can explain how functions are integrated with one another by referring to the biological mechanisms that underpin them. Accordingly, we can distinguish two basic senses of “mechanistic integration”: the integration of component operations within a mechanism (intra-level integration) and the integration of multiple mechanisms to produce a phenomenon (inter-level integration).

Individuals as integrated wholes

In philosophy and biology, the concept of the organism has often been equated with that of the biological individual, both defined as an “integrated system of interdependent structures and functions” or as “integrated wholes.” But what does it exactly mean to say that an organism is an integrated whole?

There are two main ways to answer this question. The first is to say that an individual is a physiological unit: an individual displays a coherent physiological organization that allows it to count as an integrated whole. Some authors have identified the physiological unit with metabolic capacities, others with the immune system, and still others with the absence of conflict and the presence of high cooperation among parts.

A second way to address this issue is to claim that an individual is an integrated whole if it is an object of natural selection. For example, we might say that the giraffe lineage constitutes an evolutionary individual because, over generations, it has been the object of selective pressures that shaped the elongation of the neck.

What is interesting is that functional integration appears to be the common thread between these two interpretations of individuality. On the one hand, an individual is a physiological unit because its functions are causally interdependent. On the other hand, an individual can be an evolutionary unit because it is already a physiological unit upon which natural selection can act.

Is it possible to have physiological units without evolutionary units, or evolutionary units without physiological ones? The first possibility seems plausible: consider some transient collective associations that are physiologically unified but lack evolutionary unity. The second possibility, by contrast, seems highly improbable. A full-fledged account of how an individual qualifies as an integrated whole therefore requires explaining how functional integration grounds both physiological and evolutionary individuality.

Conclusion

Functional integration is widely used in medicine and biology, where it plays an explanatory role of central importance. Nevertheless, it remains poorly characterized at the conceptual level.

In philosophy, functional integration is intimately connected to debates about the nature of biological functions, mechanisms, and individuals. Indeed, functional integration emerges as a fundamental feature of biological individuals, comprising interconnected systemic functions that rely on integrated molecular and biochemical mechanisms. This helps explain why functional integration features so prominently in explanations across the life sciences.

Several key questions arise: Which systemic functions and mechanisms must be integrated to constitute a physiological unit? What about an evolutionary unit? How does functional integration illuminate the emergence of new forms of individuality in natural evolution?

For more details see Chapter 1 of the Book Functional Integration: A Theoretical Enquiry into the Biological Unit of the Individual.

This article is part of a series on functional integration and biological individuality.