The entanglement between reproduction and development

This article examines how reproduction and development are functionally integrated, enabling lineages to persist across generations.

Introduction

In evolutionary biology, a lineage is a series of organisms that are directly connected through continuity between ancestor and descendant. So, for example, as a human being, I belong to the same lineage as my ancestors who lived in the 16th century. However, the notion of lineage is far from being trivial and raises a fundamental question: what allows a lineage to persist through time?

At its core, the answer rests on two fundamental conditions. First, organisms must be able to reproduce as integrated wholes, generating new biological systems that resemble their parents both genetically and phenotypically. Second, individuals within a lineage must not reproduce equally: some variants leave more descendants than others, selectively passing on their traits. It is this differential reproduction that drives natural selection and, over generations, shapes the evolutionary trajectory of life.

Yet behind this seemingly straightforward picture lies a deeper conceptual puzzle. What exactly counts as a reproducing individual in evolution? More specifically, must an organism reproduce as a whole in order to qualify as a unit of selection, that is, as an evolutionary individual?

This question has been at the heart of a long-standing debate in the philosophy of biology. One influential view, defended by Hull and Godfrey-Smith, holds that an organism must reproduce as a whole to count as an evolutionary individual. In contrast, Ereshefsky and Pedroso argue that evolutionary individuality does not necessarily depend on systemic reproduction, challenging the idea that a collective reproductive system is required.

The philosophical and biological debate on reproduction and individuality

Over the past five decades, debates in biology and philosophy have increasingly linked reproduction to two key dimensions of biological individuality: the evolutionary and the physiological. On the one hand, an evolutionary individual must possess the capacity to reproduce and generate offspring that can vary and be subject to selection. On the other hand, reproduction is also an integral part of the life cycle and developmental processes of a physiological individual. Reproduction thus occupies a distinctive, perhaps ambiguous, position at the intersection of ontogeny (the development of an organism) and phylogeny (the evolution of lineages).

This tension is reflected in how biologists have conceptualised the reproducibility of living systems. Dawkins and Hull were among the first to formalise the issue, introducing the concept of the replicator: an entity capable of transmitting its biological features to its descendants. More recently, however, attention has shifted from replication to reproduction, particularly because the fundamental units of life (cells) do not merely replicate; they reproduce.

Although closely related, replication and reproduction refer to different processes. Replication involves the copying of information or structure, often understood as the resemblance between an original and its copy; in genetics, it specifically concerns the transmission of DNA. Reproduction, by contrast, involves concrete biological processes (such as cellular fission and fusion) that maintain material continuity between parent and offspring. For this reason, Griesemer has proposed replacing the term replicator with reproducer to describe entities that not only multiply but also pass on their material organisation.

A related challenge arises when we consider life cycles: the series of developmental stages from birth to death. These are typically defined by a clear distinction between reproduction, which marks the transition from one generation to the next, and development, which involves transformations within the same individual. However, as Fusco and Minelli point out, this distinction is not always easy to maintain. In many cases, it is difficult to determine when a transformation preserves the identity of an individual and when it gives rise to a new one.

The problem becomes even more pronounced in organisms with complex life cycles, where individuals may undergo dramatic and discontinuous changes in form, behaviour, or environment. In such cases, the boundary between development and reproduction becomes blurred, complicating our understanding of how biological individuals persist and transform over time.

Functional integration, reproduction, and biological individuality

Reproduction operates through three interconnected levels of mechanistic integration. To illustrate them, let us consider reproduction in unicellular organisms.

The first level involves nutrient-dependent signals and regulatory molecules that link environmental conditions to cell growth and division. For example, metabolites and regulatory compounds can either promote or inhibit key processes like cell growth, DNA replication, and division. In this way, cells ensure that these phases occur only under favourable conditions and in a coordinated manner.

The second level consists of regulatory proteins that organise the cell cycle itself. These mechanisms enable both prokaryotic and eukaryotic cells to coordinate growth and division as interdependent processes. They ensure the precise timing and ordering of developmental events (such as increases in cell size, DNA replication, transcription, and chromosome segregation) with division processes like binary fission in bacteria or mitosis and meiosis in eukaryotes.

A third level of integration is provided by cytoskeletal structures, which act as internal scaffolds. These structures not only facilitate chromosome segregation and septum formation, but also ensure the spatial organisation of the cell during division. As a result, the cell reproduces as a coherent and functionally integrated whole.

Conclusion: how functional integration grounds lineages

Taken together, these three levels of integration enable a unicellular organism to reproduce while preserving its organisational structure in its offspring. Because reproduction depends on the coordination of metabolic activity, growth, and division, it is inherently sensitive to system-level conditions such as nutrient availability, energy production, cell size, and DNA replication status. In this sense, reproduction is not a single event but a tightly regulated, system-wide process.

Understanding this physiological integration is crucial for explaining the persistence of individuality across generations. It shows that reproduction is not merely the production of offspring, but the outcome of a highly coordinated network of processes that sustain the organism as an integrated whole.

For more details see Chapter 8 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.