From Groups to Individuals: The New Science of Biological Individuality
Exploring how evolutionary processes transform simple groups into integrated biological individuals
What Is an Individual, Really?
Imagine walking through a forest in Michigan and coming across a humungous fungus—a single entity spanning 1500 years and weighing more than ten tons, yet genetically identical throughout. Is this one individual or a colony of many? Or consider a coral reef, a spectacular structure built by countless polyps working with symbiotic algae. Is the reef itself a living individual? These questions lie at the heart of one of biology's most fascinating and evolving debates: what makes something a biological individual?1
For centuries, we've operated with an intuitive understanding of biological individuality. We look at animals, including ourselves, and see distinct, countable entities. However, this intuitive view is rapidly collapsing under the weight of scientific discovery. From ant colonies that function like superorganisms to our own bodies—composed of ten times more bacterial cells than human cells—biology is revealing that individuality is far stranger and more wonderful than we ever imagined1 8 .
This article explores the revolutionary new perspective that has emerged in recent decades: that biological individuality is not a fixed property but a derived characteristic that can evolve at different levels of biological organization. We'll journey through the science revealing how simple groups of organisms can evolve into integrated individuals, examine a cutting-edge experiment that demonstrates uniqueness in biological motion, and equip you with the conceptual toolkit scientists use to probe the boundaries of life's fundamental units8 .
The Paradigm Shift: From Fixed Categories to Evolved Individuality
Rethinking the Living World
The traditional view of biological individuality centered almost exclusively on organisms, particularly animals. However, starting in the late 20th century, this perspective underwent a radical transformation fueled by several key developments5 :
The hierarchical perspective
Biologists recognized that life organizes at multiple levels, from genes and cells to organisms and societies, with each level sharing functional features with others8 .
Social evolution theory
Research revealed that cooperation and conflict at one level can lead to the emergence of new individuals at higher levels8 .
Problematic paradigm cases
Scientists encountered increasing numbers of "difficult cases" that challenged traditional definitions—colonial organisms, symbioses, organisms with complex life cycles, and microbes that exchange genes6 .
These developments led to a crucial realization: individuality is not a starting point but an achievement—a product of evolutionary processes that can occur at different biological levels8 .
Two Lenses on Individuality
In contemporary biology, researchers typically approach individuality through two complementary perspectives:
| Evolutionary Perspective | Physiological Perspective |
|---|---|
| Focuses on units of selection | Focuses on functional integration |
| Asks: Does it evolve adaptations? | Asks: Is it metabolically integrated? |
| Key timescale: Evolutionary history | Key timescale: Organism's lifetime |
| Examples: Genes, virus strains | Examples: Organisms, some symbioses |
These perspectives don't always identify the same entities as individuals. For instance, a virus doesn't have its own metabolism but can evolve through natural selection, making it a Darwinian individual but not a physiological one. Some symbiotic associations function as integrated physiological wholes but haven't evolved as units of selection8 .
The Evolutionary Toolkit: How Groups Become Individuals
The transition from groups to individuals represents one of biology's most profound processes. Evolutionary theorists have identified several key mechanisms that facilitate this transition:
Alignment of fitness interests
When the evolutionary interests of lower-level units align, conflict decreases and cooperation increases8 .
Reproductive specialization
The evolution of specialized reproductive cells (like germ cells) alongside somatic (non-reproductive) cells helps maintain the integrity of the emerging higher-level individual5 .
Policing mechanisms
Mechanisms that suppress competition among lower-level units help maintain functional integration5 .
Environmental drivers
Surprisingly, recent research suggests that environmental factors—not just internal properties—can play crucial roles in individuation. Factors like ecological disturbance can sometimes act as external individuation mechanisms4 .
This framework helps explain major evolutionary transitions, such as the evolution of multicellular organisms from single-celled ancestors, or the emergence of complex societies like those of ants and bees8 .
Case Study: The Gait Signature Experiment
The Hypothesis
If biological individuality is real, we should be able to detect it in unexpected places. This premise inspired a fascinating 2023 study that asked: Can humans be identified by their walking patterns? The researchers hypothesized that if each person has a unique "gait signature," this would demonstrate biological individuality in a complex, coordinated behavior2 .
Methodology
The research team turned to machine learning to tackle this question with unprecedented scale and precision:
- Data Collection: They gathered 5,368 bilateral ground reaction force recordings from 671 healthy individuals during level overground walking from three publicly available datasets2 .
- Measurement: Ground reaction forces (GRFs) represent the forces exerted by the ground on a body during walking. These forces were measured in three dimensions: vertical, forward-backward, and side-to-side2 .
- Analysis: The researchers used four different machine learning algorithms to determine if individuals could be identified based solely on their GRF patterns2 .
Machine Learning Performance in Identifying Individuals by Gait
| Machine Learning Method | Classification Accuracy |
|---|---|
| Support Vector Machines (linear) | 99.3% |
| Random Forests | 98.7% |
| Convolutional Neural Networks | 95.8% |
| Decision Trees | 82.8% |
Results and Implications
The findings were striking: individuals could be identified with 99.3% accuracy using the bilateral signals of all three ground reaction force components. Only 10 out of 1,342 recordings in the test data were misclassified. This remarkable accuracy persisted across different days, laboratories, age groups, and sexes2 .
High Uniqueness
Human gait patterns satisfy stringent biometric demands with both high uniqueness and permanence over time2 .
Comprehensive Representation
The combination of bilateral GRF signals with all three components provides a comprehensive representation of an individual's unique locomotion signature2 .
Practical Applications
This research extends beyond theoretical interest—it has practical applications in personalized healthcare, clinical diagnosis, and therapeutic interventions2 .
The experiment demonstrated that biological individuality manifests even in complex behavioral traits, not just in static anatomical or genetic features. The ability to quantify individual differences in movement could revolutionize how we approach rehabilitation and monitor health conditions2 .
The Scientist's Toolkit: Key Concepts in Individuality Research
Modern biologists investigating individuality employ a diverse conceptual toolkit. Understanding these concepts helps illuminate how researchers approach this complex field:
| Concept | Description | Scientific Utility |
|---|---|---|
| Evolutionary Individual | An entity that functions as a unit of natural selection | Helps identify levels at which adaptation occurs |
| Physiological Individual | An integrated metabolic unit with coordinated parts | Reveals functional organization in living systems |
| Pluralism | Recognition that different definitions of individuality serve different research purposes | Allows flexibility in addressing diverse biological questions |
| Problem of Vagueness | Many biological entities exist on a spectrum between group and individual | Acknowledges the continuous nature of biological reality |
| Multiple-Criteria Problem | Different individuality criteria (genetic, developmental, immune) don't always align | Encourages precision about which aspect of individuality is relevant |
This toolkit reflects a significant shift from earlier approaches. As one philosopher of biology notes, there's now a "quasi-consensus" on several important claims: that questions about individuality are context-dependent, that we should reject anthropocentric biases, that a hierarchical approach is necessary, and that individuality comes in degrees rather than being all-or-nothing5 .
Conclusion: Individuality as Process and Relationship
The science of biological individuality has come a long way from its origins in common-sense observations about organisms. We now understand that individuality emerges through evolutionary processes when groups of biological entities align their interests and integrate their functions8 .
This perspective has profound implications. It reveals that the living world is organized not in rigid hierarchies but in fluid, dynamic systems where new individuals can emerge at different levels. From the symbiotic partnerships within our own bodies to the complex societies of social insects, nature repeatedly demonstrates that drawing boundaries around "individuals" is as much a scientific challenge as a philosophical one1 8 .
Key Takeaways
- Individuality is not fixed but evolves at different biological levels
- Groups can transition into integrated individuals through evolutionary processes
- Multiple perspectives (evolutionary, physiological) provide complementary insights
- Biological individuality manifests in unexpected places, like human gait
- Individuality is relational, not isolated
Perhaps most importantly, this revised understanding of individuality highlights the relational nature of biological reality. As research continues to uncover, biological individuals are not isolated atoms but nodes in complex networks of dependence and interaction—from the zooxanthellae that give corals both color and life to the bacterial communities without which we could not digest food or develop properly1 .
The journey from groups to individuals represents one of biology's most exciting frontiers, reminding us that in the living world, the whole is not only greater than the sum of its parts—sometimes, the whole becomes a new kind of thing entirely.