The Great Chain of Beings? Not So Fast

How Philosopher David Hull Redefined Evolution

Why the "What is a Species?" Question is Deeper Than You Think

We all know the classic story of evolution: a slow, stately procession of life, from simple organisms to more complex ones, with species as the distinct rungs on the ladder of progress. But what if this intuitive picture is fundamentally wrong? What if a species isn't a static category but a dynamic, ever-changing historical entity, more like a sprawling, branching river than a single step on a staircase?

This radical shift in perspective is at the heart of the work of David Hull, a philosopher of biology who dared to ask not just how evolution works, but what it is fundamentally made of. His ideas didn't just challenge biologists; they provided a new philosophical foundation for understanding life's messy, interconnected history.

From Static Classes to Spatiotemporal Individuals

Hull's revolutionary framework redefined how we think about species and evolutionary mechanisms.

For centuries, species were treated as "natural kinds"—abstract classes defined by a set of essential properties. Gold is gold because it has an atomic number of 79. In the same way, a tiger was thought to be a tiger because it possessed the essential "tigerness." But evolution throws a wrench in this idea. If species evolve, their properties change over time. The first mammal didn't have all the "essential" properties of modern mammals. So, what connects them?

Hull's revolutionary proposal was this: a species is not a class, but an individual.

In our everyday language, an individual is a single organism—you, or your dog. But in a philosophical sense, an "individual" is a spatiotemporally bounded entity that is continuous through time and can change its properties. A nation, like France, is an individual. Its borders, laws, and culture change, but it remains a single, historical entity from its founding to the present.

Hull argued that a species is exactly the same kind of thing. It's a single, massive entity, spread out in space and time, composed of all the organisms that are interconnected by the flow of genes and descent. The species Canis lupus (the gray wolf) is an individual that began at a point in the past, has changed in size, color, and behavior over millennia, and may one day go extinct. It's not a category defined by a checklist; it's a unique, historical lineage.

Replicators

These are the entities that pass on their structure largely intact through generations. The prime example is the gene.

Interactors

These are the entities that, as a whole, interact with their environment in a way that causes replication to be differential. The prime example is the organism.

The organism (the interactor) struggles to survive and reproduce. Its success or failure determines the frequency of its genes (the replicators) in the next generation. This cycle is the engine of natural selection. Hull's framework showed that selection doesn't act for the good of the species in any conscious way. It acts through the success of interactors, which changes the frequency of replicators, which in turn shapes the entire species-lineage over time.

In-Depth Look: A Thought Experiment in Real Time

While Hull was a philosopher, his ideas are best illustrated by real-world evolutionary studies.

Let's examine a classic experiment that perfectly encapsulates his concepts of a species as a changing lineage driven by replicators and interactors.

The Lenski Long-Term Evolution Experiment

In 1988, biologist Richard Lenski started a simple yet profound experiment. He began with twelve genetically identical populations of the E. coli bacterium and has been growing them in a glucose-limited environment ever daily, for over 75,000 generations and counting. This is a direct observation of a species-lineage (each flask population) evolving in real time.

Methodology: Observing Evolution in a Bottle

The experimental procedure is elegant in its simplicity:

Foundation

A single E. coli bacterium is used to found twelve identical populations.

Daily Cycle

Every day, a small sample from each population is transferred to a new flask with fresh growth medium, which contains a limited amount of glucose.

Freezing Fossils

Every 500 generations, a sample of each population is frozen, creating a living "fossil record" that can be revived and compared to its descendants.

Monitoring

The scientists regularly sequence the genomes of the populations and measure their fitness (reproductive success) relative to their ancestor.

Results and Analysis: A Lineage in Action

The results have been a stunning confirmation of evolutionary theory through Hull's lens.

Adaptation

All twelve populations have become significantly fitter over time, better at reproducing in their specific environment.

Divergence

Each population has taken a unique evolutionary path, with different mutations fixing in each lineage.

Major Shifts

In one famous episode, one population evolved the ability to consume citrate—a nutrient present in the medium that E. coli normally can't use.

This experiment demonstrates that a "species" (here, a flask population) is a spatiotemporally localized individual that changes its properties (becomes fitter, gains new abilities). The interactors (the individual bacteria) compete for food. Their success determines which replicators (genes and mutations) are passed on, thereby shaping the entire lineage's future. It's evolution in a nutshell, precisely as Hull described it.

Data from the Lenski Experiment

Relative Fitness Increase

The steady increase in fitness demonstrates the population as a whole (the "individual") changing its properties through time in response to natural selection.

Genetic Divergence

Each population, though starting identical and in the same environment, forged its own unique evolutionary history.

Concept Framework

Concept	Manifestation
Species as Individual	Each flask population
Interactor	Individual bacterium
Replicator	Genes & mutations
Selection	Change in gene frequency

The experiment provides a clear, tangible example for each component of Hull's philosophical structure.

The Scientist's Toolkit

Deconstructing Evolution with Modern Research Tools

Tool / Concept	Function in Evolutionary Research
Model Organism (e.g., E. coli, fruit flies)	A simple, fast-reproducing species that allows for the observation of many generations in a short time, making evolutionary change visible.
Fossil Record	Provides the spatiotemporal evidence for lineages (species-individuals) changing and going extinct over deep time.
DNA Sequencer	The ultimate tool for identifying replicators. It allows scientists to read the genetic code and track changes in gene frequency across generations.
Phylogenetic Tree	A diagram that maps the branching, historical relationships between different species-lineages, showing their common descent.
Selective Pressure	Any environmental factor (e.g., food scarcity, predators, climate) that influences the success of interactors and thus drives natural selection.

A New Lens on Life's Tapestry

David Hull's work did not discover a new gene or unearth a new fossil. Instead, he provided a new way of seeing, a philosophical lens that brings the true structure of evolution into sharp focus. By redefining species as historical individuals and clarifying the roles of replicators and interactors, he resolved long-standing paradoxes and gave biologists a more coherent framework for their work .

The next time you see a picture of the evolutionary "tree of life," remember: you're not looking at a catalog of fixed types. You are looking at a vast family history of billions of interconnected, dynamic, and unique historical individuals, each on its own incredible journey through time.

That is the powerful and elegant vision of David Hull.