Two Ways of Explaining Life
Biology is often divided into two domains.
Physiology explains how organisms function in the present: how they regulate internal conditions, acquire resources, repair damage, and maintain their biological organisation. Evolution explains how organisms change over time: how variation arises, how traits are transmitted, and how populations transform across generations.
These domains are typically treated as distinct. One concerns present function, the other historical change.
Yet both address the same underlying question:
How do living systems persist, and how does that persistence change over time?
From an APS perspective, physiology and evolution are not separate domains but different temporal perspectives on the same viability-oriented biological organisation.
This article develops the application of the APS explanatory framework. For a structured introduction to that framework, see:
→ How APS Explains Life — A Two-Step Guide
The Apparent Divide
Physiological explanations focus on mechanisms operating within individual organisms. They describe how processes are coordinated to maintain stability and function.
Evolutionary explanations, by contrast, focus on populations and lineages. They describe how heritable variation is filtered through differential survival and reproduction.
This difference in focus has led to a conceptual separation. Physiology is often treated as proximate explanation, evolution as ultimate explanation.
While this distinction is useful, it can obscure a deeper continuity.
Both forms of explanation presuppose systems that are capable of maintaining organised persistence.
Persistence as the Common Ground
Living systems persist through ongoing activity.
They maintain their biological organisation despite constant material turnover and environmental fluctuation. This persistence is not passive. It is actively sustained through coordinated processes of regulation, repair, and interaction.
Physiology explains how this persistence is achieved moment by moment.
Evolution, however, also depends on persistence. For evolutionary processes to occur, there must be systems capable of surviving, reproducing, and transmitting biological organisation across generations.
Without organised persistence, there is nothing for evolution to act upon.
Persistence is therefore not only a physiological concern. It is the condition that makes evolution possible.
Evolution as the Transformation of Persistence
If physiology explains how persistence is maintained, evolution explains how persistence is transformed.
Across generations, the biological organisation that sustains viability is modified. New forms of regulation, new structures, and new modes of interaction emerge.
Evolution is not simply change. It is the historical transformation of viability-oriented biological organisation.
In this sense, evolution is the historical dimension of physiology, and physiology is the present-time enactment of evolutionary organisation.
This transformation preserves continuity while allowing novelty. Organisms remain viable even as the ways in which they sustain viability are reorganised.
Evolution therefore extends physiology across time. It tracks how the biological organisation that supports persistence is reconfigured under changing conditions.
One Organisation, Multiple Timescales
The apparent divide between physiology and evolution arises from differences in timescale, not from differences in the underlying phenomenon.
At short timescales, we observe regulation, metabolism, and behaviour. At longer timescales, we observe adaptation, diversification, and evolutionary innovation.
These are not separate processes. They are different temporal expressions of the same viability-oriented biological organisation.
APS captures this continuity by treating process and scale as co-constitutive dimensions of the same organisation. Biological organisation unfolds across interacting temporal and spatial scales, from molecular dynamics to evolutionary history.
Explanation must therefore integrate these scales rather than treat them as independent domains.
Agency Across Time
Biological agency is central to both physiology and evolution.
In physiology, biological agency is expressed in the system’s capacity to regulate itself, respond to perturbation, and maintain viability.
In evolution, biological agency is expressed in the persistence of systems that successfully sustain themselves across generations. Lineages endure because their biological organisation continues to support viability under changing conditions.
Evolutionary outcomes are therefore not imposed on passive systems. They arise from the differential persistence of actively maintained organisations.
Agency thus connects present activity with historical transformation.
Rethinking Proximate and Ultimate Causes
The traditional distinction between proximate and ultimate causes reflects the separation between physiology and evolution.
Proximate causes explain how a trait functions within an organism. Ultimate causes explain why that trait exists in a population.
From an APS perspective, both forms of explanation refer to the same biological organisation viewed at different temporal resolutions.
Proximate explanations describe how viability is maintained in the present. Ultimate explanations describe how patterns of viability have been stabilised or transformed over evolutionary time.
The distinction remains useful, but it no longer marks a division between fundamentally different kinds of explanation.
Implications for Biological Explanation
Recognising the continuity between physiology and evolution has several consequences.
It clarifies that evolutionary theory presupposes the existence of systems capable of sustaining organised persistence. It shows that physiological processes are not isolated from evolutionary history but are themselves products of it.
It also highlights that adaptation involves the reorganisation of processes that already sustain viability. Evolution does not create biological organisation from nothing; it transforms existing viability-oriented systems.
Biological explanation must therefore account for both the maintenance and the transformation of biological organisation within a unified framework.
Conclusion
Physiology and evolution are not separate domains of biology. They are two temporal perspectives on the same phenomenon: the persistence and transformation of viability-oriented biological organisation.
The APS framework unifies these perspectives by identifying the organisational conditions that underlie both present function and historical change.
Physiology and evolution are therefore not distinct explanatory domains, but temporally differentiated expressions of a single, viability-oriented organisation. Biological explanation, in this view, concerns the integrated processes through which life persists and is transformed across time.
Key Point
APS unifies physiology and evolution as temporal perspectives on a single viability-oriented organisation—linking present-time persistence with long-term transformation within a continuous explanatory framework.