Few concepts in evolutionary biology are invoked as frequently, or with as little ontological clarity, as fitness.
It appears indispensable. And yet, it resists being said what it is.
This resistance is not accidental.
Fitness as Explanatory Pivot
Fitness is meant to explain why some variants persist while others do not. It sits at the centre of evolutionary explanation, linking variation to differential survival and reproduction.
Without fitness, natural selection collapses into mere historical narration.
And so fitness must do real work.
The Temptation to Stabilise
Because fitness is required to explain outcomes, there is a strong temptation to treat it as a property of organisms, traits, or genes.
This stabilisation feels intuitive. It makes fitness measurable, comparable, and transportable across contexts.
It also quietly misrepresents what fitness is.
Fitness Is Always Relative
Fitness cannot be defined in isolation.
It depends on:
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environment
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population structure
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timescale
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developmental pathways
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ecological interactions
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competing variants
Change any of these, and what counted as “fit” may cease to be so.
The Circularity Anxiety
This relationality makes fitness unsettling.
If the fittest are those that survive and reproduce, and survival and reproduction are explained by fitness, explanation risks becoming circular.
To escape this anxiety, evolutionary thought repeatedly attempts to ground fitness elsewhere:
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in optimal design
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in energetic efficiency
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in genetic contribution
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in long-term lineage success
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in adaptive landscapes
Each move promises to stabilise fitness as something independent of outcomes.
Each move fails.
Fitness Landscapes and Their Illusions
Fitness landscapes are among the most seductive metaphors in evolutionary biology.
They promise a terrain where fitness is intrinsic, measurable, and navigable — peaks to be climbed, valleys to be crossed.
But landscapes smuggle in what fitness cannot support.
They presume:
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a fixed space of possibilities
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a stable metric of comparison
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a single perspective across time
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continuity of identity
None of these holds.
Fitness landscapes reintroduce ground where none exists.
The Refusal Beneath the Concept
The intolerance of fitness is not a failure to define it precisely. It is a refusal to accept what its relationality implies.
If fitness is relational, then:
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it cannot be a property
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it cannot be absolute
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it cannot be detached from perspective
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it cannot ground explanation on its own
Fitness explains only within a cut.
This is difficult to bear.
Relational Fitness
From a relational stance, fitness is not a thing but a readiness.
Not readiness in the sense of intention or value, but as differential capacity for actualisation under constraint.
Why Fitness Is Protected
Fitness is often treated as more stable than it is because evolutionary explanation depends on it for coherence.
If fitness were allowed to remain fully relational, evolutionary theory would have to relinquish the fantasy of prediction without perspective.
It would have to accept that explanations are always retrospective, local, and constrained by the cuts that make them possible.
This is not a weakness.
It is a discipline.
Fitness and Value
A final temptation must be resisted.
Fitness is often tacitly moralised: success, advantage, optimisation, improvement.
These metaphors import value where there is only differential persistence.
To forget this is to mistake explanation for endorsement.
What This Intolerance Produces
The intolerance of fitness leads to familiar pathologies:
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adaptationist just-so stories
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gene-centrism as ontological shortcut
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reification of environments
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overextension of optimisation models
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confusion between explanation and prediction
Each is an attempt to make fitness tolerable by making it stable.
Fitness Without Ground
If variation has no ground, fitness cannot provide one.
Looking Ahead
If fitness cannot be stabilised, the next pressure point becomes unavoidable.
What, then, is the unit to which fitness applies?
Genes? Organisms? Groups? Lineages?
The persistence of this question is not a technical problem.
It is the next intolerance.
The following post will address The Intolerance of the Unit — and why it never resolves.
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