We have now reached the point where the gene can no longer be treated as the whole story.
The gene is inherited.
It participates in development.
Development produces organisms.
Organisms encounter environments.
Those encounters affect reproduction.
And reproduction feeds back into the distribution of hereditary variants.
This sounds straightforward.
But hidden inside it is a question that evolutionary theory sometimes leaves in the background:
What determines which forms are available for evolution to produce in the first place?
If genetic variation is the raw material of evolution, it is tempting to imagine that evolution simply receives variations and then selects among them.
But that picture quietly assumes that genetic variation maps directly onto phenotypic possibility.
It does not.
Between genetic difference and biological form lies development.
And development does something extraordinarily important.
It does not merely turn genes into organisms.
It organises the ways in which genetic differences can become organismal differences.
That means that development does not merely mediate evolution.
It helps shape the space within which evolution can occur.
The organism is not a genetic output
We have already encountered the inadequacy of the blueprint metaphor.
But now we can see why the problem matters for evolutionary theory itself.
Suppose two organisms differ genetically.
It does not follow that they will differ in every way that their genetic difference might seem to permit.
Development imposes structure.
Some genetic changes have large phenotypic consequences.
Others have very small consequences.
Some produce coordinated changes across multiple traits.
Others produce effects only under particular conditions.
Some changes may have no viable developmental expression at all.
The relationship between genotype and phenotype is therefore not a simple translation system.
It is an organised process.
And that organisation matters because evolution can only act upon phenotypic differences that actually arise.
A variation that cannot become an organismal difference cannot enter selection in the same way as one that can.
The developmental system therefore stands between what can vary genetically and what can become actual phenotypically.
That middle position is conceptually crucial.
From variation to form
We often speak of evolution as though variation comes first and selection follows.
There is variation.
Selection acts upon it.
The population changes.
But where do the forms that selection encounters come from?
They come through developmental processes.
This means that evolution does not operate upon an undifferentiated field of arbitrary alternatives.
The organism does not arrive before us as one of infinitely many possible configurations.
Its developmental organisation makes some forms easier to produce than others.
Some changes may be highly probable.
Some rare.
Some impossible.
Some possible only if other structures change first.
The distinction is subtle but important.
Evolution does not merely alter the frequency of forms.
It operates through a system that helps determine which forms can arise as variants at all.
The developmental system as a constraint
The word constraint can sound negative.
To constrain something is ordinarily to restrict it.
And development certainly does restrict biological possibility.
An organism cannot become just anything.
A mutation does not allow arbitrary redesign.
Development has architecture.
Cells interact in particular ways.
Growth occurs according to physical and biological processes.
Existing structures constrain subsequent structures.
The history of a lineage matters.
All of these things limit what can arise.
But here is the crucial point:
A constraint can be productive.
A musical scale restricts the available notes while making particular musical structures possible.
A grammar restricts the forms a sentence can take while enabling an enormous range of meaningful utterances.
A skeleton constrains movement while enabling particular kinds of movement.
Likewise, developmental organisation restricts biological variation while enabling particular kinds of biological form.
Constraint is therefore not simply the opposite of creativity.
It can be one of its conditions.
The strange generosity of constraint
This gives us another way to understand a theme that has appeared repeatedly in our earlier work.
We are accustomed to thinking of possibility as something that becomes smaller when constraints are added.
But this is only half the story.
A completely unconstrained system does not necessarily possess more meaningful possibilities.
It may possess an undifferentiated infinity of states.
A system with organisation can make certain structured outcomes accessible.
Development does something similar.
It does not merely prevent organisms from becoming arbitrary configurations.
It channels variation into forms that can actually be developed, maintained and reproduced.
So the developmental system can be understood as doing two things simultaneously:
closing possibilities
and
opening possibilities.
The same structure can perform both functions.
This is why "constraint" is such an inadequate word if it is understood only negatively.
The constraint of development is also an affordance.
It makes some transformations available precisely by making others unavailable.
Affordance without intention
The word affordance can itself become misleading if we import too much agency into it.
An affordance does not have to be an invitation deliberately offered by something.
A surface affords walking because of its physical organisation.
A branch affords perching because of its structure.
A developmental system affords certain transformations because of its organisation.
The organism does not need to "see" these possibilities in advance.
Nor does evolution need to anticipate them.
The possibilities are there as relations among structures and processes.
This gives us a naturalistic way of speaking about possibility without turning possibility into intention.
A developmental organisation makes certain forms accessible.
The environment makes some of those forms consequential.
Selection changes their representation.
The historical process continues.
No foresight is required.
Potential, possibility, actuality
We can now sharpen the distinctions we introduced in the previous essay.
A biological system can possess potential without that potential being realised.
A developmental organisation may have the capacity to generate a range of forms.
But a particular form becomes possible in a more concrete sense when the relevant conditions make its emergence accessible.
And only some possibilities become actual.
We can therefore distinguish:
These are not three names for the same thing.
And evolution depends upon their relations.
A mutation can introduce a potential difference.
Development determines whether that difference can become a phenotypic possibility.
The environment determines whether the resulting phenotype becomes consequential.
Selection affects whether variants carrying it become more or less represented in subsequent generations.
And once a new form becomes actual, it changes the conditions for subsequent potential and possibility.
So the sequence is not:
potential → actuality.
It is:
potential → possibility under conditions → actuality → transformed conditions → new possibility.
This is already beginning to look like an evolutionary process rather than a static theory of variation.
The genotype–phenotype map
Evolutionary biology has a useful mathematical and conceptual tool for thinking about some of this: the genotype–phenotype map.
The phrase itself is revealing.
A map is not the territory.
And a mapping relation is not an identity relation.
A genotype does not simply equal a phenotype.
Rather, there is some relation through which genetic organisation is associated with phenotypic outcomes under particular developmental and environmental conditions.
Different genotypes can sometimes produce similar phenotypes.
A single genetic change can sometimes affect multiple traits.
The same genetic variant can have different consequences in different environments.
The mapping is therefore structured, many-to-many in some respects, context-dependent and historically shaped.
This is precisely the kind of relation that an overly simple gene-centred picture tends to hide.
The important question is not merely:
What genes exist?
but:
What transformations of biological form does the organisation of development make accessible from those genetic states?
Now evolution begins to look less like a search through a pre-existing catalogue and more like a transformation of the relations that make biological forms possible.
The fallacy of the empty possibility space
We should therefore be suspicious of a familiar image:
Evolution searches a space of possible organisms.
It is a useful image.
But what is this space?
Where did it come from?
Does it exist independently of biology?
Does it contain every physically possible organism?
Every developmentally possible organism?
Every ecologically viable organism?
Every historically reachable organism?
These are very different spaces.
A mathematically imaginable form may be physically impossible.
A physically possible structure may be developmentally inaccessible.
A developmentally possible form may be ecologically unviable.
An ecologically viable form may be historically unreachable because the lineage lacks the structures required to get there.
So when we say that evolution "explores possibility space", we should immediately ask:
Which possibility space?
There may be no single answer.
There are layers of possibility, each structured by different relations.
And the relations themselves can change.
The possibility space is not outside history
This may be one of the most important conclusions so far.
If developmental organisation changes, then the set of forms accessible to evolution changes.
If ecological relations change, the set of advantageous forms changes.
If a lineage acquires a new structure, new transformations can become accessible.
If a lineage loses a structure, some transformations may become inaccessible.
Therefore the evolutionary possibility space is not a fixed stage upon which life moves.
It is partly historically generated.
This gives a more precise meaning to our phrase:
the transformation of possibility.
Evolution does not merely move from one actual organism to another.
It changes the conditions under which organisms can become.
And that means that evolution can alter not only:
what exists
but also:
what can subsequently exist.
That is a much stronger claim.
But it is also a claim we can begin to ground biologically.
An example: the limb
Consider the evolutionary history of limbs.
A particular ancestral structure exists.
It is inherited.
Development constrains how that structure can vary.
But variation can produce differences in length, orientation, articulation, musculature and other properties.
Some differences become actual.
Some of those actualities alter what the organism can do.
Those altered capacities change ecological relations.
And once those relations change, further forms can become advantageous or accessible.
The important point is that no single step requires the organism to know where the process is going.
A structure inherited from the past can acquire new possibilities through modification.
And once modified, it can become the basis for further modification.
The future is therefore built upon the actualities of the past.
But it is not contained within them as a blueprint.
Exaptation and the reuse of actuality
Exaptation makes this especially vivid.
A structure can arise under one set of evolutionary conditions and subsequently become useful in another.
What was once one thing becomes part of something new.
The inherited structure has not changed into nothing.
Its actuality persists.
But its possibilities have changed because its relations have changed.
This is an important distinction.
The transformation need not always be:
old structure disappears → new structure appears.
It can instead be:
existing structure → new relation → new possibility → new function.
The evolutionary novelty lies partly in the relation.
This is another reason why relational ontology is useful here.
A trait does not possess its evolutionary significance entirely within itself.
Its significance can emerge from the relations into which it enters.
The organism as a participant in its own possibility
There is a further complication.
The organism does not merely receive developmental possibilities passively.
Organisms act.
They move.
They choose habitats.
They construct environments.
They alter resources.
They interact with other organisms.
And these activities can change the selective conditions under which their descendants evolve.
This is where agency genuinely enters our story.
But notice the difference from the gene's supposed agency.
The organism can be an agent in a meaningful biological sense.
Yet its agency itself emerges from an organised biological system.
The organism does not stand outside evolution.
Its behaviour becomes part of the evolutionary process.
This gives us a much more nuanced picture:
genes participate in development;
organisms participate in environments;
environments participate in selection;
selection alters the distribution of hereditary variants;
and those variants enter future developmental processes.
Agency exists within the network of relations.
It does not replace the network.
Niche construction and the changing environment
The organism's activity can therefore modify not only itself but its surroundings.
A beaver changes a river.
Plants alter soils.
Microorganisms alter chemical environments.
Animals change the distribution of resources.
Such processes can alter the conditions under which selection occurs.
This creates another feedback:
organism → environment → future organism.
Again, we should resist the temptation to turn this into conscious design.
Niche construction does not require organisms to understand the evolutionary consequences of their behaviour.
They alter environments simply by living.
But those alterations can change the possibilities available to subsequent generations.
The environment is therefore not merely a fixed background against which organisms compete.
It can itself become part of the evolving organisation.
Possibility is relational all the way down
We can now see why possibility cannot be treated as a property possessed by an organism in isolation.
A wing affords flight only in relation to an appropriate physical environment.
A digestive structure affords the processing of particular resources in relation to an ecological context.
A behavioural capacity becomes advantageous in relation to other organisms.
A genetic variant becomes evolutionarily consequential in relation to developmental and reproductive processes.
Possibility is therefore not simply "inside" the organism.
It is relational.
This does not mean that possibilities are arbitrary.
Quite the opposite.
They are structured by the actual organisation of the world.
The organism has genuine capacities.
The environment has genuine properties.
The relations between them are real.
Possibility emerges from those relations.
What Conceptual Naturalism contributes
This is where Conceptual Naturalism becomes particularly useful.
Our conceptual vocabulary has a tendency to turn dynamic relations into static objects.
We say:
"the trait"
"the gene"
"the environment"
"the niche"
"the fitness"
"the possibility space".
These nouns are indispensable.
But they can encourage us to imagine that each item exists independently and then enters into relations with the others.
Sometimes that is exactly the right abstraction.
Sometimes it is not.
Conceptual Naturalism asks us to inspect the habits of thought that make one construal feel inevitable.
The phrase "the gene for" encourages one kind of causal picture.
"The environment" as a background encourages another.
"Fitness" as a property possessed by an organism encourages another.
"Possibility space" as a pre-existing container encourages another.
None is necessarily wrong.
But each can become a conceptual attractor.
Once the attractor becomes familiar, we stop asking what relations the abstraction has compressed.
That is precisely the kind of conceptual naturalisation we need to resist.
The shape of variation
We can now return to the central question.
What determines the shape of what can evolve?
Not genes alone.
Not selection alone.
Not development alone.
Not environment alone.
The shape emerges from their interaction.
Genetic variation provides differences.
Development transforms genetic differences into organismal possibilities.
Organisms inhabit and modify environments.
Environments alter the consequences of phenotypic differences.
Selection changes the distribution of inherited variants.
Inheritance carries altered organisation forward.
And the entire process occurs historically.
So evolution does not merely select among possibilities.
It participates in changing the conditions under which possibilities become available.
This is beginning to make our governing thesis much more concrete.
The furrow
Perhaps we can borrow a metaphor that has accompanied some of our earlier work.
Imagine a plough moving across a field.
The plough does not determine the crop that will eventually grow.
Nor does it create the soil from nothing.
But once a furrow has been made, the subsequent possibilities are not exactly what they were before.
Water may collect differently.
Seeds may settle differently.
Roots may follow the altered structure.
The next movement of the plough may be influenced by the furrow already made.
The field has acquired a history.
The past movement has changed the conditions of the next movement.
Evolution is obviously not literally ploughing a field.
But the metaphor captures something important:
history leaves structure behind.
And that structure changes what can happen next.
The furrow does not choose the future.
It does not contain the future.
But it changes the affordances of the field.
Perhaps this is one way to understand evolutionary constraint.
The past does not prescribe the future.
It shapes the terrain upon which the future can emerge.
Evolution as changing terrain
We can now make our thesis more precise.
Evolution is not simply:
variation + selection = adaptation.
It is a historical process in which:
inherited organisation shapes development; development shapes phenotypic possibility; organisms and environments interact; those interactions alter reproductive consequences; and the resulting changes in inheritance alter the conditions for subsequent development and evolution.
The process therefore changes its own terrain.
This is why the metaphor of a fixed possibility space becomes inadequate.
The terrain itself is historical.
The furrows accumulate.
Some disappear.
Some deepen.
Some intersect.
New paths become possible because old paths have been taken.
Others become inaccessible because the landscape has changed.
Again, this is not foresight.
It is history.
But have we gone too far?
We should pause here.
There is an obvious danger in all of this.
We have become increasingly attracted to the phrase:
the transformation of possibility.
It fits beautifully with our accumulated conceptual vocabulary.
It resonates with our earlier work on becoming, affordances, constraints and relational ontology.
But that very elegance should make us suspicious.
Are we actually discovering something about biological evolution?
Or are we simply redescribing evolution in the conceptual language we already happen to like?
This is a serious methodological question.
A philosophical framework should not receive confirmation merely because nature can be made to sound poetic in its vocabulary.
We need to ask whether the idea of transformed possibility provides genuine explanatory illumination.
Does it identify something that standard evolutionary concepts obscure?
Does it make predictions?
Does it connect phenomena that otherwise appear unrelated?
Does it clarify rather than merely rename?
These are tests we will have to take seriously.
For the moment, however, the biological groundwork is strong enough to justify continuing.
We have identified a real feature of evolutionary systems:
developmental organisation constrains and enables the phenotypic forms that can arise.
And historical change in that organisation can alter the range of subsequent forms.
That is not merely a metaphor.
The philosophical question is what we should make of it.
From possibility to evolvability
This brings us naturally to a concept that gives the argument a firmer biological footing:
evolvability.
Evolvability concerns, roughly speaking, the capacity of a biological system to generate heritable variation that can contribute to evolutionary change.
The concept is important because it shifts attention away from the question:
Can mutations occur?
toward something richer:
What kinds of variation can a developmental system generate and preserve?
Not every possible genetic change produces an interesting phenotypic difference.
Not every phenotypic difference is heritable.
Not every heritable difference is viable.
Not every viable difference is evolutionarily consequential.
The path from variation to evolutionary change is structured.
And the structure itself can evolve.
That is precisely where the transformation of possibility becomes an empirically interesting idea.
If evolutionary history can alter the kinds of heritable variation a system can subsequently generate, then evolution is not merely changing populations.
It is changing the capacity for future evolutionary change.
The evolution of evolvability would therefore be a particularly powerful test of our thesis.
The strange recursion of evolution
We have arrived at another recursive structure.
Evolution changes organisms.
Organisms possess developmental systems.
Developmental systems determine what kinds of variation can arise.
Evolution changes those developmental systems.
Therefore:
evolution can change the conditions of future evolution.
This is stronger than saying that evolution changes organisms.
It says that evolutionary history can modify the machinery through which evolutionary possibilities themselves are generated.
Now the phrase "the transformation of possibility" begins to acquire a more precise biological meaning.
It is not merely that one organismal form becomes another.
It is that:
the historical process of evolution can alter the range and structure of forms that subsequent evolutionary processes can generate.
That is a claim worth investigating.
And it takes us directly toward our next question.
The future evolution of evolution
If development shapes what can vary, and if evolutionary history can reshape development, then evolution is not operating on a fixed biological machine.
The machine itself changes.
The channels through which variation flows can deepen, divide, disappear or open.
The constraints can change.
The affordances can change.
The relation between potential and actuality can change.
And so the next stage of our investigation cannot simply ask:
How does evolution produce adaptation?
We must ask something more ambitious:
How can evolution change the conditions under which evolution itself can happen?
That question brings us to evolvability.
And perhaps, more surprisingly, it brings us back to the question with which we began the entire series.
If evolution has no agent, no foresight and no predetermined destination, how can it nevertheless produce increasingly rich possibilities?
Perhaps the answer will not be found in discovering the hidden agent.
Perhaps it lies in understanding how historical processes transform the conditions of becoming.
But before we allow that possibility to seduce us, we need to see whether biology itself warrants it.
That is where we go next. 🍷🙂