Before questioning the concept of the signal, we need to be precise about how it is actually used.
Biologists are not careless. The term signal is not applied arbitrarily. It is typically defined with some rigour, and within evolutionary biology it carries a fairly specific meaning.
So the task of this post is not to critique, but to reconstruct the concept in its strongest form.
Only then can we ask what kind of explanation it provides.
The standard definition
In contemporary evolutionary biology, a signal is generally understood as:
a trait or behaviour that has evolved because it affects the behaviour of other organisms.
This definition already contains several important elements:
the signal is observable (a colour pattern, a sound, a movement)
it is correlated with some underlying condition or state
it elicits a response in another organism
and crucially, it is selected for because of that response
The last point is decisive.
Signals and selection
This brings us back to familiar examples.
Similarly:
bird songs influence territorial and mating responses
warning colouration influences predator behaviour
courtship displays influence reproductive outcomes
In each case:
a trait is produced
another organism responds
and that response feeds back into evolutionary selection
The signal is thus embedded in a loop:
production → response → selection → stabilisation
Information and correlation
Biological discussions of signalling often invoke the notion of information.
A signal is said to “carry information” about:
health
strength
reproductive status
environmental conditions
This is typically grounded in correlation:
brighter colours correlate with better condition
more elaborate displays correlate with fitness
specific calls correlate with types of threat
From this perspective, the signal is informative because it is reliably associated with something else.
The role of the receiver
Equally important is the role of the receiver.
A signal is only a signal if there is:
a system capable of responding to it
in a way that affects behaviour
Receivers are often described as:
detecting
interpreting
or decoding
the signal.
But even in more restrained accounts, the key point is that:
the behaviour of the receiver is systematically altered by the presence of the signal.
Functional accounts
Putting this together, signals are typically characterised functionally:
they do something in the system
they influence behaviour
they contribute to fitness outcomes
Their existence is explained not by what they are in isolation, but by the role they play in these dynamics.
This is why signals are often distinguished from:
cues — features that provide information but have not evolved for that purpose
noise — features that have no systematic effect on behaviour
A signal, in contrast, is functionally integrated into the system of interaction.
What this account does not require
At this point, we can make an important observation.
Nothing in the definition so far requires:
representation
symbolic relation
or a system of meaning
What is required is:
correlation
responsiveness
and selection
That is enough to produce:
stable patterns
reliable responses
and coordinated outcomes
The quiet expansion
And yet, in practice, the concept of signal rarely stays at this minimal definition.
From “affects behaviour,” it expands to:
“carries information”
“communicates”
“expresses”
“represents”
This expansion is often implicit.
The same term—signal—is used across all these levels, without marking the shift.
As a result, a concept grounded in:
evolutionary function
comes to support claims about:
meaning and communication
Where we now stand
We are now in a position to state the situation clearly.
Biologists use signal to refer to:
traits that are correlated with certain states
that elicit responses in other organisms
and that are stabilised through selection because of those responses
This is a robust and well-motivated concept.
But it is also a concept that:
does not, in itself, require semiosis.
Transition
The next step is to examine how this concept is interpreted.
Or are we moving too quickly from:
- responsivenessto
representation?
To answer this, we turn to a foundational case—sexual selection—and ask what kind of explanation it actually requires.
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