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Animal signals are diverse because their functions, receivers, transmission environments and evolutionary incentives differ. Similar functions can also be achieved by different historical and developmental routes. An animal signal is shaped by selection for its effects on a receiver; an informative incidental feature can instead be a biological cue. Merely finding that another animal responds to a feature does not establish why it evolved. Diversity includes differences among signal modalities, differences among species using the same modality, geographical variation, and variation among individuals or contexts within a population.

Different problems demand different information. mate choice can favour information about identity, condition or compatibility; contests can favour assessment of fighting ability or motivation; offspring solicitation concerns need and likely returns to provisioning. Alarm communication can coordinate responses to danger, whereas recruitment can direct partners to resources. A single individual may therefore use several signals because the appropriate receiver and useful message change across activities. The efficiency of a signal is defined relative to a receiver's response and its consequences, not simply the amount of information that a human observer can extract.

Physical transmission strongly constrains signal modality. Light can provide fine spatial patterning but depends on illumination and line of sight. Sounds can travel around obstacles, yet propagation, reverberation and background noise alter what reaches a receiver. Chemical signals can persist after a signaller leaves, which is useful for some functions and poorly suited to rapid reversal in others. Substrate vibration and electrical stimulation provide additional routes where anatomy and environment permit them. The relevant comparison is the complete communication system: production, transmission, detection and action. A brightly coloured patch is ineffective if it is hidden by posture or cannot be distinguished by the intended receiver.

The environment can consequently drive divergence in signals and sensory systems. In cichlids, variation in the light environment was associated with differences in visual pigments, male colours and female preferences, supporting a role for sensory drive in divergence. ^[ref] The inference requires evidence for each link; a correlation between water colour and male colour alone would not establish evolved receiver tuning or its effect on mating. Reciprocal illumination treatments, receptor measurements and preference tests help distinguish sensory mechanisms from population history. Actual mating and gene flow provide further evidence about consequences beyond laboratory attraction.

Background interference also changes which signals work. great tits in noisier urban locations sang with higher minimum frequencies in a classic field comparison. ^[ref] This illustrates a plausible response to acoustic masking, but the association by itself does not distinguish immediate adjustment, developmental learning, habitat sorting and inherited change. Playback of controlled noise to the same birds, repeated recordings, and matched quiet sites would separate some of these explanations. Signal amplitude, distance to the microphone and recording conditions also require control. Diverse responses to the same disturbance can reflect different receiver sensitivities or different costs of modifying a signal.

Selection by intended receivers can oppose selection by unintended ones. Male guppy colour patterns are associated with mating advantages but can also increase detection by predators. Experimental changes in predation and visual background altered colour-pattern evolution, showing that ecological conditions can change the balance rather than imposing a single universally optimal ornament. ^[ref] The intended receiver and predator may have different colour vision, so conspicuousness should be modelled and tested separately for each. Increased mating attraction does not establish a net lifetime advantage if survival is substantially reduced. signal eavesdropping thus helps explain why closely related populations can evolve different compromises.

Receiver psychology supplies further routes to diversity. A preference may originate in another task and subsequently be exploited by a new display, a process of sensory exploitation. Alternatively, a signal and preference may coevolve through sexual selection. These hypotheses have different historical predictions, requiring independent phylogenetic evidence about when the preference and display arose. Learned recognition can produce local conventions, while social learning can maintain differences without an initial genetic difference in the signal. A novel signal can be favoured when familiar ones lose effectiveness, but novelty is not always beneficial: unfamiliar warnings or mate signals may instead be ignored or mistaken for something else.

signal honesty also permits several designs. Reliability can follow aligned interests, physical links between production and an advertised property, or differences in the costs and benefits of expression. An index signal is constrained by its production mechanism; a condition-dependent signal may be easier for some individuals to sustain. In costly signalling, an expense is relevant when it changes incentives to exaggerate. Showing that a display consumes energy is not sufficient: an equally expensive display could still be advantageous to an unreliable signaller. Tests should manipulate both the signal and the advertised property, then measure receiver responses and the resulting fitness effects. Different information problems need not share the same honesty mechanism.

Conflict can maintain deceptive alternatives alongside informative signals. fork-tailed drongos use varied alarm mimicry to obtain food from other foragers, and changes in signal type can restore responses when targets become less responsive to repetition. ^[ref] This demonstrates how receiver experience can favour behavioural variation rather than one permanently effective deception. To separate signal-specific learning from declining general responsiveness, experiments need controlled playback order, repeated genuine alarms and measurements of target state. Deception also depends on an otherwise useful response remaining sufficiently common; a response that never benefits its receiver is unlikely to be maintained indefinitely.

Finally, developmental mechanisms and evolutionary history limit the available solutions. Related species inherit similar production structures and sensory systems, so some resemblance reflects biological homology. Different lineages may reach similar functional outcomes through convergent evolution. multimodal signalling can supply redundancy under variable conditions or combine different information, but additional components also carry production and detection costs. Comparisons should therefore measure both performance and history, account for phylogenetic nonindependence, and distinguish within-individual flexibility from inherited divergence.

Signal diversity is the outcome of several interacting filters: what benefits receivers, what signallers gain, what travels and can be perceived, and what inherited mechanisms can produce. No single explanation such as mate attraction, environmental adaptation or honesty is expected to account for every component of that diversity.

There is no general basis for assigning behaviour a smaller causal role in macroevolution. Its immediate effects are usually easier to measure within populations, but those effects can alter lineage formation and persistence. The comparison needs a definition of “contribution”: the proportion of evolutionary changes initiated by behaviour, their magnitude, their necessity, or the strength of available evidence would give different answers. Microevolution concerns change within biological populations; macroevolution concerns patterns at and above the level of species, including speciation, adaptive radiation and extinction. They are connected processes with different objects of measurement.

At the population scale, animal behaviour changes who encounters which resources, survives, mates and reproduces. Habitat preference changes exposure to ecological natural selection; mate choice changes reproductive success; dispersal changes the spatial distribution of alleles. Behaviour can therefore affect the evolution of other traits even if the behaviour itself does not evolve. Conversely, an environmentally induced change in behaviour is not automatically evolutionary change. Learning and phenotypic plasticity can occur within one lifetime, whereas an evolutionary claim requires persistence through inherited differences or an explicitly specified process of cultural inheritance.

A simple population genetics calculation makes the distinction concrete. Let $x_i$ be an individual's number of copies of a focal allele, $w_i$ its contribution to reproduction, and $p=\mathbb{E}(x)/2$. With unbiased Mendelian segregation and no other transmission change, the expected allele frequency after selection satisfies:Behaviour can change $w$ and thereby change the covariance, whether selection acts directly on a behavioural difference or on another trait whose consequences depend on behaviour. The equation does not imply that every correlation between a behaviour and reproductive success is genetic: common environment, age and condition can generate such correlations. A common garden experiment, a pedigree and measurements across environments help identify the inherited component.

Behaviour may expose a previously uncommon phenotype to selection. Foraging in a new habitat can make a different resource profitable, after which selection can refine feeding structures or behavioural response thresholds. Laboratory competitions between spadefoot tadpoles from populations differing in expression of a carnivore ecomorph found differences in resource capture and morphology consistent with adaptive refinement of a formerly plastic response. Expression without the inducing cue also supported genetic assimilation. ^[ref] This is a combination of behaviour and development, not proof that behaviour universally comes first. Genetic accommodation requires evolution of the response, rather than assuming that an acquired habit is copied directly into DNA.

The opposite effect is also possible. Through phenotypic buffering, movement or flexible foraging may keep individuals in suitable conditions and reduce selection on physiological or morphological differences. Thus flexibility can promote occupation of a new environment, delay genetic divergence, or prevent local extinction. The sign depends on whether behaviour changes exposure to selection or allows animals to avoid it. A short-term adaptive response is consequently not evidence for a universal acceleration of evolutionary change.

At the boundary between populations and species, the behavioural effects are particularly direct. Assortative mating, host choice and learned signals can reduce gene flow. In the Big Bird lineage of Darwin's finches, field observations and genomic ancestry documented an immigrant Geospiza conirostris breeding with a resident Geospiza fortis and the formation of an endogamous descendant lineage. Song and distinctive morphology contributed to mating within that lineage. ^[ref] The evidence links immigration, hybridization and mate recognition to rapid reproductive separation; it does not show that song alone caused isolation or establish the lineage's indefinite future persistence. Nevertheless, behaviour participates in an event conventionally classified as macroevolution.

Over longer periods, these mechanisms can influence the rates of lineage origination and disappearance. Behaviour that permits colonization can expose populations to ecological opportunity and eventual adaptive radiation. Specialization in host use or mating signals can strengthen reproductive barriers. Conversely, costly reproductive competition or inflexible resource use can increase vulnerability. In fossil ostracods, greater sexual dimorphism was associated with greater estimated extinction risk, using carapace differences as a proxy for male reproductive investment. ^[ref] This is unusually direct evidence about actual lineage loss, but an anatomical proxy does not preserve courtship or experimentally isolate sexual selection from every correlated attribute. It supports a possible connection rather than a general law that strong sexual selection causes extinction.

Evaluating a claim about relative importance also requires correcting unequal evidence. Contemporary behavioural experiments can manipulate cues and follow reproduction, whereas most ancient behaviour must be inferred from traces, anatomy or living relatives. Living-only comparative studies omit extinct lineages and can confuse a trait associated with diversification with a trait that evolved after diversification. Comparisons should allow for shared ancestry, environmental opportunity and incomplete sampling, and distinguish an increased speciation rate from a reduced extinction rate. Counting observed behavioural changes over years against anatomical changes over millions of years is not a meaningful common denominator.

Behaviour has demonstrable population-level effects and plausible, sometimes well-documented effects on speciation and extinction. The stronger conclusion is that its evolutionary influence is context-dependent and transmitted across scales, while the evidence becomes harder to identify over deep time. A qualified statement about evidential accessibility is defensible; a general ranking of causal importance is not.