Sensory Drive Hypothesis
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The sensory drive hypothesis is a hypothesis in
population ecology Population ecology is a sub-field of ecology that deals with the dynamics of species populations and how these populations interact with the environment (biophysical), environment, such as birth rate, birth and death rates, and by immigration an ...
that posits that when local environmental conditions differ between conspecific populations,
communication systems A communications system is a collection of individual telecommunications networks systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interoperation to form an integrated whole. Commun ...
will adapt to these conditions. Sensory drive predicts that both communication signals and perceptual systems will adapt to these local environmental conditions. Divergence will then occur based on the intensity and direction of selection on the mating signals and on the sensory systems acquiring information regarding predators, prey, and potential mates. The sensory drive hypothesis has two primary assumptions. The first is that greater sensory stimulation results in preferences for mates with the stimulating trait, meaning exaggerated traits are expected to have greater signal value and generate more mating because they cause a stronger response from the sensory system. In this sense, it is possible for sensory drive to contribute to the formation of runaway traits when
sexual selection Sexual selection is a mechanism of evolution in which members of one sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex ...
is working in the same direction as the sensory biases. The second assumption is that biases are relatively fixed and show limited
developmental plasticity Developmental plasticity refers to changes in neural connections during growth, influenced by environmental interactions and learning. Similar to brain plasticity, it specifically involves how neurons and synapses adapt during development. Most of ...
.
Mating In biology, mating is the pairing of either opposite-sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. '' Copulation'' is the union of the sex organs of two sexually repr ...
signals, perceptual systems, and
behavioral Behavior (American English) or behaviour (British English) is the range of actions of individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as well as the inanimate p ...
responses are not independent. As a result, as one of these diverges based on selective pressures, the others should also diverge unless under independent selection against divergence. While sensory drive is likely not the primary driver involved in
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
, it is able to set the initial evolutionary trajectory of an aspect involved in these communication signals that can be acted on by evolutionary forces to drive
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
. To find support for the sensory drive hypothesis, there needs to be a match between the biases of the sensory system predicted by environmental constraints and a match between signals and that sensory bias. As of 2018, support has been found for the sensory component with there being a strong bias towards aquatic systems (57% of studies) and visual signals (83% of studies). There is also support for the signal component with more bias towards terrestrial systems (71% of studies) and visual signals (57% of studies). There have also been papers focused on identifying support for both the sensory and signal components; of these papers, only those with support for the sensory drive hypothesis have been published. Similar to the studies focusing on the sensory component, there is a bias towards aquatic systems (62% of studies) and visual signals (86% of studies) with the majority of these being focused on fish taxa (55% of studies).


Visual signals and vision

In a typical
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...
,
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
passes through the ocular media (
cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...
,
lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
, and
vitreous humor The vitreous body (''vitreous'' meaning "glass-like"; , ) is the clear gel that fills the space between the lens and the retina of the eyeball (the vitreous chamber) in humans and other vertebrates. It is often referred to as the vitreous humo ...
) before reaching the
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
, where it passes through several layers of
cells Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a d ...
before finally reaching the photoreceptors. Transmission properties in each layer vary according to
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
. One study found that
cone cell Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...
sensitivities vary between individual guppies, which influenced Endler's original hypothesis in 1992. Scientists do not yet understand how color preferences are impacted by this variation. Additionally, UV waves may be transmitted through the eye differently in different individuals and/or populations; this has been documented in birds, especially
warbler Various Passeriformes (perching birds) are commonly referred to as warblers. They are not necessarily closely related to one another, but share some characteristics, such as being fairly small, vocal, and insectivorous. Sylvioid warblers T ...
s. This may be selectively advantageous for a species if its predator is incapable of seeing UV wavelengths, allowing for conspecific communication without drawing attention from predators. Sensory drive could explain differences in UV preference if properties of the ocular media are not uniform between populations. For example, two closely related species of ''
Anolis ''Anolis'' is a genus of anoles (), iguanian lizards in the family Dactyloidae, native to the Americas. With more than 425 species, it represents the world's most species-rich amniote tetrapod genus, although many of these have been proposed to b ...
'' lizards differ such that the males of the species that is more sensitive to UV wavelengths reflect UV light on their
dewlap A dewlap is a longitudinal flap of skin or similar flesh that hangs beneath the lower jaw or neck of many vertebrates. More loosely, it can be various similar structures in the neck area, such as those caused by a double chin or the submandibula ...
s (used for signaling to mates) more than the males of the species that is less sensitive to this light. In general, marine
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
s tend to have a greater capacity for visual tuning than terrestrial vertebrates, which could partially explain why most of the studies that find evidence of sensory drive focus on aquatic systems. There is a great diversity of colors in the animal kingdom, which is potentially explained by color perception and how the environment drives its evolution. One aspect of this is color contrast (when an object's color appears to be different when seen in front of varied backgrounds). For instance, an animal will perceive the color of a yellow bird differently when it is viewed in front of a dark brown tree trunk versus a bright blue sky. This connection between environment and detection is an integral part of the sensory drive hypothesis. Three mechanisms for the evolution of visual perception have been proposed relating perception, signal, and environmental factors. The first ignores the effects of environmental conditions and proposes that color and perception traits coevolve as a result of
social selection Social selection is a term used with varying meanings in biology. Joan Roughgarden proposed a hypothesis called ''social selection'' as an alternative to sexual selection. Social selection is argued to be a mode of natural selection based on r ...
, e.g., females’ preferences for certain colors in males. For more on this, see
Fisherian runaway Fisherian runaway or runaway selection is a sexual selection mechanism proposed by the mathematical biologist Ronald Fisher in the early 20th century, to account for the evolution of ostentatious male ornamentation by persistent, directional f ...
. The second mechanism also predicts coevolution of color and perception, but the direction of evolution is influenced by environmental factors, according to the sensory drive hypothesis. For a given male trait, the strength of female preference depends on the honesty of the trait in conveying the male's quality and the detectability of the trait. Variation in detectability is expected to significantly influence color diversification; this variation often is due to the features of the habitat occupied by the population in question. Even if males’ colors accurately represent their quality as a mate, poor visibility as a result of environmental factors could result in females choosing males of poorer quality. This could lead to the evolution of preferences for other, more easily detected, male characteristics. The final mechanism proposes that evolution carries on linearly, without any feedback loops: environmental conditions define the selective pressures that change color perception, which in turn drives the evolution of colors. The most obvious example of this is found when analyzing the need for organisms to detect and hide from predators and prey. The amount of visible light as well as the range of colors in water decreases sharply with depth and other abiotic and biotic factors that contribute to
turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wa ...
. Studies have found that these gradients correlate with several color perception traits, including those related to
opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...
s. For example, in
surfperch The surfperches are a family of percomorph fishes, the Embiotocidae. They are mainly found in northeast Pacific Ocean (as far south as Baja California), but a few species (genera '' Ditrema'' and '' Neoditrema'') are found in the northwest Pacifi ...
living in
kelp forest Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on E ...
s,
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
has likely favored refined color discrimination, which allows for easier detection of prey against the background. Sensory drive explains male
cichlid Cichlids () are a large, diverse, and widespread family of percomorph fish in the family Cichlidae, order Cichliformes. At least 1,760 species have been scientifically described, making it one of the largest vertebrate families, with on ...
coloration and female preference in
Lake Victoria Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropics, tropical lake, and the world's second-largest fresh water lake by surface are ...
, where the light gradient is highly predictable. Threespine sticklebacks (''
Gasterosteus ''Gasterosteus'' is a genus of ray-finned fishes belonging to the Family (biology), family Gasterosteidae, the sticklebacks. These fishes are found in freshwater, brackish water and marine habitats in the Holarctic realm, Holarctic region. Speci ...
'' spp.) are another example of coevolution of male signals and female perception driven by environmental factors. Males either display red or black nuptial colors, and there is a direct correlation between female sensitivity to red light and preference for red males. Since such steep gradients of light availability and quality are not observed in terrestrial environments, it is less likely to find support for sensory drive here as well as in aquatic environments that are less optically constrained. In cichlids and guppies, opsin expression can be explained both by phenotypic plasticity and genotypic variation between populations and species, though scientists have not yet determined which is more important. One problem for future work to address is that it is difficult to link
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
s for opsin tuning and expression with behavioral
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
s, which is a key component of the sensory drive hypothesis.


Acoustic signals and hearing

The
auditory system The auditory system is the sensory system for the sense of hearing. It includes both the ear, sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, incre ...
s of animals are impacted by both the biotic and abiotic conditions in their environment. Under conditions of high background noise, there is evidence of perceptual tuning to better perceive and respond to
conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organism ...
signals. For example, in
anurans A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough skin texture due to ...
(frogs and toads) there is evidence of perceptual tuning of the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
sensitivity to best perceive conspecifics and to prevent hybridization. Additionally in anurans, there is evidence for perceptual tuning contributing to
directional selection In population genetics, directional selection is a type of natural selection in which one extreme phenotype is favored over both the other extreme and moderate phenotypes. This genetic selection causes the allele frequency to shift toward the ...
on
acoustic signals Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician ...
. An organism's auditory system can also be tuned to perceive signals produced by both prey and predators. For example, by an organism's perceptual tuning to perceive echolocating bats. This anti-predator strategy is also seen in some spider systems, such as '' Deinopis spinosa'', which have evolved higher perceptual capabilities to perceive avian predator cues. In addition, there is evidence that predator species will use auditory cues to detect prey items as well as
parasitoid wasp Parasitoid wasps are a large group of hymenopteran Superfamily (zoology), superfamilies, with all but the wood wasps (Orussoidea) being in the wasp-waisted Apocrita. As parasitoids, they lay their eggs on or in the bodies of other arthropods, ...
s that can detect host cricket species. Acoustic signals are considered to be most affected by biotic noise rather than abiotic noise compared to other signal modalities. In a study of tropical bird species, it was determined that divergence in bird song during the dawn chorus was more strongly predicted by the composition of competing songs rather than phylogenetic relatedness or physical location. This has also been tested and agreed upon in insects and frogs, as well as other bird systems. Evidence for sensory drive in acoustic signaling and perception is lower than that in visual signaling. In a recent
meta-analysis Meta-analysis is a method of synthesis of quantitative data from multiple independent studies addressing a common research question. An important part of this method involves computing a combined effect size across all of the studies. As such, th ...
, they determined that of the studies showing no support for sensory drive, 95% of them were testing the acoustic signal modality. However, of the studies testing the sensory drive hypothesis in acoustic signaling, only two tested both components of sensory drive (signal and sensory). The majority of acoustic studies tested the signal component of sensory drive and there were many fewer studies testing the sensory component, suggesting this is an understudied modality. Support for both components of the sensory drive hypothesis has been identified in birds, specifically ''
Parus major The great tit (''Parus major'') is a small passerine bird in the tit family Paridae. It is a widespread and common species throughout Europe, the Middle East, Central Asia and east across the Palearctic to the Amur River, south to parts of No ...
'', and frogs, specifically '' Amolops tormotus''. In both cases, background noise is the driver underlying the sensory biases contributing to sensory drive and signal divergence. There is also support in acoustic systems for sensory drive contributing to
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
and the diversification of lineages. For example, the ''Paragalago zanzibaricus'' species complex shows support for speciation by sensory drive by looking at responses to
conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organism ...
and
heterospecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organism ...
signals and ear morphology. Additionally, a study in tropical birds found that habitat characteristics predict acoustic divergence, suggesting sensory drive is playing a role in this divergence and potentially
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensu ...


Chemical signals and chemosensation

Chemical signals are among the most prevalent signal modalities, but there is a lack of research focusing on sensory drive in this modality. This is likely driven by three main complications when studying chemosensation: complex chemical backgrounds, complex chemical signals, and complex genetic basis for chemosensation.   Recent work has discovered that chemical signals, similar to the other modalities discussed, are also affected by environmental conditions. Under higher temperatures, chemical signals travel faster but also fade faster. Similarly, in nutrient rich or chemically complex environments, chemical signal detection can be greatly decreased. These signals can also be subject to different interferences such as masking by other chemicals, disruption of the biological signaling pathway, or interference with the chemosensor The majority of support for sensory drive in chemosensation is considered a by-product of research that is not explicitly exploring sensory drive. For example, in the ''
Podarcis hispanicus ''Podarcis hispanicus'', also known as Iberian wall lizard, is a small wall lizard species of the genus ''Podarcis''. It is found in the Iberian Peninsula, in northwestern Africa and in coastal districts in Languedoc-Roussillon in France. In Span ...
'' species complex divergent environmental conditions (cold and humid vs. wet and dry) have led to divergence in the chemical composition of the signals. This divergence is likely to contribute to producing more efficient and stable signals in their respective environments. These signals are used to mark male territory and attract female mates and both males and females show stronger responses to the conspecific signals from their respective environments. While this is only a single example, this modality may be more important for sensory drive and the process of speciation than the lack of literature suggests.


References

{{Reflist Animal communication Population ecology