Articles With our reliance on vision and hearing and our weak vibration sense, humans have only recently developed ways to mine the information provided by the soundless vibration of the surfaces around us (box 1). Nevertheless, the information is there: geophones show ongoing tremors of the earth's surface, stethoscopes bring the rhythm of our heart and lungs to a doctor's ears, and laser vibrometers let surveillance teams listen to conversation faithfully reproduced in the vibration of a windowpane. Vibration-sensitive species, including insects and spiders, can mine this wealth of information directly. They not only monitor vibrations to detect predators or prey but also introduce vibrations into structures to communicate with other individuals. In this article we provide evidence of the importance of this form of signaling, review what we know about vibrational signaling in insects, and discuss ecological sources of selection on vibrational communication systems.Whether counted by species, family, or phylogenetic distribution, vibrational signaling is prevalent in insects (figure 1). Indeed, it is the most common form of communication among the insects that use some type of mechanical disturbance propagating through a medium; this includes airborne and underwater sound, substrate vibrations, and water surface ripples (Greenfield 2002). Of the insect families in which some or all species communicate using such mechanical channels, 80% use vibrational signals alone or in combination with other mechanical signals, and 74% use vibrational signals alone (figure 1). At the species level, we estimate that 92% of such species-over 195,000 described taxa-use vibrational communication alone or in concert with other forms of mechanical signaling, and that 71%-150,000 species-use vibrational signaling exclusively (figure 1). These estimates are probably low: many if not most insect species remain to be described, and vibrational communication is probably even more taxonomically widespread than the current literature suggests.Accompanying the high species diversity of vibrational signalers is a fantastic diversity of signals. Humans can experience these signals by broadcasting them through a loudspeaker as airborne sound, a process that leaves their pitch and timing intact. One dramatic contrast between communication systems that use substrate vibration and those that use airborne sound is immediately obvious: substrate-borne signals give the impression of being produced by a large animal. This phenomenon, often startling to those listening to vibrational signals for the first time, arises from a relaxed relationship between the size of the signaling animal and the frequency (pitch) of the signal produced. When communicating with pressure waves traveling through air, small animals cannot efficiently broadcast low-frequency signals (Bennet-Clark 1998). As a consequence, only large animals can effectively produce low-frequency sounds. However, the physical constraints responsible for this relationship do not exist for subs...

The large body of theory on speciation with gene flow has brought to light fundamental differences in the effects of two types of mating rules on speciation: preference/trait rules, in which divergence in both (female) preferences and (male) mating traits is necessary for assortment, and matching rules, in which individuals mate with like individuals on the basis of the presence of traits or alleles that they have in common. These rules can emerge from a variety of behavioral or other mechanisms in ways that are not always obvious. We discuss the theoretical properties of both types of rules and explain why speciation is generally thought to be more likely under matching rather than preference/trait rules. We furthermore discuss whether specific assortative mating mechanisms fall under a preference/trait or matching rule, present empirical evidence for these mechanisms, and propose empirical tests that could distinguish between them. The synthesis of the theoretical literature on these assortative mating rules with empirical studies of the mechanisms by which they act can provide important insights into the occurrence of speciation with gene flow. Finally, by providing a clear framework we hope to inspire greater alignment in the ways that both theoreticians and empiricists study mating rules and how these rules affect speciation through maintaining or eroding barriers to gene flow among closely related species or populations.

Mate choice is considered an important influence in the evolution of mating signals and other sexual traits, and-since divergence in sexual traits causes reproductive isolation-it can be an agent of population divergence. The importance of mate choice in signal evolution can be evaluated by comparing male signal traits with female preference functions, taking into account the shape and strength of preferences. Specifically, when preferences are closed (favouring intermediate values), there should be a correlation between the preferred values and the trait means, and stronger preferences should be associated with greater preference-signal correspondence and lower signal variability. When preferences are open (favouring extreme values), signal traits are not only expected to be more variable, but should also be shifted towards the preferred values. We tested the role of female preferences in signal evolution in the Enchenopa binotata species complex of treehoppers, a clade of plant-feeding insects hypothesized to have speciated in sympatry. We found the expected relationship between signals and preferences, implicating mate choice as an agent of signal evolution. Because differences in sexual communication systems lead to reproductive isolation, the factors that promote divergence in female preferences-and, consequently, in male signals-may have an important role in the process of speciation.

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