Category: Senses
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An organism's senses connect it to the outside world and are essential in allowing it to react adequately to environmental stimuli. As the structure of sensory organs is limited by the physical properties of the signals they are supposed to pick up (e.g. light, sound), it is not surprising that they represent prime examples of evolutionary convergence.
With respect to vision, classic cases of convergence include the camera eye (which has not only been invented by cephalopods and vertebrates but also by several other groups) and the compound eye (famously found in arthropods as well as in other invertebrates). Colour vision has evolved multiple times in insects and vertebrates, but remarkably is also found in mantid shrimps, highly effective predators with complex compound eyes. Vertebrates are primitively tetrachromatic (having four different types of cone cells with different light absorption spectra), but most mammals are dichromatic, which might reflect a transition to a nocturnal way of life. In the largely diurnal primates, however, trichromatic vision has re-evolved, allowing them to detect coloured fruit and leaves. In many animals, both vertebrate and invertebrate, vision extends into the ultraviolet part of the electromagnetic spectrum. For example, ultraviolet plumage ornaments are often involved in bird courtship, kestrels prey on voles by following their UV-reflecting urine trails and numerous insects use UV marks on flower petals to find nectar.
Although to date there is no direct evidence for infrared vision, a number of animals have independently evolved systems of infrared detection (that in a number of respects are closely analogous to the eye) based on heat. Many snakes (e.g. pythons, pit vipers and rattlesnakes), vampire bats and insects such as bedbugs detect infrared emitted as heat from warm-blooded prey, whereas several beetles as well as hemipterans are attracted to the heat of forest fires (they lay their eggs in burnt wood). The general ability to sense thermal radiation has mainly been studied in insects and mammals. In some butterflies, for example, the antennae are equipped with thermoreceptors that seem to have some similarities to the pit receptors of snakes.
A sense of balance is essential in a three-dimensional world, and an almost universal, but convergent, method to detect changes in orientation is with the help of statoliths. These are small grains attached to fine hairs, the movement of which then triggers nerve impulses. Statoliths are employed in cephalopods, cnidarian jellyfish, crustaceans and other invertebrates, whereas in vertebrates, balance is achieved by using the semi-circular canals of the ear. Remarkably, a strikingly similar canal system operates in some crabs.
Hearing has evolved independently in a number of groups, notably in the insects and vertebrates. Many animals are able to hear very high frequency (ultrasound) or very low frequency sounds (infrasound) that lie well outside the range of human hearing. Most famous amongst those sensitive to ultrasound are the bats, which use echolocation for prey detection and navigation. But echolocation has also evolved in cetaceans (whales and dolphins), other mammals such as shrews and tenrecs, as well as some birds. In addition, a species of frog is capable of ultrasonic communication. Low frequency sounds travel partly by air and partly through the ground, which is the basis for seismic communication. It entails quite long distance transmissions (e.g. in elephants) or shorter distances (e.g. in the many insects that sense vibrations).
Other pressure-sensitive systems also show convergence. Fascinating examples include the lateral-line system, a specialised system for pressure detection under water, and mechanosensory systems based on small corpuscular touch receptors (often linked to seismic communication). The lateral-line system is best known in aquatic vertebrates (mostly fish as well as some amphibians), but analogues have evolved independently in some cephalopods, a few aquatic mammals and, in an intriguingly separate way, in some penaeid shrimps. Mechanosensory systems in different groups of animals employ different, and convergent, types of touch receptors, such as Herbst corpuscles in the bills of birds and equivalent Pacinian corpuscles in mammals. Equally noteworthy are the tactile Eimer's organs in moles and convergent push-rods in monotreme mammals. Closely associated with the mechanosensory system of monotremes is their electrosensory system. Electroreception, i.e. the ability to perceive weak electric fields, has also been demonstrated in fish (where it evolved at least twice) and amphibians, relying on different types of receptors and different brain regions for processing the electric signals.
Many features of olfaction strongly suggest that there really is an optimal solution to detecting smells. The common design of olfactory receptors has evolved independently many times and olfactory processing circuits in the brain are convergent between insects and mammals. Pheromone recognition, widespread in vertebrates and insects, employs closely analogous, but convergent, systems. Olfaction furthermore exemplifies that a reduction in sensory capacities can be convergent, too. Apes and humans show a loss of olfactory genes, which is likely to be linked to the development of acute vision, as do whales, which lost their terrestrial olfactory capacities upon returning to the oceans. Although the capacity to taste is clearly very ancient, it still provides interesting insights into convergence. A bitter modality has independently evolved in humans and chimpanzees, and in a striking example of molecular convergence, the gustatory proteins of arthropods are effectively identical in structure to those of other animals, but clearly have a completely different origin.
