Convergent Evolution
There is a pervasive occurrence of convergence across the tree of life. Many roads can lead to evolved similarity in disparate branches. ~ Hungarian evolutionary geneticist László Nagy
Convergent evolution (aka parallel evolution) is the acquisition of the same trait in unrelated life forms (different clades). (Selfsame trait evolution is considered parallel between organisms if both ancestors had an antecedent similarity that can be pointed to as the genomic foundation for the trait’s development, or convergent if not. Considering our limited knowledge of the genetic dynamics involved in adaptation, making such a distinction is often problematic.) Instances of convergent evolution are voluminous.
The fact that many solutions are used over and over again by completely unrelated species suggests that the evolutionary path is repeatable and predictable. ~ Peter Andolfatto
Convergence in Nature is more common than many biologists would have wagered not long ago. Nonetheless, convergence is not inevitable – in many cases, lineages adapt in different ways to the same environmental conditions. ~ American evolutionary biologist Zachary Blount et al
Plants
Despite striking differences in climate, soils, and evolutionary history among diverse biomes ranging from tropical and temperate forests to alpine tundra and desert, there are similar interspecific relationships among leaf structure and function and plant growth in all biomes. This demonstrates convergent evolution and global generality in plant functioning, despite the enormous diversity of plant species and biomes. ~ American plant ecologist Peter Reich et al
Plants present a litany of convergent evolution. To begin, there were at least 5 independent evolutions of single-celled photosynthetic organisms into multicellular plant forms by 600 MYA. The best known are brown, red, and green algae. Green algae alone gave birth to the land-based life commonly called flora.
Despite substantial variations in the morphology of the lineages of plant forms, they share common functionalities, and means of achieving them. Kelp, which are members of the brown algae group, possess trumpet cells which are like phloem, the conductive tissue found in green plants that transports sugars from sunlit tissue to those below which live in perpetual shade.
Selfsame problems gave rise to similar solutions between evolutionarily remote plant groups. The non-vascular structure of some brown algae is strikingly like the arrangement of leaf-stem-root in vascular plants.
Algae live liquidly, where the needs of life are readily available in the immediate surroundings. The move to terra firma required extensive modification in body plan, and strategies to cope in a completely different habitat. Needed were novel methods of CO2 and nutrient acquisition, dealing with desiccation, body support, and reproduction. These were first met with economy, and later succeeded with embellishment.
As all plants are photosynthesizers, they share the same requirements to capture light and internally exchange water and nutrients. Relationships in body plan geometry, such as surface areas and volumes, are crucial, and thereby constrained in terms of practical possibilities.
A critical feature in plant physiology is the presence of a cell wall. Beyond securing internal contents from environmental exposure, cell walls provide rigidity and strength to hold shape and protect against mechanical stress. Plant cell walls, along with characteristics dependent upon cell cycle and growth conditions, including rigidity-control mechanics, evolved independently many times.
If one were to design an organism to optimize light collection, a branched structure with flat-bladed leaves might seem obvious. There are constraints in the anatomical and biomechanical features of leaves and petioles (leaf-supporting stalks). These again exhibit convergent evolution in various plant lineages.
Another driving factor arose from competition for access to sunlight. The ultimate achievement would be a perennial lifestyle: literally being able to perpetually stand one’s ground, and growth potentiality to lift one’s leaves above others. Thus, in various forms and means, evolved trees. Many employ woods, but others, such as palms and bamboo (an ambitious grass), do not.
Acquiring carbon dioxide for photosynthesis through controllable pores (stomata) causes water loss by transpiration. Although plants evolved various mechanisms to alleviate this problem, one well-established solution is C4. This technique separately evolved at least 45 times, starting 32 MYA, and most recently 4 MYA.
The familiar fixation of nitrogen via symbiotic microbes independently evolved in over a dozen families of flowers. Their accommodation by plants varies, indicating loose constraints to a decided functional need.
Convergence is also shown with parasitic and carnivorous plants. There are at least 4,000 species of parasitic plants distributed among 19 different flowering families, with 6 different lifestyles which establish the type of parasitism. All parasites use a haustorium to penetrate host tissue and connect their vascular system to that of the host.
Some 600 species of carnivorous plants are known, in 6 angiosperm families, including both monocots and dicots. Typically living in bright, low-nutrient, water-logged environs, many produce adhesive areas on stems and leaves to lure, capture and digest insects. Sundews are exemplary. With at least 194 species, they are the most specious genus of parasitic plant. Their flypaper lifestyle independently arose 5 times. Similarly, the pitfall traps of pitcher plants convergently evolved in 4 families (3 dicot and 1 monocot).
Over 1,064 species in 7 genera of euphorbias are succulents that store large volumes of water in their short, broad stems, which are green and photosynthesize. Desert conditions led to the same adaptations employed by the unrelated cactus family, with over 2,000 species in ~175 genera. Similarly, conifers in the forests independently converged on the same genetic changes to better withstand cold weather.
Further demonstration of plant convergence is that vegetation in selfsame biomes tends to be similar. The Mediterranean climate – mild, wet winters and hot, dry summers – occurs in California where chaparral plants live, South Africa, central Chile, and southern Australia, as well as the Mediterranean. In all these areas, native vegetation is a dense scrub, dominated by woody evergreen sclerophyllous shrubs. Sclerophyll is a type of vegetation that has hard leaves oriented parallel or oblique to direct sunlight, with short internodes (the distance between leaves along the stem).
Likewise, Alpine plants typically have small, hairy leaves and prostate stems; a response to often perpetual brisk winds and soil mineral deficiency.
Genera with very different flower shapes are often very closely related, and genera with highly similar flowers share such similarity via convergent evolution. Floral traits are more prone to rapid evolutionary changes in response to local ecological conditions, whereas vegetative and fruiting traits are more conserved and not readily shaped by local conditions. ~ Brazilian botanist Domingos Cardoso
Digestive Flexibility
Homogenous test tube populations of E. coli were fed easy-to-digest glucose and harder-to-stomach acetate. In each test tube, 2 groups emerged, each specializing in 1 of the 2 food sources.
Then they gained the ability to better switch between meal types. This last adaptation would have not been useful until the specialization had emerged, which helped exhaust food supplies faster. The different bacterial populations genetically came about their adaptations in the same way and in the same order.
The results provide empirical evidence of adaptive diversification as a predictable evolutionary process. ~ Matthew Herron & Michael Doebeli
Foraging
Foraging is a universal behavior in motile organisms. Animals that hunt sparse prey over large areas often traverse terrain in a pattern known as a Lévy walk, which is characterized by several small steps interspersed with infrequent long steps.
This Markov process optimizes covering ground. Albatrosses searching for squid over the open ocean nowadays and sea urchins on the seabed 50 million years ago both took a Lévy walk.
The efficacy of a Lévy walk is appreciated by cells. Food is transported within a cell using the same mathematical pattern that animals employ when foraging.
Blood Suckers
Blood is over 95% water. The rest comprises proteins, a sprinkling of sugars, minerals, and other small molecules, but almost no fat.
Tiny creatures do fine feeding on such light fare, which is why the overwhelming majority of blood suckers are arthropods: bedbugs, ticks, chiggers, female mosquitos. For larger sanguivores, hematophagy is as much of a challenge to survive as it is to adaptively acquire.
Lacking dietary fat, vampire bats cannot pack on reserves, and so must consume half their 1-ounce body weight in blood every night or risk starving to death. Because the water in a blood meal would make the bats too heavy to fly, vampires freely urinate as they feed.
Despite the challenges, several animals evolved a taste for blood, and the equipment and behaviors that make it a feasible lifestyle, including hatpin teeth, clot buster chemicals, and pain deadeners.
Bedbug habits and senses are well-adapted to find and reside in the living quarters of their prey. Bedbug mouthparts deliver painkillers and anticoagulants to prime victims.
In between feedings, bedbugs prefer nooks and crannies where they are not easily dislodged. Aquatic leeches aim for pockets and crevices on their victims when attaching themselves for a feast.
A vampire bat does not suck the blood of its victims. The bat instead lets subtle physics do the sucking for it. Relying upon capillary action, a vampire bat’s cleft lower lip, perfectly spaced lower incisors, and doubly grooved tongue jointly form a tube through which a victim’s blood is readily pulled up. The anticoagulants in bat spit are so potent that a host animal often continues to bleed long after the vampire has feasted its fill and flown on.
Hematophagy is a difficult, dangerous trade. Blood feeders must be stealthy and good at escaping the swats and fury of their often much larger hosts.
The common vampire bat, which feeds on large land animals, creeps along the ground like a spider. As well as its flight capabilities, a vampire bat can spring straight up a meter to attach itself.
The white-winged vampire bat approaches a potential host chicken so softly and lovingly that a hen may be deceived and sweep it up to its brood patch as though to warm its own chick.
The candiru is a tiny catfish found in the Amazon and Orinoco Rivers. They are enticed by the scent of urine. Fish urinate through their gills. The modus operandus of a candiru is to infiltrate its host’s gill slits, grasp the flesh inside, rupture blood vessels, pump out as much blood as it can in a minute or 2, and dart out again. Larger catfish are candiru’s favored prey.
Vampire finches on the Galápagos Islands live mostly on seeds, nectar, and eggs. They supplement their diet with occasional iron-rich snacks by persistently pecking at the wings and tail region of one of the blue-footed boobies that live there. Once the finch draws blood, other finches line up like customers at a deli counter. Curiously, victimized boobies do not offer much resistance.
The oxpecker is famed for living aboard large mammals – rhinos, buffalo, and giraffes–and plucking ticks off their hosts’ hides. This symbiosis is sanguinary. Oxpeckers press their beaks in the wounds where ticks are lodged and take nips of blood from the beasts they reside on: trading one blood sucker for another.
Swimming Ants
The tropics are lush because they are often awash in rain. Tropical ants must adapt to the sometimes-soggy conditions, even those that live in trees, as they may be dropped into the drink.
Not surprisingly, dozens of different tropical ant species independently evolved the ability to swim, though some do so better than others. Some ants swim with front and middle legs in a synchronized manner. Others are less coordinated. A few species can stand up and literally walk on water. Once dunked, all ants proceed in a determined and directed way.
Rove Beetles
Rove beetles are an ancient family, primarily distinguished by a short shard (wing cover) that typically leaves over half their abdomens exposed. Emerging over 200 million years ago during the Triassic, rove is the most specious family of beetle, with over 63,000 species in thousands of genera. Rove are roving in every biome where beetles live.
Most rove are invertebrate predators, especially of other insects. They often live in obscure places: forest leaf litter, under stones, and around freshwater margins. Around 400 species reside on ocean shores that are submerged at high tide.
A social insect colony is an energy-packed resource with lots of different niches. If you manage to get in, the payoff is very high. ~ American evolutionary biologist Daniel Kronauer
Army ants are notoriously fierce. They specialize in coordinated mass attacks. Yet within the bivouacs of all 340 known species of army ant live tiny rove beetles, surreptitiously stealing food from their unsuspecting hosts.
These flea-sized parasites lost their beetleness to look, smell, and behave so much like ants that they can pilfer food stores or eat ant larvae with impunity. This remarkable feat of evolutionary deception independently evolved dozens of times.
The common ancestor of the parasitic lineage of rove beetles evolved a gland at the tip of the abdomen that can squirt noxious compounds – quinones – at attackers. This was a useful head start.
It’s a pre-adaptation that allows you to undergo some extreme adaptations. ~ American entomologist Joseph Parker
Since then, parasitic beetles evolved variant glands with new functions. One group sprays ant alarm pheromones that send would-be attackers scattering. Another secretes chemicals that ants use to recognize colony mates.
With the help of a raft of morphological features – including narrowed waists, longer legs, antennae like ants, and even an antlike gait – interloping rove beetles persuasively fool their fearsome hosts into feeding, protecting, and transporting them.
High-Altitude Hummingbirds
With elevated aerobic metabolism, hummingbirds have exceedingly high oxygen demands. Yet 63 species thrive at high-altitude in the Andes, where oxygen is scarce.
These different hummingbirds independently evolved hemoglobins with enhanced oxygen-binding. They repeatedly did so via the same molecular changes.
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While hummingbirds have convergently adapted to high altitudes similarly, other birds have not. Avian adaptation to alpine air has transpired innumerable times, but the molecular means to do so has differed considerably. These discrepancies reflect differences in evolutionary history.
Even in cases where adaptive phenotypic change is predictable, the molecular pathways to these changes may not be. ~ Indian biologist Chandrasekhar Natarajan et al
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Efficiencies are not just chemical. There are economies of scale in social organization. These can be cued chemically.
Insect Eusociality
Social insect queen pheromones likely evolved from preexisting fertility signals directed at males. ~ Belgian evolutionary biologist Annette Van Oystaeyen et al
Eusocial wasps, bees, and ants possess unsurpassed coherent societies: so organized as to make human civilization look like mindless chaos. Hundreds of thousands, sometimes millions, of eusocial insects build elaborate nests which are run at an epitome of cleanliness and efficiency.
The lynchpin to the success of eusociality is reproductive division of labor. A queen devotes herself to laying eggs while her daughters take care of all else.
The key to this role relegation are fertility-related pheromones that arose are 145 million years old in the common ancestor to eusocial insects. A queen exudes a long-chained hydrocarbon that suppresses worker reproduction.
Employing selfsame pheromones, reproductive division of labor arose independently at 3 times via a highly conserved gene set. This distinctly led to eusociality at least 10 times.
Blue Tarantulas
Even within monophyletic lineages, tarantulas have evolved strikingly similar blue coloration through divergent mechanisms. ~ Chinese biologist Bor-Kai Hsiung et al
Among tarantulas, blue hair evolved independently at least 8 times. Why remains a mystery.
Recent research suggests that tarantulas may have color vision. If so, the blue may be a mating signal. That explanation is not wholly convincing, as these spiders are almost entirely crepuscular, if not nocturnal. But then, if tarantulas do see in color, they may be able to do so in very low light.
The blue may sufficiently resemble flower petals to lure some insects within striking distance. Blue may help hide tarantulas from predators in the green-tinted light of the forest. Or it may look enough like the color of some fierce wasps, and so give hungry predators pause before attacking.
Certainly, blue hair among tarantulas is no coincidence. All of them reflect blue light within a narrow ~10 nanometer range at the 450-nm wavelength.
The blueness comes solely from their hair. Beneath even the most eye-popping sapphire locks lies a dark, dull cuticle.
Tarantula hair blueness is produced by embedded nanoscale structures that vary widely among spiders. Some species produce an ordered phalanx of micro pancake stacks: cuticle layers alternating between thin air pockets. Others have subtle arrays of quasi-ordered spongy structures like those of bird feathers.
Structures and evolutionary histories differ, but tarantulas are “evolving the same blue over and over again,” observed American biologist Todd Blackledge.
{Note that this section was updated since the book was published, as new information became available.}
Dragonfish
In the dark, deep sea food is very scarce. ~ American ichthyologist David Johnson
Besides the dark and crushing pressure, the deep sea is a dangerous place. Lacking light, many deep-sea animals independently evolved bioluminescence to help them see, and to communicate with conspecifics.
The risk of shedding (or reflecting) light is marking yourself as prey. Dragonfish cultivate this defect to their advantage by having a bioluminescent bulb at the end of a tether below their head that acts as a lure. These lie-and-wait ambush predators save energy by drawing dinner to them. The glowing lure is bait for a fish that is sublimely equipped to capture and eat more than its fair share.
Dragonfish have an array of traits that render them tiny sharks of the dark with their efficient design. (Dragonfish only grow at most to 46 cm; many are only pencil length.) Their skin is a nonreflective matt black, achieved by a film of microscopic melanin granules which soak up light. 2 noteworthy peculiarities – loose jaws and distensible stomachs – borrow from the playbook of snakes.
Loose dragonfish jaws are achieved by replacing neck vertebrate with flexible tendons. The rubber neck lets dragonfish open their mouths over 120°, allowing them to swallow prey as long as and larger than they are; hence the distensible stomachs.
To secure their catch, dragonfish have long saber-like teeth on jaws that snap shut with astonishing speed. The teeth are an engineering marvel. Dragonfish teeth are sharper than piranha teeth and as hard as the teeth on great white sharks. To avoid reflecting light, dragonfish teeth are transparent. The effect is achieved by miniscule nanocrystals spread throughout the enamel which pass light through without scattering, thereby eliminating reflection. The transparent teeth and non-reflective skin mean that dragonfishes’ bioluminescent lure doesn’t give them away to potential prey.
Camouflage
As a protective stratagem camouflage independently evolved in various sea urchins, gastropods, crabs, insects, amphibians, cephalopods, reptiles, birds, and mammals.
Japanese quail understand the value of camouflage. Hens know the patterning of their eggs and carefully lay them in locations that hide them best.
Camouflage has a variety of forms. The best known is coloration, which may change seasonally and/or at separate stages of life, if not at will, as with coleoids (squid, cuttlefish, and octopuses). Some cuttlefish and octopi can create exquisitely detailed and exact colored patterns via their minds in milliseconds despite being color-blind.
Clupeids are a family of ray-finned fishes that includes herrings, sardines, and shades. Silvery clupeids convergently evolved a multilayer skin made of 2 types of guanine crystals, each with distinct optical properties. The different crystals are meticulously arranged to cancel the polarization of light reflected off the fish.
These fish evolved this particular multilayer structure to help conceal them from predators, such as dolphin and tuna. They found a way to maximize their reflectivity over all angles they are viewed from. This helps the fish best match the light environment of the open ocean, making them less likely to be seen. ~ English physicist Nicholas Roberts
Concealment camouflage is often coupled with behaviors to secure the effect, such as prey staying stock still, or predators slowly stalking.
With mimesis a creature’s camouflage makes it look inconsequential. Posturing as a plant is a common ploy. Both prey and predator make such a masquerade. To avoid being munched, a grasshopper looks like a leaf. A stick insect passes for a twig. As a predatory ploy, a flower mantis climbs to the top of a suitable plant and appears to be in bloom, awaiting prey.
The orchid mantis specializes in pink orchids. A mantis sways side-to-side, as in the breeze. Various small flies land on and about it, attracted by a black spot at the tip of its abdomen that resembles a fly; exactly what the mantis is hoping to nab.
There are numerous cryptic behaviors employed situationally. Most mimic the environment. Leafy sea dragons sway like the seaweed among which they rest.
As stealth hunters, dragonflies employ motion camouflage: flying in way that hides them from their prey.
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The expression of evolution begins at the molecular level. Convergent evolution occurs there as well.
Endogenous Retroviruses
Endogenous retroviruses are transposable elements (TEs) that resemble retroviruses. TEs can change their positions within a genome, and so have been called “jumping genes.”
Transposable elements can drive evolution by creating genetic and epigenetic variation. ~ Japanese cytologist Tokuji Tsuchiya & American cytologist Thomas Eulgem
The basic process of using the information encoded in DNA – transcription – employs RNA as an intermediary. Retrotransposons are TEs that employ RNA to copy themselves. Endogenous retroviruses independently evolved into retrotransposons multiple times; an example of convergent evolution at the molecular level.
Adipose Fins
Vertebrates in general have conserved body plans. New appendages, whether fins or limbs, evolve rarely. ~ American evolutionary biologist Michael Coates
An adipose fin is a small, soft, fleshy fin found on the back of a fish, behind the dorsal fin and just in front of the caudal fin. The adipose fin is a sensory organ, though its exact employment is not known. Over 6,000 species of fish have adipose fins, including salmon, trout, catfishes, and characids.
The adipose fin did not descend from a common ancestor. Instead, it repeatedly arose. Moreover, the adipose fin convergently evolved with different structures, indicating selfsame functionality obtained via various physical means.
Flight
Powered flight is a classic example of convergent evolution. Insects were the first land animal to take to the air but the first flight may have taken place in the water.
Sea butterflies are a small swimming predatory sea snail. These zooplankton flap their wings in the same way that insects do, albeit much slower: 4–5 wing beats per second versus hundreds for a small insect.
From a hydrodynamic standpoint, water is a viscous version of air. Sea butterflies flap in a figure-8 pattern that provides lift in the water like tiny insects do in the air. To do so, the snails rotate their bodies along with the clap and fling of wings characteristic of fruit fly flight.
The earliest vertebrates known to have powered flight were pterosaurs, a cousin to dinosaurs. Down the evolutionary line, a cousin evolved in a way that would cause a flap.
Some flightless dinosaurs lived in trees. Getting around, and quickly down to the ground, led to feathered wings. In a word: the bird.
On a completely different linage, bats are mammals whose forelimbs formed webbed wings between the fingers.
It’s shocking how incredibly soft and compliant bat wings are. The wings are geometrically complicated because they have all these joints. ~ American evolutionary biologist Sharon Swartz
Whereas birds flap their entire forelimbs, bats flap their fingers, which are long and covered by a thin skin (patagium) or membrane. Yet, due to biomechanical constraints and the evolutionary vector of limb modification, the wings of birds and bats are similar in construction.
Pterosaurs had yet a different flight path to the same effect. Their wings evolved from a greatly lengthened 4th of 4 digits, with an arm membrane (brachiopatigium) extending from the 4th finger to the body, creating a wing. The other digits were part of the wing in some pterosaur species. Many pterosaurs, if not all, also had webbed feet.
Powered flight evolved independently in pterosaurs, birds, and bats, each of which has a different configuration of the bony elements and epidermal structures that form the wings. ~ Chinese vertebrate paleontologist Min Wang
Flightlessness
Convergence toward gigantism and flightlessness was facilitated by early Tertiary expansion into the diurnal herbivory niche after the extinction of the dinosaurs. ~ Australian evolutionary biologist Kieren Mitchell
Sometimes flight becomes an unnecessary extravagance, even if you are a bird. All the flightless birds are lumped together as ratites. Ratites tend to be sizable birds. Several are fast runners, as befitting being large and living on a savanna. The ostrich can outrun a horse.
It’s relatively easy to lose flight. ~ American evolutionary biologist Scott Edwards
Flightlessness convergently evolved at least 5 times, consistently doing so through delicate changes in regulatory genes focused on limb development. More disruptive mutations were done to facilitate optimal metabolism for the fleet-but-flightless lifestyle.
Swimming
In that it overcomes a stronger gravitational force by occurring in air, flight is the fancy version of swimming. Unsurprisingly, selfsame biomechanics of swimming through viscous media convergently evolved innumerable times.
Undulatory locomotion, a gait in which thrust is produced in the opposite direction of a traveling wave of body bending, is a common mode of propulsion used by animals in fluids, on land, and even within sand. ~ American biomechanics physicist Daniel Goldman et al
Vocalization
As animals vocalize, their vocal organ transforms motor commands into vocalizations for social communication. ~ Danish zoologist Coen Elemans et al
Sound is a vibration propagating as an audible wave of pressure. Vocalization is sound production by an animal through its respiratory system. Audition is sound perception.
Humans and other mammals vocalize via their vocal cords, which are a folded membrane across the larynx. Sound ushers forth when muscles regulate the voice box as air from the lungs rushes past.
Birds vocalize through their syrinx, which has no vocalizing membrane. Instead, avian vocalizations are made by muscling the walls of the syrinx during airflow.
The syrinx is a tiny box of cartilage. It reinforces the airway, and when air passes over the folds in it, it produces a sound: birdsong. ~ American zoologist Chad Eliason
Structurally, the syrinx is highly diverse across bird species. Some birds, such as condors, lack a syrinx. Their communications are restricted to throaty hisses.
Whereas the mammalian larynx is above the trachea, the syrinx is below the windpipe. Because the syrinx is located where the trachea forks into the lungs, songbirds can produce more than 1 sound at a time (lateralization): separate sounds from each bronchus.
Despite using disparate organs independently evolved, mammals and birds converged on the same mechanism for vocalization, corresponding with the myoelastic-aerodynamic theory.
Expiratory airflow is mechanically converted by vocal folds into pulse-like airflow, which causes air pressure disturbances constituting the acoustic excitation of the system. The mechanical properties and recruitment of different layers of vibrating tissues affect their resonance properties, which in combination with aerodynamic driving forces determine the frequency and mode of oscillation. ~ Czech biophysicist C.T. Herbst et al
Vocalization involves physiological structures and regulatory systems outside (ex vivo) the sound-producing organs themselves (in vivo). The entire vocalization system in birds and mammals is an instance of convergent evolution.
There is substantial redundancy in the control of key vocal parameters ex vivo, suggesting that in vivo vocalizations may also not be specified by unique motor commands. ~ Coen Elemans et al
Infant Distress
The infant distress calls of many mammal species – from deer to marmots, bats, cats, dogs, fur seals, and humans – are sonically similar: sharing acoustic elements that make them recognizable across species.
A deer understands what a human baby’s cry means. This suggests that the emotional makeup of animals is selfsame.
These mammals are separated by over 90 million years of evolution. The basis for infant cry characteristics may have originated cladistically but it is also an instance of convergent evolution, as the vocal production apparatuses for these diverse creatures are quite different.
Singing
The capacity to modify vocal syntax to changes in social context is an important component of vocal plasticity and complexity in adult vertebrates, especially in human speech. ~ American zoologist Kirsten Bohn
Flight is not the only thing that birds and bats have in common which each independently evolved.
The free-tailed bat is a fast-flying species, able to flit up to 9 meters per second. A male may only have 1/10th of a second to attract a female to his roost for some romance.
The male free-tailed bat sings beautifully: as well as any songbird in quality and creativity. Once a female is attracted, a male changes his tune: displaying versatility so as to enamor her into mating. Flying is not the only thing that free-tailed bats are fast at.
Songbirds are not born knowing the songs they will sing as adults. Like human infants picking up language, birds listen and imitate to learn the tunes they croon.
Songbirds and hominids descended from a common ancestor 300 million years ago. Birds and hominids independently acquired the ability to vocalize.
~80 genes create similar pathways in the brains of songbirds and humans; pathways which are active when imitating sounds and singing. This genetic activity is not resident in birds that cannot learn songs or mimic sounds.
(All biological functions are controlled by a mind within. The mind may be associated with a molecule, a cell, or an organism. Any reference to physiological activity, such as brain pathways, is merely artifactual to the generative mentation.)
Hearing
Though variations are possible, some biological imperatives have an optimal vector of solution. Audition is exemplary.
Hearing universally involves 3 stages: sound collection, impedance conversion, and frequency analysis. In mammals, the eardrum, middle ear, and cochlear serve those respective functions.
Katydids, which have some of the smallest of ears of all organisms, have the same tripartite structure for audition, albeit with much different components. Each part performs the identical function as in mammals.
Toxic Adaptation
Numerous insects have independently evolved the ability to feed on plants that produce toxic secondary compounds and sequester these compounds for use in their defense. ~ Chinese evolutionary biologist Ying Zhen et al
Plants produce vast arrays of chemical protection for themselves that are toxic to would-be predators. In an evolutionary arms race, animals may overcome this barrier to consumption, at least to some tolerance. Some animals even adapt to employ plant toxins to their own advantage.
Alkaloids
Some 3% of flowering plants produce pyrrolizidine alkaloids, which are a nasty trick. To an herbivore, the alkaloids go down well enough, but are converted into toxins as metabolized. Both the African grasshopper and European cinnabar moth caterpillar, by evolving the same specific enzyme, adapted to safely store the toxin themselves. This adaptation conferred protection from predation.
Separated by 300 million years of evolution, 14 species of insect – from aphids to beetles to butterflies – convergently evolved the same molecular mechanism to handle the cardenolides that are the natural defense for dogbane, milkweed, and other plants. This steroid cardiotoxin usually cripples the ability of cells to regulate sodium-to-potassium ratio. The different insects independently tweaked the gene that produces the protein which handles this chore in the same way, to nullify the effect that cardenolides normally have; identical convergent evolution.
Glycosides
Cardiac glycosides are toxic molecules that perturb the cell membranes of animals, causing cardiotoxicity. Several plants and some toads produce these glycosides as a defense against predation.
Widely divergent insects, amphibians, reptiles, and mammals have overcome the toxicity to dine on these glycoside producers. All of them evolved resistance via 2 identical amino acid changes in a specific portion of a single gene. (There were possible alternate molecular routes to this common trait.) The gene produces a protein that is essential to the binding site that makes the glycosides toxic. The adaptation renders the glycosides harmless.
The predictability of this convergent evolution is underlined by its reversal in carnivorous lizards that migrate to toad-free areas. These lizards energetically economize by not producing unnecessary proteins.
Similar selection pressures have resulted in convergent evolution of the same molecular solution across the breadth of the animal kingdom, demonstrating how a scarcity of possible solutions to a selective challenge can lead to highly predictable evolutionary responses. ~ Australian biologist Beata Ujvari
Vision
What it means to be an eye is so much broader than we originally thought. ~ American zoologist Sönke Johnson
Light is a rich information-carrying medium, so it is unsurprising that vision evolved early and in most forms of life. Vision independently evolved over 50 times. Though sight itself is functionally convergent, there are an astonishing variety of vision systems, including eyes that are not at all apparent.
Many invertebrates and vertebrates have different vision subsystems, each tuned to one specific task. ~ English biologist Innes Cuthill et al
Erythropsidinium is a single-celled alga. It has an organelle eye that it uses to catch prey and avoid predators. The eye detects polarized light. Erythropsidinium‘s eye is so extraordinary that no one believed the biologist who first described it over a century ago. But then, it is hard to explain how an organism can see when it has no nerves nor a brain.
Sea Urchins
Sea urchins are mobile pincushions. Their spiny defense system dominates their appearance. They have no eyes per se. Instead, a sea urchin is an eye: a very prickly one.
The tube feet that propel urchins possess opsins, which are the proteins that make animal eyes possible. Opsins at the base of the tube feet are complimented by a 2nd set of opsins scattered over an urchin’s surface.
Altogether, an urchin’s mind has enough pixelated information to put together a rough image: good enough to tell light from dark and detect movement; sufficient sight to know when to put one’s spines up or run for cover.
Sea urchins have no brain. They do have a simple nervous system that rings their mouth: right where the action is.
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All animal eyes may have had a common origin: a proto-eye that could resolve something more than light from dark. Sophisticated optical systems seem to have appeared abruptly ~540 million years ago, in the Cambrian explosion of life forms.
Rapid eye evolution was dictated by competition between predators and prey. Good eyesight quickly became essential. As well as gross morphology, nimble innovation in fine-scale anatomy appeared during the Cambrian.
Eyes for vision arose independently in cnidarians (e.g., jellyfish), echinoderms (e.g., sea stars), cephalopods (e.g., squid), invertebrates (e.g., insects), and vertebrates. The last common ancestor of these divergent phyla may have had at most a photosensitive spot; or maybe not.
Chiton
Nature provides a multitude of examples of multifunctional structural materials in which trade-offs are imposed by conflicting functional requirements. ~ Chinese materials scientist Ling Li et al
Chitons are marine mollusks that arose 500 MYA. They have a segmented shell connected by muscular tissue that allows them to roll up into a protective ball. Chitons can move with surprisingly speed. If they choose, chitons can cling tightly to irregular surfaces.
Chitons do not have a brain and not much of a nervous system. But they are aware of their surroundings and act accordingly: getting themselves picked up by passing waves when they want, using ambush tactics on prey, and reacting to threats of predation.
What Nature has perfected is to use comparatively simple, cheap starting materials and turn them into an exquisite, multifunctional material. ~ German materials scientist Peter Fratzl
While most chiton are nocturnal, some have busy days. Of these, 100 of the 940 extant species have up to 1,000 aragonite eyes embedded in shell crevices. This plethora of peepers let chitons watch the action around them: their minds putting together the multitude of inputs into a single image.
It actually forms a shockingly clear image. ~ Sönke Johnson
Aragonite is a variety of calcium carbonate (aka calcite or calspar) and is one of the most common minerals on Earth. Calcite is a major constituent of limestone, chalk, and marble. It is also the building material for animal shells, exoskeletons, and eggshells. And, in the case of chiton, eyes.
There is a drawback to chiton calcite eyes. The lenses create weak spots in the shell’s armor.
In order to see, they had to back off on mechanical protection. ~ American evolutionary biomechanist Sheila Patek
Chiton eyes nestle in protective grooves, with shell protrusions that partly compensate for the eye spots.
Chitons might see better with slightly larger eyes. But that apparently would compromise shell integrity too much.
Sometimes we assume Nature is perfect. But more often than not it is a perfect compromise. ~ English biologist Andrew Parker
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Adaptive pressures led to progressive refinements in vision equipment, though not always obvious improvements. Vertebrate eyes oddly have blood and nerve vessels coming from the front. The cephalopod eye has a more logical wiring, with the vessels from the back.
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Wasp eyes illustrate how sociality can drive senses.
Paper Wasps
Whereas dark North American paper wasps each make their own nest, golden paper wasps form small colonies by building a joint nest where they lay their eggs.
Clustering queens squabble and determine a dominance hierarchy. A social hierarchy is only possible if individuals can recognize one another.
Compound eyes are great for detecting motion but yield poor resolution. Comparatively, larger diameter lenses collect more light, providing sharper vision.
Many flying insects have a high-acuity zone within their compound eye, outfitted with larger diameter lenses. The lenses with heightened acuity face forward, which is the typical flight direction.
Social wasps had 2 adaptations which facilitated individual recognition: singular looks and the sight to see it.
Stiff exoskeletons do not afford facial expressions but patterns on the face allow an individual look. Social wasps all have distinctive faces.
Whereas large wasps tend to be solitary, smaller ones socialize. Smaller social wasp eyes are proportionately large, delivering sight just as good as their bigger cousins. Those with variable facial patterns have larger lenses in their acute zone than those that do not have variable faces.
Social wasps evolved improved acuity relative to size to discriminate among different individuals in the colony. ~ American zoologist Michael Sheehan
As they live within a social hierarchy, social paper wasps can infer rank by watching how other wasps behave around each other: an acumen called transitive inference. Honeybees, with equivalent brains, do not posses this skill; but then, they don’t need to, as they don’t have a social hierarchy.
Wasps use known relationships to make inferences about unknown relationships. The capacity for complex behavior is shaped by the social environment in which behaviors are beneficial, rather than being defined by brain size. Miniature nervous systems do not limit sophisticated behaviors. ~ American evolutionary biologist Elizabeth Tibbetts
Echolocation
This highly specialized life trait is affecting vast portions of the genetic makeup of the organism. ~ English biologist Joe Parker
Echolocation is a complex trait, involving production, reception, and sensory processing of ultrasonic pulses. It is used for orientation, avoiding obstacles, communication, and hunting.
Echolocation evolved in bats as a way to fly at night without banging into things. Fruit bats, also known as the flying foxes, produce clicks with their wings that allow them to hunt in darkness via echolocation. This is more rudimentary than the oral biosonar of other bats, which arose independently at least twice.
Echolocation also convergently evolved in dolphins and toothed whales 34 MYA. Narwhals, the unicorns of the sea, are superb practitioners: equal to bats in precise employment of echolocation.
Echolocation is not a matter of a few genes. It involves multiple genetic complexes ranging over hundreds of genes, geared to the numerous traits required for an organism to echolocate.
In echolocating mammals there were identical changes in nearly 200 genes – incredible! ~ Joe Parker
Like vision, echolocation beautifully illustrates adaptation as a teleological exercise.
Suction Vision
Blind cavefish nimbly navigate the pitch-black caves where they live, never bumping into obstacles. In a liquid variant of echolocation, cavefish see through suction.
Sucking water into its mouth, a cavefish creates currents in the surrounding water that flow around and reverberate off objects. A fish detects these subtle water movements via lateral line sensors on it flanks.
As with other fish, the blind cavefish mentally maps nearby terrain by subtracting its own wave production while swimming.
When approaching large objects, cavefish suck more often: getting better resolution by feedback from more frequent pressure waves. From a physics standpoint this is the same technique that echolocating animals employ.
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Convergent evolution raises the issue of evolutionary inevitability: that functional facets of life are constrained to emerge similarly.
Nervous Systems
The earliest-evolved animals include sponges, placozoa, cnidarians, and ctenophores (comb jellies). Though they somehow relate to the original metazoan, their lineage remains uncertain. Though comb jellies are more complex than sponges, modern ones retain the most ancient DNA. These 4 may well all be basal: originators of their respective clades.
Sponges have unspecialized cells which can migrate and transform into the needed type. Sponges do not have circulatory, digestive, or nervous systems. They do possess an immune system.
Placozoa are a basal invertebrate: one of the simplest non-parasitic metazoa. These small flattened animals have specialized cells, including fibers to interconnect cells for communication and coordination. But they lack neurons.
Cnidarians include jellyfish and anthozoa (coral, sea anemone and sea pens). Cnidaria have various cell types, including nerve cells.
Comb jellies (ctenophores) have an elementary brain with nerve cells which are linked by complex synapses to muscles. These jellies have a nervous system like no other, with unique neurons and communication signaling molecules.
Cnidarians and ctenophores do not share a common ancestor. Nervous systems evolved at least twice.
Everyone thinks this kind of complexity cannot be done twice. But ctenophores suggest that it happens. There is more than one way to make a brain, a complex neural circuit and behaviors. ~ Russian biologist Leonid Moroz
The comb jelly genome is radically different from other animals. Besides nervous systems, comb jellies are distinct from other genomes in their immune system and developmental genes.
They are the aliens of the sea. ~ Leonid Moroz
Intelligence & Brains
Intelligence is a sine qua non for every entity that must be responsive to its environment. From viruses and single cells on up, awareness, memory, pattern-matching, and decision-making are essential. Every cell, every organism has a mind.
Complex cognitive abilities evolved multiple times in distantly related animals with vastly different brain structures in order to solve similar socioecological problems. ~ English zoologist Nathan Emery & English psychologist Nicola Clayton
That birds have considerably different brain structures than other modern vertebrates illustrate that evolutionary drives are to functional solutions, not biological forms: which is a caution to not confuse substance with essence. A brain is merely the physical substrate of the mind: a material mirage for intelligence which is energetically based. Plants are more intelligent than animals yet possess no identifiable physiological system for their acumen.
Understanding that intelligence is a universal aspect of life, the idea of convergent evolution in intelligence becomes nuanced. Cognition did not independently evolve multiple times to handle similar socioecological problems.
All thought is symbolic manipulation employing memory. Only the context differs. Specific sets of cognitive frames put an organic entity in a proper mental milieu to deal with its world.
Whereas bodies and behaviors are all that we see, we know that organisms are operationally defined by their minds. Psychology is both species-specific and individual. These principles of cognitive schemas are selfsame throughout the tree of life.
Intelligence is not a product of brains. Intelligence is instead a display of the mind, which was crafted from an evolutionary perspective to certain possibilities, inclinations, and limitations.
