Category: Physiology

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Physiology refers to how living systems function from molecules to cells, tissues, organs and whole organisms. Key physiological mechanisms are explored at the level of molecules and biochemistry, for example how membrane proteins and ion channels work, how enzymes catalyse critical reactions, how toxins affect cell chemistry and how plants fix nitrogen and carbon. In turn, these molecular mechanisms underlie the more tangible physical features of living organisms, from the achievement of balance through tiny particles (statoliths) in the ears to hormonal changes that make insects moult or the development of muscles specialised for flight or high-speed swimming.

As may be imagined, the fundamental nature of physiology means that all biological convergences can ultimately be correlated with shared physiology at some level. Selected examples of convergence relating directly to physiological mechanisms are listed below, and you are welcome to explore further physiology topics within the related categories of Anatomy, Biochemistry, Enzymes, Locomotion, Metabolism, Molecular Biology, Photosynthesis, Proteins and Respiration.

For a brief impression of the diversity of physiological convergences that may be associated within even a single organism, a few key examples involving the cephalopod molluscs are summarised here. At the molecular level, most molluscs use haemocyanin, a respiratory protein based on copper. Haemocyanin independently evolved in arthropods and molluscs for oxygen transport, as clearly shown by the similar active site but very distinct molecular structure of each of the two proteins. Various aspects of cephalopod anatomy betray physiological convergence with vertebrates. Octopus arms have been found to function via 'pseudo-joints' in a remarkably similar way to vertebrate arms, with a shoulder-plus-elbow arrangement apparently leading to an optimal solution. Cephalopod hearts are strongly convergent with vertebrate hearts in being of an effectively closed vascular system and having an aorta equipped with elastic proteins for pumping at high pressure in haemodynamic patterns shared with the vertebrates. Another striking convergence is of course the camera eye, highly vertebrate-like in octopus, with a lens, pupil, sensitive retina, extra-ocular muscles, capacity for accommodation and correction for spherical aberration. On the theme of eyes, certain squid have bioluminescent 'reverse' eyes (as do some fish) while others put on a defensive bioluminescent light show when threatened in the deep sea. Bioluminescence is observed in various animals both marine and terrestrial, each species dependent on the independent recruitment of the enzyme luciferase. Cephalopods (and other animals) achieve balance via tiny grains or 'statoliths' located inside 'statocysts'. Their function is analogous to that of the vertebrate semi-circular canal, where positional orientation is linked to physiological change and neural activity. When it comes to buoyancy in the oceans, females of one species of Octopus (Ocythoe) have evolved a swim-bladder just like the swim-bladder found in most bony fish (which incidentally evolved on multiple occasions to form lungs).

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Topic Title Teaser text Availablity
Antifreeze proteins n/a Not Available
Echolocation in bats

How can bats navigate in total darkness amongst trees and branches, but still locate a tiny, fluttering insect with extraordinary acuity? All made possible through echolocation, an astonishing sensory mechanism…

Monochromacy in mammals

Underwater environments are dominated by blue light. Ironically, whales and seals cannot see blue, because they have independently lost their short-wavelength opsins.

Luciferases n/a Not Available
Solar powered animals n/a Not Available
Daily torpor in birds and mammals n/a Not Available
Endothermy (“warm-bloodedness”) n/a Not Available
Bioluminescence in marine animals n/a Not Available
Carnivorous plants

All plants are harmless? Well, not quite - at least not when you're an insect...

Carbon dioxide concentration in plants n/a Not Available
Thermogenesis in plants and algae n/a Not Available
Mycorrhizal fungus associations in plants n/a Not Available
Fin collagen in tuna, dolphins and sharks n/a Not Available
Pressure sensitivity and the tactile sense (excluding the lateral line)

The star-nosed mole is famous for, well, its nose, but do you have any idea what these peculiar 'tentacles' are for? The answer is rather touching and, of course, convergent...

Reversion from xylem vessels to tracheids

In three plant taxa that evolved in environments with frequent freeze-thaw cycles (Winteraceae, Trochodendraceae and cold desert Ephedra), vessel evolution has been reversed independently in favour of a return to a tracheid-based vascular system.

Foam nests in animals

Nests crop up everywhere, but one made out of foam? Might not sound like a great idea, but it is. And no surprise, it has evolved several times...

Hydrogenosomes and mitosomes n/a Not Available
Mitochondrial genome convergences

Most likely, mitochondria have a single evolutionary origin, but that doesn't mean they are immune to convergence...

Myelinated nerves in vertebrates, annelids and crustaceans

Myelinated nerves are an excellent biological solution and needless to say have evolved independently in several groups other than vertebrates. In each case myelination is associated with very rapid nervous conduction and often escape reactions.

Explosive discharge in fungi and plants

The very rapid release of reproductive bodies is perhaps most famous in the fungi, where several methods of flinging spores at high velocity have evolved independently.

Collagen in animals and bacteria n/a Available
Flight muscle (arthrin) n/a Not Available

Not many foods served in a restaurant can kill you, but pufferfish is the exception. Tetrodotoxin, the toxin responsible for such culinary fatalities, reveals a fascinating story of convergent evolution...

Desert plants with succulent stems

Fleshy, succulent stems have evolved in several distantly related desert plant families, including cacti, certain species of Euphorbia and two genera of the family Asclepiadaceae, Hoodia and Stapelia.

Desert plants with succulent leaves

Perhaps the most striking case of convergence among leaf succulents occurs between Agave and its relatives Yucca and Hesperaloe in the Americas and Aloe and its relatives (e.g. Haworthia and Gasteria) in Africa.

Succulent desert plants

Classic examples of convergence in desert plants include the so-called 'stem succulent' cacti in the Americas and cactus-like Euphorbia species in Africa and South Asia, and also the striking similarity between 'leaf succulent' Agave and Yucca of the Americas and Aloe and its close relatives in Africa.

Saxitoxin synthesis: from molluscs to algae

Saxitoxin has a similar molecular structure to tetrodoxin and a wide distribution amongst living organisms, with evidence that is has been recruited independently several times.

Echolocation in toothed whales and ground-dwelling mammals

Given the extraordinary powers of echolocation in bats, it is not surprising that this group has received the most attention. However, they are not the only mammals to have evolved echolocation. Who invented sonar millions of years before the Navy?

Extremophiles: Archaea and Bacteria

Surely, no organism can survive in boiling water or brines nine times the salinity of seawater? Wrong - some archaea and bacteria have independently evolved adaptations to such extreme environments...

Sodium voltage-gated ion channels

Sodium voltage-gated ion channels are vital to electric signal transmission, but it is less widely appreciated that they are convergent and have evolved at least twice in groups outside the animals.

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Gliding lizards, frogs and ants

Tree-dwelling (‘arboreal’) ants capable of controlled gliding do so when dislodged or threatened by predation. Gliding species include members of three disparate families: Myrmicinae, Pseudomyrmecinae and Formicinae.

Gliding in feathered reptiles

A number of reptile species have been discovered in the Mesozoic fossil record, bearing feathers that were apparently used to support gliding locomotion, rather than true, powered flight as we see in present day birds.

Gliding mammals

Gliding mammals rely primarily on extensive skin membranes or ‘patagia’ that stretch between fore- and hind-limbs, creating a wing-like structure.

Gliding reptiles

In the reptiles, different forms of skin membrane (called ‘patagia’) and in some extinct species, primitive feathers, have evolved convergently as adaptations for gliding.

Sand-dwelling (psammophilous) lizard ecomorphs

Desert sand dunes represent an extreme environmental setting in which selective forces have apparently generated dune ‘ecomorphs’ in six lizard families. – Lamb et al. (2003) Biological Journal of the Linnean Society, vol. 73, p. 253

Ecological adaptations in Moloch and Phrynosoma lizards

Lizards of the genera Phrynosoma and Moloch have been considered a classic example of convergent evolution J. J. Meyers & A. Herrel (2005) The Journal of Experimental Biology, vol. 208, p. 114

Drinking adaptations in desert lizards

Both Moloch horridus and [...] Phrynosoma cornutum have the remarkable ability to transport water over their skin’s surface to the mouth where drinking occurs. Sherbrooke et al. (2007) Zoomorphology, vol. 126, p. 89

Mammal-like placentation in skinks (and fish)

“Only two types of vertebrates [have] evolved a reproductive pattern in which the chorioallantoic placenta provides the nutrients for fetal development. One is [...] the eutherian mammals […], and the other, a few lineages of the family Scincidae.” A.F. Flemming (2003) J Exp Zool 299A 33-47

Trichromatic vision in mammals

Who has not enjoyed the splash of colour in a market: gorgeous red peppers, the green of basil and what on earth are these purple vegetables over there? All thanks to trichromatic vision, another story of convergence.

Olfaction: insights into convergence

Although olfaction is very widespread, there is abundant evidence for repeated convergence of key features, strongly suggesting that there really is an optimal solution to detecting smells.

Mitochondrial lens formation in flatworms

In some of the flatworms (platyhelminthes) the lens is formed from mitochondria, and it is intriguing to speculate whether a mitochondrial enzyme has been co-opted to provide a crystallin.

Hearts in cephalopods and vertebrates

There is a striking convergence between the aorta of the cephalopod and vertebrate heart, notably in its structure and the employment of elastic proteins.

Lipocalins for milk and pheromone transport

Lipocalins are proteins that bind to and transport small hydrophobic molecules such as lipids and steroids, and have been associated with biological processes such as milk production, pheromone transport and immune responses.

Animal haemoglobins

There is good evidence for convergence in animal haemoglobins because even though the protein itself is ancestral to all animals, during its evolution various episodes of gene duplication have led to a number of different varieties, notably the β-globins.

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Innate and adaptive immune systems

A vile cough, soaring temperature? When attacked by nasty microbes, our immune system comes in handy. Surprisingly (or not), plants have come up with a very similar solution to dealing with pathogens, but independently...



Adhesive pads: from geckos to spiders

In terms of adhesive pads we find they have a remarkably wide distribution evolving in at least four distinct groups, including members of the reptiles, amphibians, arthropods and mammals, with tentative parallels in sea urchins.

Nitrogen-fixing bacteria in legumes

One convergent avenue to obtaining nitrogen is to employ symbiotic bacteria that typically are found in root nodules, perhaps best known in the legumes.

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Mussel attachment and the Pinna byssus

It is clear that the Pinna byssus has unusual properties in comparison to its equivalent in the bivalve mussel, and is conspicuously different in terms of crystallinity.

Bacterial carboxysomes (and other microcompartments)

It is now clear that the cellular construction of at least the eubacteria is more complex than realized, and includes organelle-like structures known as microcompartments, of which the best known are the carboxysomes.

Enzymes: convergence on active sites and reaction types

Enzymes make the world go round, each an evolutionary marvel - and convergent.

Respiration in myriapods


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Gut fermentation in herbivorous animals

Ever tried eating a newspaper? Don't. Plant cell walls contain cellulose, which is notoriously difficult to digest. Considering that all vertebrates lack the enzymes to attack this polysaccharide, how do so many of them manage to survive on a plant diet?

Carbonic anhydrase in vertebrates, plants, algae and bacteria

Carbonic anhydrase is extremely convergent and may have evolved as many as six times. The most familiar variants are α, β and γ carbonic anhydrases.

Electric fish: insights into convergence

Ever seen an electric eel in an aquarium? Don’t dare putting your hand in the tank...

Hummingbirds and hummingbirdoid moths

Like other birds hummingbirds are warm-blooded, but so independently are the hawk-moths, which like a number of insects have evolved thermoregulation.

Swimming and thermoregulation in sharks and tuna

Thunniform swimming depends on a large, lunate tail that is joined to the rest of the body via a narrow peduncle. Whilst the tail flicks backwards and forwards, so propelling the animal, the rest of the body hardly moves sideways.

Haemocyanin in arthropods and molluscs

The degree of similarity between the active sites in arthropod and molluscan haemocyanin has been called “remarkable” and “startling”, but actually suggests that wherever in the universe life employs copper for aerobic respiration it will call upon haemocyanin.

Elastic proteins

What do rubber bands and fleas have in common?

Crystallins: eye lens proteins

Whereas typically technology demands furnaces, so that the glass for a lens is produced at hundreds of degrees Celsius and then requires most careful grinding, so nature calls upon proteins known as crystallins.

Alcoholism in mammals and flies

Identification of alcohol tolerance (or lack thereof) in different animal groups is important because alcoholism in humans may have some genetic basis.

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Statoliths and balance in animals

An almost universal, but convergent, method to detect changes in orientation is for small grains (statoliths) to be attached to fine hairs, whose movement triggers nervous impulses.

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Hearing and ears in animals

Hearing has evolved independently in a number of groups, notably in the insects and vertebrates.

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Defence in frogs: toxins and camouflage

The many striking examples of convergence most famously include the case of mimicry, but the question of defence also extends to the use of toxins (and venoms), such as alkaloids, where we also find molecular convergence.

Swim bladders of fish and the octopus Ocythoe

Swim bladders have evolved independently in fish and in Ocythoe octopus females.

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Moulting in arthopods, annelids and other animals

Moulting has, however, evolved independently in other groups, including the annelids where some polychaetes shed their jaws.

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Reflective tissues

Other cephalopods achieve reflectivity by employing collagen fibrils, of which the deep-sea Vampyroteuthis is perhaps the most striking example.

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