Snowy owls (Bubo scandiacus)…
…and Polar bears (Ursus maritimus) inhabit Arctic and sub-Arctic regions.
Both species are predators…
…that rely on surprise to catch their prey.
Both species have white coats which make them effective hunters able to stalk their prey unnoticed…
…by blending into the harsh environments in which they live.
The ancestral color of Snowy owls and Polar bears was brown; their white coloration is an adaption that both species have evolved.
Snowy owls are descended from brown coated ancestors that lived in North Africa four million years ago. Snowy owls are the only owls of the Bubo genus to have evolved white coats.
When their brown coated ancestors started hunting in Arctic habitats 3.5 million years ago, it was the light coated individuals that were the more successful hunters.
The evolutionary pressure was to evolve lighter and lighter colored coats until a new species with a white coat appeared -the Snowy owl.
Similar selection pressures drove the evolution of the Polar bear. It is believed that the Polar bear split from a brown bear ancestor during the Paleolithic era between 479,000 and 343,000 years ago.
It was at that time that the Polar bear’s ancestral species, the brown bear (Ursus arctos)…
…. started hunting in Arctic environments. Those brown bears with the lighter colored coats were the more successful hunters better placed to breed and pass on their lighter coated genes to subsequent generations.
Eventually a new species of bear, the Polar bear, evolved.
The white coloration of Snowy owls and Polar bears is one example of convergent evolution.
Convergent evolution occurs where two or more completely unrelated species evolve similar traits or adaptions. The ancestors of neither species possessed the trait or adaption.
Species acquire identical traits or adaptions when living in similar habitats or facing similar environmental conditions. The trait or adaption they evolve makes them better placed to hunt, survive and breed.
There are many, many examples of convergent evolution in existence. Some other noteworthy examples are discussed below:
The Ichthyosaur, a marine reptile, became extinct 95 million years ago. It had a streamlined fusiform shaped body…
… very similar to the fusiform shaped body of a shark…
…and that of the dolphin.
In addition to their fusiform shapes, all three organisms evolved similar dorsal, pectoral and tail fins. They used these fins to maintain balance and help them maneuver quickly through the water
The ichthyosaur, shark and dolphin all evolved their body forms independently of each other. The ichthyosaur evolved from a land reptile, the shark from a fish, and the dolphin from a land mammal.
Marsupial echindas (Tachyglossus aculeatus) from Australia…
… and mammalian hedgehogs from Europe (Erinaceus europaeus) are not related.
Both have independently evolved ‘spines’ on their backs. Spines are tough, modified, hollow hairs…
…stiffened with keratin- the same substance that fingernails are made of.
Both animals range far and wide searching for small invertebrates to eat. They are vulnerable to attack from larger predators as they hunt for food. Their spines are an effective defense against being attacked.
Bats hunt at night and often live in dark caves.
Dolphins sometimes hunt in murky waters where it is difficult to see.
Both animals have independently evolved an incredibly sophisticated way of ‘seeing’ in conditions where there is little light.
….before returning to the animals’ auditory systems for the information to be processed.
Using ‘ecolocation’ these animals are able to build up a ‘sonic map’ of their surroundings.
Bats’ echolocation sytem not only allows them to successfully hunt prey; it allows them to communicate with each other and ‘see’ where they are flying in the dark.
Likewise the echolocation system of dolphins allows them to ‘see’ in murky water, navigate their surroundings, communicate and hunt.
Chickens have an amazing ability to sleep and stay awake at the same time. This chicken shows how it is done! It has one eye open and one eye shut.
The left side (or hemisphere) of the chicken’s brain is asleep while the right hemisphere is fully awake and alert to danger. Chickens have evolved the ability to watch out for predators even when sleeping!
But when they trust someone they shut both eyes- indicating that both brain hemispheres are capable of sleeping at the same time!
Other species that have developed this extraordinary ability to shut down one half of their brains at a time (called ‘unihemispheric slow-wave sleep’ or ‘USWS’) include dolphins….
Unihemispheric slow-wave sleep is especially useful to aquatic mammals; it gives them the ability to sleep while at the same time knowing when they have to rise to the surface to breathe.
Plethodontid salamanders from forest habitats of north-west America….
…… and Parson’s chameleons (Calumma parsonii) from the rainforests of Madagascar have independently evolved the ability to catch prey with their l-o–n–g sticky tongues!
The tongue of a Plethodontid salamander can accelerate at a speed of 1,740 meters per second squared! Protraction and retraction of the tongue can occur within 20 milliseconds! (twenty thousandths of a second). The tongue of the Parson’s chameleon is equally fast!
Believe it or not human beings…
have independently evolved advanced camera eyes that are remarkably similar.
The whole evolutionary process started when the remote ancestors of humans and octopuses developed simple camera eyes…
…while swimming in Cambrian seas 544 million years ago.
The octopus can trace the evolution of its advanced camera eyes back to the two tentacled Nectocaris pteryx that lived 505 million years ago.
The evolution of advanced camera eyes in humans can be traced all the way back to the common ancestors of all mammals, fish, reptiles and amphibians that first crawled out of the sea some 375 million years ago.
The need to evolve advanced camera eyes resulted from evolutionary pressures to improve vision in order to see both predators and prey in complex sea and land environments.
It just so happens that advanced camera eyes evolved in octopuses in the sea and in tetrapods (including us humans) on land!
The green sawfish (Pristis zijsron)….
….is unrelated to the Bahamas sawshark (Pristiophorus schroederi)
Both species of fish have evolved long noses (‘rostrums’) edged with teeth which they use to slash and disable their prey.
They also use their rostrums to dig for prey that may be hiding. Their rostrums are also useful tools which they use to defend themselves against predators.
Both species live in similar habitats -on or near the sea floor.
The third fingers of aye-ayes (Daubentonia madagascariensis) from Madagascan rainforests…
… and the fourth fingers of striped possums (Dactylopsila trivirgata) from Australian rainforests are very long.
Both animals use their elongated fingers in identical ways to hunt for prey.
They tap on the bark of trees and listen out for a returning echo which informs them about the presence of wood boring insect lavae under the bark.
Once they have located the lavae they use their sharp teeth to gnaw at the bark to create a small hole.
They then use those same long middle fingers to hook the lavae out of the hole…
…which they then eat.
The non venomous Scarlet King snake (Lampropeltis elapsoides) ….
….has evolved to look very similar to the venomous Eastern Coral snake (Micrurus fulvius).
Both species live in the south-east of the United States.
By evolving to resemble Coral snakes, Scarlet King snakes are less likely to be eaten by predators!
Children living in areas of the United States where both snakes live are taught to identify the difference between the two species.
In this example of convergent evolution, a harmless species ( the Scarlet King snake) has evolved to look like a harmful species (Eastern Coral snake). This phenomenon is called Müllerian mimicry.
The Monarch butterfly (on the left) has evolved to closely resemble the Viceroy butterfly (on the right)
Predators try to avoid eating both species because they taste so disgusting!
Take this Blue jay bird; in this image it can be seen eating a Monarch butterfly and finds the taste so revolting that it is promptly sick.
Supposing, the next time it goes out hunting,the Blue jay finds a Viceroy butterfly to eat instead of a Monarch. What will it do?
Ignore the Viceroy butterfly and eat peanuts instead!
Saforrest at wikpedia GFDL/CC-by-SA 3.0
The Blue jay would avoid eating the Viceroy butterfly because it so closely resembles a Monarch butterfly.
Two similar looking species of butterfly increase their chances of survival because predators learn to avoid eating both species.
Once a predator has had a bad experience eating one species, it is less likely to make the same ‘mistake’ again! This is an example of the phenomenon we call Batesian mimicry.
A swim bladder is a gas-filled organ that helps an organism control its buoyancy…
…and stay at its chosen depth without wasting energy swimming.
Swim bladders have independently evolved in bony fish….
...tuberculate pelagic octopuses…
…and Portuguese men ‘o war (Physalia physalis). The Portuguese man o’ war lives on the surface of the ocean. Its gas-filled bladder remains on the surface…
while its venomous tenacles stay submerged.
Convergent evolution is also evident in plants. Take the cactuses (Cactaceae) of the Americas….
…and the euphorbs (Euphorbiaceae) of southern Africa.
Both species evolved in hot, waterless arid environments. Both have developed thick, fleshy stems in which they store water.
Their true leaves have changed into spines that, among other things, reduce the amount of water the plant loses through evaporation.
Spines provide some protection against hungry herbivores -but not always!
Some plants are able to survive in nutrient deficient habitats such as wet, boggy, acidic soils. (see Spores, sphagnum moss and wetlands)
Plants living in nutrient deficient habitats have become carnivorous; this has given them the ability to acquire the vital nutrients they need.
Carnivorous plants trap insects in different ways.
Insects become trapped on the sticky tentacles of the Cape sundew (Drosera capensis)….
drown in the water-filled cups of the Tropical pitcher (Nepenthes lowii)…
or become emprisoned in the toothed, modified leaves of the Venus fly trap (Dionaea muscipula).
Pillbugs and pill millipedes live in very similar habitats; they inhabit moist, humid forests and feed on dead plants on forest floors.
When threatened by predators they curl up into defensive balls.
Curling up into balls protects their soft undersides and presents predators with their more formidable exterior plated armor.
Of course, convergent evolution does not just occur in living species. There is much evidence for convergent evolution in species that are now extinct.
The herbivorous Doedicurus clavicaudatus was a heavily armored south american glyptodont that became extinct only 11,000 years ago
It had a tail club which was probably used in territorial and mating displays. It may have also been used for defense.
The Anklyosaurus magniventris became extinct 66 million years ago. Like the Doedicurus it was a herbivore; it had extensive body armor and a tail club which it used to defend itself.
This Wikipedia article has many examples of convergent evolution you could research further.
Watch this Plethodontid salamander catch its prey with its amazing tongue!
Marvel at this aye-aye in action!