Why hasn't evolution created yet another platypus?

Original author: Jonathan Losos
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Is evolution predictable or is it random


Sniffing through the undergrowth, a small shaggy creature wanders through a forest night, poking its nose in one place and then in another, looking for the smell of its soft-bodied dinner. It is dark in the forest, this creature has poor eyesight, but a long mustache and good sense of smell allow it to navigate. In the event of a threat, it is able to develop dizzying speed, rush through vegetation, dive into holes, and quickly disappear from sight.

Absolutely unoriginal style of life. Many animals walk at night in the woods, and similarly look for small prey: hedgehogs, shrews, weasels, and besides them large animals - possums and even pigs. The world is full of such animals.

But this animal is not like that. All the rest are hairy. The hairline of this animal is also soft, consisting of millions of thin strands. But this is not hair. All the rest move on four legs and bear living offspring. But not that.

Scratching, studying the environment, sniffing, this animal sometimes creates a duet with its couple, having a call, staying in touch while passing through its territory. The cry of the male gives his name: "Kii-vii, kii-vii."

The filters separating food from water in whales and whale sharks have a completely different structure.

We are in New Zealand, and this is a nocturnal insectivore - a bird with stumps instead of wings, a mustache, like a cat, with soft feathers, and, unlike other birds, with nostrils located on the tip of the beak. Many call it an "honorable mammal."

New Zealand is packed with unusual creatures. But what’s still unusual is what’s not there: mammals. On the islands there is hardly a scrap of wool. Apart from fur seals filling the beautiful beaches of New Zealand, the only local mammals are the three bats - and even they are strange.

On the other half of the globe, in Cuba, there are oddities. An owl the size of a first-grader, fed, among other things, by young giant ground sloths, unfortunately, has died out (like giant sloths, comparable in size to a gorilla), but a hummingbird-sized bird still lives on the island; a crabfish, an archaic mammal, as if descended from the pages of children's books, with poisonous saliva and a long, flexible, mustachioed nosyar; something like a beagle the size of a guinea pig, capable of climbing trees and delivering green poop in the form of a banana in abundance.

Even small islands have their own unusual wonders. On the island of Lord Howe with an area of ​​14.5 square kilometers crescent-shaped, located in the Tasman Sea, live “wood lobsters,” which, despite the name, are obese and hefty members of the insect family, usually characterized by thin, rod-like representatives. In the Solomon Islands, in the south of the Pacific Ocean, there lives a lizard posing as a monkey: a giant chain-tailed skink is a shiny, slender, 70 cm lizard with a tenacious tail, which it clings to tree branches in search of fruit. And everyone heard about the once-existing dodo bird from the island of Mauritius in the Indian Ocean. She did not know how to fly, had no feathers, ate fruit, was the size of a turkey, reached a meter in height, and weighed up to 20 kg.

Among the small islands, the first place in the number of oddities is occupied by the Hawaiian Islands: dragonflies, which have equal wings, whose larvae, usually aquatic, live on land; gluttonous carnivorous caterpillars; fruit flies switching from fruits to decaying plants; another fruit flies with a head in the form of a hammer, which protect their territory, butting, as if rams. The flora of Hawaii is just as strange - the primacy is held by a wonderful brigamia that looks “like a pin, topped with a lettuce”.

And there is Madagascar, which is sometimes called the eighth continent because of its distinctive flora and fauna. There is a dwarf hippo; adaptive radiationlemurs (including one weighing 35 kg hanging from branches like sloths, and the other, looking like an overgrown koala - but both of these species were destroyed by humans); three-meter 500-pound elephant birds (extinct; the heaviest of all ever existed birds); half of all world species of chameleons that shoot tongues that are twice as long as their body length; fossil frogs the size of a large pizza; vegetarian crocodiles; beetles with a neck, like a giraffe. Madagascar's plants are no less strange, including desert forests, consisting of tall and thin stems dotted with thorns, and thick baobab trees that look like they were stuck in the ground upside down.

Last, but not least, are the wonders of Australia: platypus, kangaroo, koalas - in the rest of the world there is nothing like it.

Flying in birds and bats developed independently. Both of them adapted their forelimbs to wings, only birds use feathers, and bats use skin.

What it all boils down to: The Islands give us a glimpse of alternative evolutionary worlds that could exist if life turned in a different way. What if mammals died out at the end of the Cretaceous with dinosaurs? New Zealand shows how it might look. Where would the evolution of primates lead if monkeys did not evolve? Pay attention to lemurs that are found exclusively in Madagascar.

The islands give us a recipe book for evolution. The resulting dishes tell us that it is impossible to predict what happens in the oven. Change the ingredients or the sequence of their addition, add heat, do not put anything, drop one pinch of salt instead of two - and the result may be completely different. The island recipe book is replete with examples of contingencies and unforeseen circumstances, and the variety of results suggests that it is very difficult to predict what exactly will appear on any of the islands as a result of evolution.

For many decades, the conventional wisdom of evolutionary biology, formulated by Stephen Jay Gould, was one of these accidents: change any event in the history of life, and all life can become completely different. The existence of such a modern life, as it is, is not inevitable and not even most likely - all this is just a game of chance.

But in recent years, the backbone of scientists has appeared, led by Siman Covney Morris, who have taken the opposite point of view - they argue that certain evolutionary decisions are quite likely. In the course of evolution, completely different species constantly develop similar adaptive solutions to the problems encountered in their environment - for example, a very similar structure to the eyes of people and octopuses. These recurring decisions are called evolutionary convergence (convergence). From this point of view, randomness in history has little effect, and their effects are erased by the predictable pressure of natural selection.

We can easily understand convergence - species that equally adapt to the same situations. But what about one-of-a-kind evolutionary solutions? Why didn't other species come up with similar solutions during evolution?

One explanation for the exceptions to evolution is the appearance of unusual species in unique environmental conditions. Perhaps they had no analogues because no one fell into similar circumstances. This may explain the koala. Her whole life is tied to eucalyptus trees and eating their leaves, which contain many poisons. As a result, the digestive system of the koala is extremely long, which gives it a lot of time to detoxify the leaves and extract nutrients. The slow passage of food, combined with its low nutritional value, means that koalas have to minimize energy consumption and sleep most of the day. Eucalyptus trees come from Australia, therefore, it is possible that the originality of the koala reflects the uniqueness of its environment.

But it seems to me that in most cases this explanation is not suitable. Platypuses are found in streams, ponds, lakes and rivers in eastern Australia, where they can eat crayfish and other aquatic invertebrates, which they search for, snooping around the bottom, and feeling the victim with the help of electroreceptors located on their beaks. The rest of the time they retire to their chambers at the ends of long burrows dug on the river bank. This lifestyle seems possible in many places besides Australia. The streams on which they live are very similar to the stream that passed behind my friend’s house when we grew up in St. Louis. North America is full of crayfish streams, many of which are located in climates similar to the one in which the platypus lives, in the absence of more terrible predators than those living in Australia. So where is our double of the platypus? Why hasn’t anything else appeared somewhere else? Or a kangaroo, or any other of the examples I have listed - they all live in habitats found elsewhere.

Other explanations for evolutionary exceptions are that natural selection is either not as predictable or not as powerful as some believe. That is, even living creatures living in similar conditions can evolve in different ways.

The main reason for the lack of convergence is the presence of several ways to adapt to the tasks that the environment poses. Think about how vertebrates swim. Many use their tail, but the tails are all different. In fish, the tails are flattened vertically and move left and right. Crocodiles swim the same way. But in whales, the tails are flattened horizontally, and move up and down. Other animals, such as snakes or sea snakes, make wave-like movements with their whole body. Some birds, such as cormorants and loons, can quickly move under water, furiously rowing webbed feet. On the other hand, some species swim with the help of changed forelimbs - sea lions have such flippers, and penguins use wings for this. However, the most amazing swimmer may be the tree sloth, whose long forelimbs, evolved to hang from branches, help him crawl. Invertebrates have even more solutions for moving in water, for example, the reactive movement of octopuses and squids.

This list of different methods of fast movement in water raises a natural question: how similar should the properties of the two species be to be considered convergence? Squids and dolphins use a very different anatomical system for fast movement in water - they clearly do not converge. And another dissimilar mode of transportation is rowing with webbed feet in some aquatic bird species.

Other examples are not so obvious. What about the flat tails of whales and sharks - they are similar in structure and work, only one of them is vertical, and moves left and right, and the other is horizontal, and moves up and down? Are these features small variations in convergence or non-convergent solutions with similar functionality? I suspect that most people will find horizontal and vertical flat tails essentially one solution.

Let us move one step back to properties that lead to similar functional results, but show great anatomical diversity between species. Among vertebrates, active flight appeared three times during evolution: in bats, in birds, and in pterosaurs (large reptiles that conquered the skies in the era of dinosaurs). They all changed their forelimbs to the state of wings, and they fly (or fly, in the case of pterosaurs) essentially the same way, swinging the light structure down to produce lift and acceleration forward.

But a careful study shows that the wings of these flying vertebrates are very different in structure. The most obvious difference is the aerodynamic surface itself. Birds use feathers that grow separately from the bones of the hands. The wing profile of bats and pterosaurs consists of thin, but very strong skin, stretched between the bones of the fingers and the body, and in some cases even connected to the hind legs. The anatomy of the skeleton of the wings in these three groups is also very different.

So are the wings, grown from the forelimbs of birds, bats and pterosaurs, convergent adaptations for active flight, built in different ways? Or do they represent alternative, non-convergent methods for generating active flight during evolution?

One more example. The largest fish in the sea is an 18-meter whale shark. Like whales, it feeds through a filter, swallowing huge amounts of water with its massive mouth and filtering out tiny food. But the similarity ends here. Baleen whales - blue, humpback, gray and others - catch prey by pushing water through hard plates of a crest-like whalebone that forms a veil hanging from the upper jaw. Any piece of food is larger than the cracks in the mustache, retained by its inner surface, and then digested. Conversely, in whale sharks, water passes through gill slots located on the sides of the back of the head. Cartilage filters are arranged in such a way that water passes between the filters, through the gills and goes into the ocean, and food particles continue to move past the gill slots, and form a mass in the throat, which is then swallowed. So baleen whales and whale sharks are large aquatic creatures that use huge mouths to draw water and filter small prey. But the specific structure of the filters differs in design, location and operation. Is convergent adaptations for filtered nutrition or not?

It is possible to draw a line between convergence and its absence in structures that coincide in many respects and lead to similar functional advantages. I tend to consider the wings of birds, bats and pterosaurs convergence. In the same way, I believe that baleen whales and whale sharks are convergent because they have large mouths and feed on plankton. However, I consider their filtration and nutrition systems themselves not as convergent, but as alternative adaptations to such nutrition. But in such cases, there are no right or wrong answers.

Cheetahs and hyena-like dogs prey on the same animals, using different strategies and anatomical adaptations

In other cases, species can adapt, evolving in clearly different ways, producing non-convergent phenotypes with similar functionality. My favorite example of this phenomenon is associated with the underground life of rodents. More than 250 species from the rat clan spend most of their life underground, moving through independently dug tunnels. Such digging of holes in the process of evolution constantly appeared in rodents, but was achieved by different methods. Many rodents dig holes in the usual way, loosening the ground in front of them with their forelimbs, and throwing it back. The forelimbs of these species are strong and muscular; claws are long and strong. Other species use teeth instead of claws to remove soil. As expected, their teeth are long and prominent, even by the standards of rodents, and the muscles of the jaws and skull are massive. Most dental diggers get rid of the soil, pushing it back with the forelimbs, but some rodents show another variation - they ram the softened soil into the walls of the tunnel with the help of blows of a long, muzzle-like shovel. The differences in the anatomy of these diggers are an obvious illustration of non-convergent adaptations leading to similar functional results.

Convergence may not occur for another reason. Often there are several different functional ways to adapt to the environment. For example, see how species that serve as potential prey for predators can adapt to the presence of a predator like a lion. One option is to develop running capabilities in the process of evolution to overtake them, but there are other options. This is camouflage, passive protection or active protection. The resulting adaptations will undoubtedly be inconvergent, for example, the horns of an African buffalo, the body armor of armadillos and turtles, the long legs of an impala, porcupine spikes, the venom and accuracy of spitting of a spitting cobra, and the motley skin of a forest antelope.

Many solutions to the same problem are not limited to protection. Cheetahs and hyena-like dogs prey on the same animals, but feline dogs do it with short throws at great speed, and dogs run slower, but longer, exhausting the victim. And their adaptation accordingly differs: very long legs and a flexible spine of the cheetah allow it to accelerate to 110 km / h; the excellent endurance of hyenoid dogs allows them to run at a constant speed of 50 km / h long enough to tire the victim (and cheetahs can run at high speed only for short distances).

Or consider the adaptation of animals to produce nectar. Plants often produce a sweet-smelling, sweet liquid to lure insects, birds, and other animals to aid in the reproductive process. When an animal sticks its head or the whole body into a flower to enjoy nectar, it becomes covered with pollen. When moving to the next flower, part of the pollen falls off and pollinates the ovules of the plant.

Many flowers have long tubules with nectar at the bottom - thus the flower restricts access to and pollen to several specific species that are well adapted for use with this flower, for example, moths with long proboscis and hummingbirds with the same long beaks and tongues. Such species, due to adaptation, do not often visit other flowers, which limits the likelihood that pollen from them will fall into a flower of another species and disappear in this way.

But not all creatures that feed on nectar play by the rules. Some species of insects, birds and mammals gnaw a hole in the base of the flower, bypassing the petals and their pollen, thus not fulfilling their role in the co-evolutionary transaction. To do this, these nectar thieves use very different adaptations. They do not need long tongues and parts of the head to get to the bottom of long tubes, they develop properties that improve their ability to break through the material of the flower. Some hummingbirds have nicks in their beaks for this purpose. The hookbeak bird at the end of the top of the beak has a hook used to cut flowers. In these many examples, it is clear that for the solution of the tasks posed by the environment, there are often several evolutionary options. But the fact that there are many of them does not mean that as a result of evolution all the options will appear. Conway Morris and his team argue that usually one option has an advantage over others, and therefore the same properties convergent appear again and again. However, convergence does not always appear. Why doesn't natural selection use the same property every time?

It may happen that two or more properties are equivalent. Camouflage or the ability to run away at high speed can be equally successful ways to avoid predators. Or one way will be more successful than the other for a specific purpose, but with other disadvantages outweighing its advantages. Running away from an approaching predator can be a good way to escape, but camouflage can improve the ability of animals such as snakes to lurk their own prey. When survival and reproduction are summed up, individuals with camouflage can be as successful as those that rely on speed, and they also, through reproduction, pass on their genes to the next generation. As a result, natural selection does not prefer one over the other. The appearance of properties can be a matter of chance, a question of which mutation will occur first,

Conversely, the evolution of a property may depend on the initial phenotype and genotype of the species. In general, an active species may be predisposed to develop properties that affect the increase in speed when a predator appears, and less mobile species may develop camouflage. None of the options prevails over the other, but the evolutionary result can greatly depend on the initial conditions.

It may be that one solution is preferable, but in some cases it is easier to develop not the most optimal solution. French scientist Francois Jacob, who received the Nobel Prize for researching the work of DNA, proposed an analogy explaining why natural selection does not always lead to the appearance of a perfectly constructed organism. Jacob says that natural selection is not like an engineer designing the optimal solution for an existing problem. Better imagine a home-made man, jack of all trades, using the materials that he has at hand to create a viable solution - not the best possible, but the best available in the circumstances.

Imagine birds caught in a lake full of slowly swimming fish. They can begin to dive for food, and eventually adapt to the aquatic lifestyle, producing large and powerful hind legs, like a cormorant, or changing the shape of their wings and bringing them closer to the flippers, like penguins. Let’s say that the best way to swim quickly and deftly is to move a strong, muscular tail in the water, waving it left and right, or up and down - the fastest swimmers do it. But the birds do not have long tails - they lost them at the beginning of evolutionary history, more than a hundred million years ago, and they have only a small remainder of fused bones (the “tails” of birds consist only of feathers, but not of bones). I’m not saying that it’s impossible to redevelop a long tail as a result of evolution - but natural selection, jack of all trades, most likely will not go this way. Birds already have wings and legs capable of providing a driving force. It seems more likely that natural selection will work to improve the swimming functions of existing structures than that it will develop a new structure from scratch, even if a new bird with a bone tail - which looks like a hybrid between a loon and a crocodile - will swim much better .

But still, if the crocodile bird is better adapted - it will be the best swimmer - why do not waterfowl evolve in this direction? Perhaps sometimes it is impossible to go in any direction: it is difficult to go through evolution from one adaptive form to another because the intermediate conditions will turn out to be unsuitable. A long powerful tail is good for fast swimming, but a short tail can only get in the way and reduce the swimming speed. Natural selection has no foresight - it will not play in favor of a harmful property just because it is the first step on the path to excellence. For a property to appear as a result of natural selection, each step on the path should be an improvement over the previous one - natural selection will never prefer a deterioration, even if it is only a transitional evolutionary phase.

What do we get? Is convergence a fundamental force, a demonstration of the structure of the biological world, guided by the predictable influence of natural selection along the path to predetermined environmental outcomes? Or are convergent evolution examples exceptions, specially selected illustrations of biological predictability in a random world in which most species have no evolutionary parallels?

On this subject, one can argue until hoarseness. I will give an example of a platypus, you will give a counterexample in the form of convergent hedgehogs; I will answer with a unique tree sloth; you will respond with a two-legged mouse that independently appears as a result of evolution on three continents. That is how this debate developed historically - by compiling lists and telling stories.

Conway Morris and colleagues need to be praised for pulling convergent evolution to the forefront. Convergence was known to all of us as a cunning trick of natural history, a prime example of the possibilities of natural selection. But Conway Morris and colleagues clearly showed that evolutionary copying is much, much more common than we thought. Now we understand that it is very common in nature, and you can find full of examples. And yet she is not omnipresent.

It seems that just as often, and maybe more often, species living in similar conditions do not adapt convergently. Now we need to move on from describing historical patterns and collecting examples. We need to ask if we can understand why convergence occurs in some cases and not in others - which explains the extent to which convergence can occur and to what extent it cannot, why rodents jumping on two legs independently appeared in the deserts of the whole world, and the kangaroo - only once. And for this we need to not just add a few additional examples to the list. We need to test the hypothesis of evolutionary determinism directly.

Evolutionary biology later entered the experimental game - the legendary slowness of evolution did not allow the ideas of experiments to develop. Now we know that this view is erroneous, that evolution can go very quickly. And this understanding opens up new possibilities in the study of evolution.

Jonathan Salmon - Professor of Biology, Director of the Salmon Laboratory at Harvard University, curator of the herpetology department of the Harvard Museum of Comparative Zoology. Author of the book “Lizards in the Evolution Tree: Ecology and Adaptive Radiation of Anoles”. An excerpt from the book “Unlikely Fates: Fate, Chance, and the Future of Evolution” is given.

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