What is convergent evolution 1

Evolution: the principle of convergence

Many animal species have developed the same organs independently of one another. A basic principle of nature is hidden behind the phenomenon.

Among the hundreds of species of cichlid that live on the edge of Lake Tanganyika in central Africa, there are some wondrous shapes. For example, the bulge-lip cichlids, whose lips are enlarged to form a kind of pouty mouth. This probably protects against injuries when the fish pull their prey out of cracks in the stone.

The algae rasps, which chisel plants from the rocks with their teeth, are speckled blue. Bright yellow snail crackers use pointed eating tools to pull the mollusks out of their shells, and slim, turquoise shimmering scale eaters nibble on the flanks of their fellow species.

Bulgeous cichlids, algae rasps and snail crackers can also be found in Lake Malawi, 300 kilometers away. And also in Lake Victoria, another 300 kilometers northeast. For decades, some researchers suspected that some specimens of each species might have swum across rivers from one lake to the other two. The species had evolved there, but the respective adaptation - such as a pointed snout - only emerged once.

Convergence: The same conditions produce the same shapes

But when scientists working with the evolutionary biologist Axel Meyer from Constance analyzed the genetic makeup of the fish a few years ago, they were amazed: The various cichlids in the three lakes are similar - but they do not come directly from one another. Rather, the fish owe their identical appearance to a fundamental principle of evolution: convergence. This is what biologists call it when species develop amazingly similar characteristics even though they are not or only distantly related to one another.

The thick lips of the cichlids, for example, have evolved separately from each other in all three lakes - because their shape offers a survival advantage: They are apparently a good adaptation for those fish that suck their prey out between sharp stones and protect their snouts from injury .

Animals and plants live across all continents and they resemble each other in astonishing ways because they developed convergently. The correspondences arise when different living beings have to adapt to the same environmental conditions - to a hot climate, to prey that is difficult to access or to special habitats - and evolution then finds the same solutions.

The mole and pouch mole use the same shovel technique

The European mole, for example, gets along well in its habitat, because its front legs have been transformed into mighty grave shovels. So he can dig his way through the earth. The bag muck, which also lives underground in Australia, needs the same ability. Although the pouches and moles are descended from different ancestors and their habitats are 13,000 kilometers apart, the burial shovels of the two species look quite similar - because their shape is optimized to clear soil.

The hummingbird warmer, a butterfly, seems to stand still in the air when it sucks nectar from a blossom with its long trunk. It flaps its wings more than 70 times per second. He can also fly backwards. The same art is mastered by the hummingbird - a bird that can stand still in the air with up to 80 wing beats per second and with its extremely long tongue sucks nectar from deep flowers. It is a very similar flight technique - but realized one time with a chitin insect wing, the other time with the bird's feathered arm.

The harsh living conditions of desert plants have also led to similar developments: Certain cacti in Mexico, for example, have produced shoot axes whose cells store fluid and which can reduce their surface area in times of drought so that less water evaporates. They also store red pigments in their trunk, which make them appear dead, and they have short thorns that protect against predators.

Apparent relatives grow in African deserts - prickly plants that look deceptively similar to Central American cacti. But they are milkweed plants that have little in common with them in terms of development history.

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Even when it comes to injecting toxic substances, living things of the most diverse origins - jellyfish, scorpions, insects, snails, fish - have all developed the same weapon: the poison sting. And the eye was created more than 50 times across the animal kingdom. The lens eye is a particularly powerful form of the organ of vision. The lens captures the light, bundles it and provides an image of the outside world. It allows you to focus near and far and to track movements.

Although this eye is immensely complex, it has spawned several groups of animals, including octopuses, vertebrates, some jellyfish, and even annelids. And from very different components. In humans, the retina, i.e. the light-sensitive part of the eye, grows from cells of the nervous system. This happens when a part of the brain in the human embryo protrudes and becomes an eye cup. In the cuttlefish, on the other hand, the light-sensitive layer arises from the outer cell layer, which sinks in the shape of a cup to form the pit of the eye.

Organs that arise from convergent developments can therefore - like the eyes - look very similar, but differ greatly in detail and development.

Homology: Two animals are related but look different

But there is also the reverse case: body parts look completely different and have different functions - but are related in terms of development. The smooth front fin of a dolphin, for example, is ideal for swimming, the leathery, sweeping wing of a bat for flying and the human arm with its gripping hand for handling. Their shapes and functions are therefore very different - nevertheless, there is always a large bone inside, which is connected to two other and several small bones, which are connected to fingers.

Behind this development lies another elementary principle of evolution: homology, the counterpart to convergence. Homology means that body parts, although they differ greatly in form and function, have the same basic structure because the species in question descend from the same ancestor.

The fins of some primeval fish, for example, have changed into four limbs as some of them adapted to life outside the water. All terrestrial vertebrates are descended from them - and they inherited the common basic plan. From this an astonishing variety of limbs has developed over the course of millions of years. The bat's arm bones became shorter and their fingers lengthened considerably, so that there was space in between for the flight membranes. In the frog, on the other hand, the leg bones elongated, partially fused together, and enabled it to move in long leaps.

Teeth developed from flakes of skin

However, homologies are not limited to extremities. A human tooth, for example, is homologous to the scales on the skin of sharks: both are made of tooth enamel and have a medullary cavity inside. From the scales of a common ancestor, the teeth of those fish developed that later conquered the country and became, among other things, the ancestors of man.

Fish embryos have four bulges and furrows on each side of the head, the gill arches. As the fish grows, the depressions open so that water can flow through - the fish has created gaps between the gills.

The human embryo also has such furrows, but they remain closed. Instead, the first two bulges form the jaws, ossicles, muscles and nerves.

The jaw bones are thus homologous to the gill arches.

It is the same with the human lungs. It is homologous to the swim bladder, which gives many fish the buoyancy they need. Both organs emerged from a protrusion of the foregut. In those animals that left the water at some point, complex lungs developed from this. In other fish, however, the protuberance later turned into a swim bladder.

If you look at a fish, you always see a piece of yourself. And vice versa: In almost every part of your body you can recognize memories of prehistoric times. It carries the history of the living within itself.

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