[Animals] The incredible evolutionary convergence: why did animals of different lineages evolve in similar ways?

[#Wittels-]

How is it possible that animals with such different origins as birds, bats and pterosaurs have the same structure that allows them to fly? And why do most marine animals have the same body plan, whether they are fish, reptiles or mammals?

 

As established by the theory of evolutionary synthesis, the variations that occur in po[CENSORED]tions of living beings are subject to selective pressure, which determines whether these variations are fixed as adaptations or discarded. When, over time, adaptations accumulate, significant changes occur in species and groups of living things, which acquire new traits. That is, ultimately, the origin of the differences between the different groups of living beings.

But sometimes it happens that groups of living beings of different origin and ancestors acquire similar traits independently. Evolution provides similar solutions to the same problem, in different organisms. This process is called 'evolutionary convergence', and it has given rise to really curious situations.

Many marine animals have the same body pattern even though they have different evolutionary origins. — solarseven/iStock

Some examples of evolutionary convergences
Today, we are all clear that dolphins and whales are mammals. However, it is inevitable to find more similarities between cetaceans and some fish than with other mammals. When looking at the fossil record, animals such as ichthyosaurs are found, whose resemblance to fish is such that they even receive their name, from the Greek ιχθυς, ichthys, 'fish' and σαυρος, sauros, 'lizard'.

Fish such as tuna or sailfish share certain traits with dolphins and ichthyosaurs, adaptations that allow them to spend their entire lives in the sea. Large pectoral fins, a caudal fin for propulsion, and a dorsal fin that stabilizes swimming, join a fusiform, hydrodynamic body shape, and tapered head. However, they are animals with very different origins. The closest evolutionary relatives of dolphins and other cetaceans are hippos and ruminants; and ichthyosaurs are reptiles, more closely related to lizards, crocodiles, or birds than to any group of fish.

This phenomenon, far from being exceptional, is usually common. Another well-known case is that of the wings. One of the traits that characterize birds is that their forelimbs have evolved into the shape of wings, an adaptation that, along with other modifications to their skeletal, muscular, and respiratory systems, allows most of their species to take flight.

 

It is a capacity that bats also have, a group of mammals more closely related to zebras or tigers than to birds. It is true that, in this case, although the wings of both are formed by the forelimb, their shape is completely different; the surface of the wing in the bird is formed by feathers, while in the bat it is a skin membrane that extends between the fingers. But there are at least two groups of prehistoric animals that, without direct relation to bats —and without even coinciding in time—, have the same type of wing.

Bats have the same type of wings as pterosaurs and scanscansoriopterygids — JAH/iStock

Pterosaurs arose in the Triassic and became extinct in the Cretaceous. They are not dinosaurs, although they are related to them. And they have, like bats, wings formed by a membrane that extends between the body and the fingers of the forelimbs. Another group, also independent, with the same type of wing was the Scansoriopterygids, a small group of dinosaurs, closely related to birds, and represented by the very strange Yi qi, the dinosaur with the shortest scientific name.

Although perhaps the most extraordinary case of evolutionary convergence can be found in the eyes. It is well known that the vertebrate eye has a single evolutionary origin; all vertebrates descend from a common ancestor that existed in the Cambrian period, about 500 million years ago. Our eyes have that origin.

But vertebrates are not the only animals with eyes. Insects, arachnids, myriapods, crustaceans, tunicates, flatworms, annelids, and mollusks have eyes, and even some groups of unicellular algae, such as euglenophytes, have a pigmentary spot that performs the function of a highly simplified eye.

The evolution of the eye has happened multiple times independently in many groups of living beings, unrelated to each other. And the most fascinating case is probably that of cephalopods and their similarity to the vertebrate eye. The eye of octopuses and squids is formed from the epidermis of the embryo, unlike the case of vertebrates, whose origin of embryonic development is found in the nervous tissue. Furthermore, cephalopods do not have a blind spot. Apart from this, their structure is surprisingly similar: both have a retina, vitreous humor, lens and sclera. The optic nerve of cephalopods is very short, since the information from the eye flows directly into a cerebroid ganglion.

Octopuses have eyes similar to those of vertebrates — OscarPaunerRamirez/iStock

How can the same organs arise in such different groups?
Evolution is not omnipotent. It is not an imaginative entity that deliberately seeks creative solutions to a certain problem. The evolutionary process lacks intentionality, and presents limitations given by biomechanics and phylogeny.

The phylogenetic limitation tells us that an organism belonging to a certain lineage, such as a mammal, cannot develop specific organs of a different lineage, such as feathers. The only way to break this rule is for there to be a massive horizontal transfer of genes, for these genes to be successfully integrated into the gene pool of the recipient po[CENSORED]tion, and to be favorably selected. Something very unlikely.

However, what can happen is that some pre-existing structure undergoes modifications that end up resembling the structures of another group. This process, in biology, is called analogy. Using the example of wings, pterosaurs, dinosaurs and mammals start from a common pattern, a front leg made up of a humerus, a forearm made up of an ulna and a radius, carpals, metacarpals and phalanges.

 

Each of these groups, independently, can be subjected to the same selective pressure; and given that there is a phylogenetic limitation that does not allow them to develop structures typical of other groups —they will not have insect wings—, and a biomechanical limitation that prevents them from developing impossible structures, it should not be surprising that evolution ends up finding the same solution. the same problem. Even though this problem occurs in different lineages, with a different evolutionary history.

References:
McGhee, G. R. 2011. Convergent Evolution: The Most Beautiful Constrained Forms. MIT Press.
Solomon, E.P. et al. 2013. Biology (9a). Cengage Learning Publishers.
Stayton, C. T. 2015. What does convergent evolution mean? The interpretation of convergence and its implications in the search for the limits of evolution. Interface Focus, 5(6), 20150039. DOI: 10.1098/rsfs.2015.0039
Stern, D. L. 2013. The genetic causes of convergent evolution. Nature Reviews Genetics, 14(11), 751-764. DOI: 10.1038/nrg3483

 

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