Convergent evolution

Convergent evolution is when different kinds of living things develop similar traits all on their own, even though they are not closely related. This happens because these traits help the animals or plants survive in similar ways. For example, birds, bats, pterosaurs, and insects all learned to fly even though they started out very different.

When animals or plants develop these similar traits through convergent evolution, the traits are called analogous structures. These structures look or work alike but they did not come from a common ancestor. A good example is the wings of birds and bats. Their wings are analogous because they evolved separately, but their forelimbs are homologous because they share a common ancestor even though they do different things.

Convergent evolution is different from divergent evolution, where related species change in different ways. It is also similar to parallel evolution, where two species change in the same way and end up looking alike. Plants also show convergent evolution, like how some plants developed ways to spread their seeds by making fruity parts that animals like to eat, or how some plants learned to catch and eat small animals.

Overview

Further information: List of examples of convergent evolution

Sometimes, different animals that live in similar places or have similar ways of living develop similar traits, even if they are not related. This happens because they face the same challenges in their environment and find similar solutions. For example, birds and bats both have wings for flying, but they evolved these wings separately.

Scientists have noticed this happening in many areas of life, not just in body shapes. Even in the tiny workings of cells, similar patterns can appear independently in different groups of organisms. Some thinkers believe that if we could start life over again under the same conditions, it might take very different paths. Others think that certain solutions are so good that life will always end up finding them, leading to similar results again and again.

Distinctions

Cladistics

Further information: Cladistics

In a way of studying life called cladistics, a homoplasy is a feature that two or more groups share for reasons other than having a common ancestor. These groups that share ancestry belong to the same group called a clade. Cladistics tries to organize them based on how related they are to show their family tree. When features look similar because of convergent evolution, it can make this analysis tricky.

Atavism

Further information: Atavism

Sometimes it’s hard to know if a feature disappeared and then reappeared in a new way, or if a gene just turned off and then turned back on. When a feature comes back like this, it’s called an atavism. Over time, genes that aren’t used have less chance of staying able to work again. How long this takes can differ a lot among different groups of animals.

Parallel vs. convergent evolution

When two species look similar in some way, scientists call it parallel evolution if their ancestors were also similar, and convergent evolution if their ancestors were not. Some scientists think there’s a smooth middle ground between these two ideas, while others believe there are still clear differences.

If we don’t know the ancestors or the traits well, telling the difference between parallel and convergent evolution can become more guesswork. For example, the similar shapes of animals that give birth to live young on different continents are often called convergent evolution. This is because each group of animals had been evolving separately for a long time before a big event changed the world.

At molecular level

Proteins

Tertiary structures

Many proteins share similar shapes that developed separately in different living things. These shapes help the proteins do their jobs, even though they weren’t inherited from a common ancestor.

Protease active sites

Some enzymes that break down other molecules show very clear examples of convergent evolution. Because of the same chemical rules, these enzymes often end up with the same basic setup to work effectively, even when they come from very different sources.

Serine and cysteine proteases use different parts of molecules to kickstart their reactions. They arrange certain building blocks in the same way to make this happen, and this arrangement has evolved independently more than 20 times in different enzyme families.

Threonine proteases use a different molecule called threonine. Because of its shape, threonine has limits on how it can arrange itself, leading most of these enzymes to use a specific position to avoid problems. Different enzyme families have independently evolved to use this same position, showing convergent evolution in their active sites but with different overall structures.

Cone snail and fish insulin

A type of snail makes a version of insulin that looks more like the insulin in fish than like the insulin in its closer relatives, suggesting convergent evolution. It might also be due to a process where genes move between species.

Ferrous iron uptake via protein transporters in land plants and chlorophytes

Plants and certain green algae have evolved similar protein structures to take up iron efficiently, even though their amino acid sequences are very different. This shows convergent evolution at the molecular level.

Na⁺,K⁺-ATPase and Insect resistance to cardiotonic steroids

Insects have evolved resistance to certain toxins at the molecular level. One example is resistance to cardiotonic steroids through changes in a specific protein. These changes have occurred independently in many insect species across different groups, often at the same positions in the protein.

Nucleic acids

Convergent evolution also happens at the level of DNA and the sequences that make up proteins. Studies have found similar amino acid sequences in bats that can echolocate and in dolphins, as well as in certain mammals and between different species. Convergent evolution has also been seen in parts of DNA that don’t code for proteins, suggesting either strong selection for certain traits or less strict rules for how these parts evolve.

In animals

Bodyplans

Animals that swim, like fish such as herrings, marine mammals such as dolphins, and ichthyosaurs from the Mesozoic, all developed the same streamlined shape. Even molluscs, like Phylliroe, have a similar shape and swimming adaptations. This fusiform body shape, which is a tube tapered at both ends, helps them travel at high speed in a high drag environment. Similar body shapes are seen in earless seals and eared seals, which have four legs modified strongly for swimming.

The marsupial animals of Australia and placental mammals of the Old World have several similar forms, even though they evolved separately. For example, the body and skull shape of the thylacine (Tasmanian tiger or Tasmanian wolf) look very much like those of Canidae such as the red fox, Vulpes vulpes.

Echolocation

Echolocation has evolved separately in cetaceans (dolphins and whales) and bats. They developed this ability from the same genetic changes.

Electric fishes

The Gymnotiformes of South America and the Mormyridae of Africa independently evolved passive electroreception around 119 and 110 million years ago, respectively. About 20 million years later, both groups evolved active electrogenesis, creating weak electric fields to help them find food.

Eyes

Main article: Eye evolution

One of the best-known examples of convergent evolution is the camera eye of cephalopods (such as squid and octopus), vertebrates (including mammals), and cnidarians (such as jellyfish). Their last common ancestor had only a simple light-sensitive spot, but over time, they all developed advanced camera eyes. However, there is one big difference: the cephalopod eye is wired backward, with blood and nerves entering from the back of the retina, unlike vertebrates. Because of this, vertebrates have a blind spot.

Flight

Further information: Flying and gliding animals § Evolution and ecology of aerial locomotion

Birds and bats both have limbs that came from the same ancient four-legged animals, but their wings work in different ways. Their wings are similar in function but built differently. Bat wings are made of skin stretched over very long fingers and legs. Bird wings are made of feathers attached to the forearm and fused wrist and hand bones, with only small bits of two fingers left. Even though their wings work similarly, they are not built the same way. Birds and bats also have more of a certain skin substance that helps their wings stay flexible, which is helpful for flying. The extinct pterosaurs also developed wings on their own, and insects have wings that came from different body parts.

Flying squirrels and sugar gliders look alike with their gliding membranes stretched between their limbs, but flying squirrels are placental mammals while sugar gliders are marsupials, evolving separately.

Hummingbird hawk-moths and hummingbirds have evolved similar ways of flying and feeding.

Insect mouthparts

Insect mouthparts show many examples of convergent evolution. Different insect groups have mouthparts made from the same basic parts, but these have changed to suit their diets. For example, flower-visiting insects like bees and flower beetles have long proboscis for reaching nectar, while blood-sucking insects like fleas and mosquitos have biting-sucking mouthparts for feeding on blood.

Intelligence

Further information: Cephalopod intelligence

Advanced intelligence has evolved separately in cephalopods and vertebrates. Octopuses have shown skills in solving problems, learning, and thinking, similar to mammals. Unlike other smart animals, octopuses usually live short lives and have different social behaviors, with much of their brain power spread between their head and arms.

Opposable thumbs

Opposable thumbs that allow grasping objects are most known in primates like humans, apes, monkeys, and lemurs. But giant pandas also evolved opposable thumbs, though theirs are built differently, developing from a wrist bone instead of fingers.

Primate phenotypes

Further information: Human skin color § Genetics of skin color variation

Convergent evolution in humans includes blue eye colour and light skin colour. When humans moved out of Africa to areas with less sunlight, having lighter skin became helpful. It seems that some skin lightening happened before European and East Asian groups separated, as they share some of the same genetic changes for lighter skin. After they separated, both groups developed even lighter skin through different genetic changes.

Lemurs and humans are both primates. Most primates, including their ancestors, have brown eyes. In humans, blue eyes are known to be influenced by genetics, with one gene area responsible for much of the difference. In lemurs, the genetics of blue versus brown eyes are not fully known and involve a different gene area than in humans.

In plants

Plants can develop similar traits even if they are from different families. This is called convergent evolution. One example is the annual life cycle, where plants live for only one growing season. This way of living appeared separately in over 120 plant families, especially in hot and dry places.

Plants have also found different ways to capture and use carbon. A process called C₄ photosynthesis has evolved independently up to 40 times. Many plants, including some grasses like maize and sugar cane, use this method.

Fruits like apples and tomatoes have also evolved to be tasty to animals, which helps spread their seeds. Some plants even eat animals! Carnivorous plants, like the Cephalotus follicularis, Nepenthes alata, and Sarracenia purpurea, have developed ways to break down and use nutrients from small creatures. They do this by making special substances that help digest food.

Methods of inference

Scientists study how similar traits develop in different species using special methods. Some methods look at how traits look and change over time, while others look at why these traits might have developed in similar ways. These studies help us understand how nature can lead different animals to develop similar features.