Have you ever wondered why butterflies and birds both have wings even though they're not the same class of animal? Their wings are analogous structures: characteristics that evolved in two different organisms to serve the same purpose. Keep reading for analogous structures examples in animals and plants around the world.
Unlike creatures with homologous structures, where organisms' shared traits can determine a common ancestry, organisms with analogous structures may not be related at all. Birds and insects, for example, are part of different animal classes within phylum Chordata and have no common ancestor, but they can each use their wings to fly.
Analogous structures can also occur at different times in different organisms. Birds and bats developed forelimbs from a common ancestor, but they each developed the ability to fly long after evolving from that ancestor (known as convergent evolution). Bats developed skin membranes between their finger bones over 100 million years before birds became able to fly.
Fish and penguins are both vertebrates that use fins to swim in deep waters. However, penguins come from a flying ancestor. They lost the ability to fly around 65 million years ago when their wings became heavy enough to act as flippers when swimming. Fish evolved into having fins several million years earlier for the same reason: navigating through water.
The duck-billed platypus, which is a mammal, seems like a perplexing example of evolution. But when you consider analogous structures, it makes a lot more sense. Like ducks and other waterfowl, the platypuses have a bill that makes it easier for them to catch fish and dive underwater. Both duck bills and platypus bills are sensitive to detect the movement of prey or threatening animals. But both animals developed their bills long after evolving from their common ancestor around 315 million years ago.
What do a cactus and poinsettias have in common? They share a similar desert environment — though not the exact same environment — as cacti from genus Astrophytum are found in North America, and poinsettias from genus Euphorbia are from Africa. Both types of plants develop in round shapes to conserve water and have thick skins that can harm anything that tries to eat them. Although cacti have spines and poinsettias and other euphorbias have thorns, both evolved this trait for the same purpose.
Both crabs and turtles have shells that grow from their body and that cannot be removed. The shells protect them from predators and allow them to keep their fleshy bodies safe. Crabs and turtles also both shed the outer layers of their shells during their lifetimes as they grow. However, crabs are invertebrates and turtles are vertebrates — they developed their shells on completely different sides of the animal kingdom.
Shells aren't the turtle's only analogous structure! Both turtles and tortoises eat with hard beaks that are similar to a bird's beak, and used for the same purpose: to cut and chew food. Turtles and birds evolved into beaks at different times, as did other animals that have historically used beaks to eat, including dinosaurs such as triceratops and cephalopods such as octopus and squid.
Above the octopus's bird-like beak are two human-like eyes. Like other cephalopods, the octopus eye has a "camera eye" — it includes an iris, a circular lens, photoreceptor cells, a vitreous cavity, and a retina. The octopus eye is very large for its size, and despite being more movement-based than a human's eye, is quite similar to the eyes of many vertebrates. Although cephalopods and humans have a common ancestor, scientists believe that their eyes developed independently.
An interesting example of analogous structures is the shared coloring of sharks and dolphins. Even though these two predators share the same oceanic habitat, they are from different animal classes (fish and mammal). But the coloring of a shark and a dolphin is remarkably similar despite their different genetics. The top of a dolphin and a shark is darker to blend in with dark ocean waters when seen from above, and their undersides are white to blend in with the shallow ocean water when seen from below. Their coloring helps them remain undetected to potential prey as well as possible predators.
If there's one consistent trait in nature, it's that it's always changing. The animals we know today were quite different 100 million years ago — and who knows what the world will look like 100 million years from now? Explore examples of evolution in biology for more ways that organisms have changed over time. Or, take a look at these unique examples of animal adaptations that explain how some species survive their harsh environments.