Analogous structures are similar structures that evolved independently in two living organisms to serve the same purpose.
The term “analogous structures” comes from the root word “analogy,” which is a device in the English language where two different things on a basis of their similarities.
Analogous structures are examples of convergent evolution, where two organisms separately have to solve the same evolutionary problem – such as staying hidden, flying, swimming, or conserving water – in similar ways. The result is similar body structures that developed independently.
In the case of analogous structures, the structures are not the same, and were not inherited from the same ancestor. But they look similar and serve a similar purpose.
For example, the wings of an insect, bird, and bat would all be analogous structures: they all evolved to allow flight, but they did not evolve at the same time, since insects, birds, and mammals all evolved the ability to fly at different times.
Examples of Analogous Structures
Wings Through The Ages
As mentioned above, many creatures have independently developed wings. All wings were evolved in order to solve the same problem: how to fly through the air. But they have evolved on several different occasions throughout history.
Insects were the first organisms to evolve structures which could push air down in order to propel their bodies through the air. Insects probably evolved flight by using parts of their protective exoskeletons to propel themselves through the air.
Millions of years later, reptiles learned to do the same thing- pterosaurs evolved a skin membrane, stretched between their finger and ankle bones, which was capable of propelling them through the air.
Millions of years later still, dinosaurs separately evolved flight – using the feathers they had developed to keep warm in order to push them into the sky. In the process, these small, feathered dinosaurs evolved into birds.
Mammals solved the problem of flight yet again about 100 million years after birds first appeared, with bats using a similar solution to that of the pterosaurs: skin membranes stretched between long finger bones.
In this way, we have at least four different types of wings in the fossil record which are analogous: they serve the same purpose, but were not inherited from the same ancestor.
The “Duck-Billed” Platypus
When the first specimen of a platypus was sent to a British museum by an Australian explorer, they tried to pry it apart to prove it was a fake! British scientists were sure that someone had simply stuck a duck’s bill onto the body of a beaver-like animal.
However, the truth was much more interesting: platypi had evolved almost exactly the same structure evolved by ducks to solve the problem of gathering food such as fish and aquatic plants from water.
Ducks and platypi could not possibly be related – platypi are mammals, and they evolved long after birds and mammals went their separate ways on the evolutionary path. Yet both evolved very similar solutions when they moved from land back into the water!
Cacti and Water Conservation
Some members of the plant genres Euphorbia and Astrophytum look extremely similar.
Both have round, ball-shaped bodies divided into eight equal wedges; both have hard, pointy thorns sticking out in a row along the middle of each wedge, protecting them from animals who might try to eat them. To the untrained eye, they may be mistaken for members of the same species.
This is particularly remarkable because these two genii are only distantly related, and they live in two completely different parts of the world.
Astrophytum evolved in North America, and all members of its genus are cacti that live in the southwestern deserts.
Euphorbia, on the other hand, is a plant genus that includes poinsettias – as well as certain cacti found in the deserts of Africa.
Both the African and North American cacti conserve water by minimizing their surface area – resulting in a round, ball shape – developing a thick, waxy skin, and placing prickly deterrents on its skin at its most vulnerable places to discourage animals from trying to eat it for its moisture.
The result is two plants which look nearly identical – but which have very different ancestry!
Difference Between Analogous and Homologous Structures
The difference between homologous and analogous structures can be thought of in terms of ancestry and function:
• Analogous structures have different ancestry, but the same function.
These can be thought of in terms of the literary device of “analogy,” where two different things are compared based on their similarities.
• Homologous structures have the same ancestry, but may no longer serve the same function.
For example, the bones that make up human fingers were inherited from an ancestor that’s shared by all mammals. Bats, dogs, and whales also have these bones, but bats use them to spread their wings, dogs walk on them, and whales do not use them for anything since they are encased inside their fins.
These structures are therefore homologous – there is a clear relationship and similarities between them, even though they are not used for the same purpose.
The existence of homologous structures is strong evidence for the theory of evolution, since there is no reason why a whale should have the same bones in its fin that a bat has in its wings, unless they both evolved from a common ancestor.
These can be thought of in terms of the literary device of “homonyms,” where two words sound the same, but have different meanings.
Identifying Analogous Structures
Scientists usually identify analogous structures by looking at the known relatives of the two species being studied.
If a line of common inheritance can be found – such as humans and monkeys both having fingers, when we have a fossil record showing that humans and monkeys shared a common ancestor, who also had fingers – the structures are not considered analogous.
But if no common ancestor which shares these features is found – such as in the case of bats and insects, whose shared ancestor did not fly at all – the structures would be considered analogous.