They are similar structures that evolved independently in two living organisms to serve the same purpose.
The term “analogous organs” comes from the root of the word “analogy,” which is a device in the English language where two different things are based on their similarities.
Analogous structures are examples of convergent evolution , where two organisms have to separately solve the same evolutionary problem, such as staying hidden, flying, swimming, or conserving water in a similar way.
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, as insects, birds, and mammals developed the ability to fly everywhere at different times.
Examples of Analog Organs
Example 1: Wings through the ages
As mentioned above, many creatures have independently developed wings. All wings were developed 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 develop structures that could push air downward to propel their bodies through the air. Insects likely evolved by flying using parts of their protective exoskeletons to propel themselves through the air.
Millions of years later, reptiles learned to do the same: pterosaurs developed a skin membrane, stretched between the toe and ankle bones, that was capable of propelling them through the air.
Millions of years later, dinosaurs evolved separately, using the feathers they had developed to keep warm in order to push them skyward. In the process, these tiny feathered dinosaurs evolved into birds.
Mammals solved the flight problem once again, some 100 million years after birds first appeared, with bats using a solution similar to that of pterosaurs: skin membranes stretched between the long bones of the birds. fingers.
In this way, we have at least four different types of wings in the fossil record that are analogous: they serve the same purpose, but they were not inherited from the same ancestor.
Example 2: The “duck-billed” platypus
When the first specimen of a platypus was sent to a British museum by an Australian explorer, they tried to separate it to prove it was a fake!
British scientists were sure that someone had simply pasted a duck bead onto the body of a beaver-like animal.
However, the truth was much more interesting: the platypi had developed almost exactly the same structure developed by ducks to solve the problem of collecting food such as fish and aquatic plants from the water.
Ducks and platypi cannot be related, platypi are mammals and evolved long after birds and mammals went their separate ways on the evolutionary path. However, they both developed very similar solutions when they moved from land to water.
Examples 3: Cactus and water conservation
Some members of the Euphorbia and Astrophytum plant genera look extremely similar.
Both have round, ball-shaped bodies, divided into eight equal wedges; both have hard, pointed spines that stick out in a row along each of the wedges, protecting them from animals that might try to eat them.
To the untrained eye, they can be mistaken for members of the same species.
This is particularly notable because these two geniuses are distantly related, and they live in two completely different parts of the world.
The Astrophytum evolved in North America, and all members of its genus are cacti that live in the deserts of the Southwest.
Euphorbia, on the other hand, is a genus of plants that includes poinsettias as well as certain cacti found in the deserts of Africa.
Both African and North American cacti conserve water by minimizing its surface, resulting in a round ball shape, developing a thick, waxy skin, and placing prickly elements on the skin in the most vulnerable places to prevent animals from trying to eat it for its moisture. .
The result is two plants that look almost identical, but have very different ancestors.
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 ancestors, but the same function
This 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 shared by all mammals.
Bats, dogs, and whales have these bones too, but bats use them to spread their wings, dogs step on them, and whales don’t use them at all, as they are enclosed within their fins.
These structures are therefore homologous; that is, 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, as there is no reason for a whale to have the same bones in its fin as a bat in its wings, unless both have evolved from a common ancestor.
This can be thought of in terms of the literary device of “homonyms,” where two words sound the same, but have different meanings.
Identify analogous structures
Scientists generally identify analogous structures by looking at the known relatives of the two species being studied.
If a common line of inheritance can be found, such as humans and monkeys that have 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 is found that shares these characteristics, as in the case of bats and insects, whose shared ancestor does not fly at all, the structures would be considered analogous.