Biology

Homologous organs: from the same origin to a different function

The definition of homologous organs is that they are organs of different species that have a common evolutionary origin and a similar basic structure, although later they may or may not have different functions and an equally different appearance (at the end of the article there is a summary table). This phenomenon is called homology and is the object of study in evolutionary biology.

That said, throughout the article we are going to illustrate it to make it clearer.

It should be clarified that homology not only refers to homologous organs, but this concept also applies to homologous structures and even homologous genes that will not be the subject of this article.

Example of homologous organs

Homologous organs
Homologous organs

What do the arm of a human being and the wing of a bird have in common? What if we add the leg of a dog and the fins of a whale?

In all these cases, the base is built on the same pattern, on a structure of similar bones . And it is that, after an evolutionary analysis, it has been observed that they derive from a common ancestor that had five fingers. At present, they (the fingers) have them at some point in their life cycle, in a similar position but with a different appearance to perform a different function.

In the case of the human being to grasp objects, in the dog to support and walk, in the case of the bird to fly and in the case of the whale to swim; they are different functions of a homologous organ , the hand and its fingers. In some they have been lengthened, others have been shortened, or they articulate differently or they merge or lengthen as in the case of birds, and therefore show the different appearance that the homologous organs have.

In the previous example, we observed that the chosen species continued to maintain the five fingers, but we also commented that in the adult stage they did not have to maintain them.

For example, it is the case of the following image where the human hand (I) is compared with the forelimbs of the dog (II), the pig (III), the cow (IV), the tapir (V) and the horse (SAW). In all these cases, we can observe a similar base structure of an adult, with different function, a different appearance (different number of fingers) and a common origin (during embryonic development they have five fingers, it does not appear in the image).

Homologous organs derive from a specific organ of a common ancestor, although later they may have led to very different functions and appearance thanks to a so-called divergent evolution due to the evolutionary pressure of the environment in which they live.

Another example of homologous organs is found in insects. The same common structure has given rise to the flying wings of dragonflies, to the pair of hard wing covers (elytra) of beetles, and in flies to a second pair of wings that serve to maintain balance.

Homologous organs in plants

Homology is not something that is present only in animals. In plants we also find homologous organs that have a common origin, with a similar base structure but different appearance and function.

A fairly clear and easy-to-see example happens in succulent plants. The leaves have evolved into water storage structures such as Aloe vera or defensive structures such as thorns.

In other species such as carrots the root has become a thickened root or in potatoes where tubers appear that are modified roots as well.

Analogous organs

Homologous organs
Homologous organs

The opposite of homologous organs are analogous organs, which are those that are similar to each other with a very similar or equal function but have a different structure and evolutionary origin. We will see them more in depth in another article.

Summary of homologous organs and difference from analogous organs

Homologous Organs
(Homology)
Analogous organs
(Analogy)
Appearance Different Similary
Function Different Similary
Basic structure Similary Different
Evolutionary origin Common with divergent evolution Different with convergent evolution

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