ANALOGOUS ORGANS: EVOLUTION, CHARACTERISTICS AND EXAMPLES - BIOLOGY - 2025

The similar bodies are body structures similar to the naked eye in form and function, but whose evolutionary origin is different. A simple example of this concept is the case of wings; These appendages are observed in various groups of animals such as birds, insects, bats, etc., but they do not share the same origin.

Although two or more groups of living beings have similarities in some parts of their body, this is not an evidence or sign that these groups are evolutionarily close to each other, or that they are closely related.

Analogous and homologous organs (Source: Vanessablakegraham Via Wikimedia Commons)

In evolution, analogy and homology do not mean the same thing. The term homology refers to the existence of structures, similar or not from the morphological and functional point of view, that are the product of a common evolutionary origin, from a trait in a common ancestor that suffered modifications related to the environment when which was adapted.

An example of homology can be the fin of a dolphin and the arm of a human being; These are forelimbs in both vertebrates, but they serve somewhat different functions.

The analogy, on the other hand, refers to the “superficial” similarity between living beings or parts of these, without any value from the phylogenetic point of view to carry out kinship studies between species.

Evolution

When we refer to the evolution of analogous organs, we are necessarily talking about convergent evolution, since, according to this definition, the same trait arose independently at different times in the natural history of living beings to exercise the same function in evolutionarily different species.

To understand the subject more deeply, it is important to distinguish between similarities or similarities that are the result of descent (homology) and those that are due solely to functional similarities (analogy).

The wings of a fly and those of a bird are functionally equivalent, since they both serve to fly; however, these are not the product of a common descent, which means that the wings of a bird and those of a fly are not modified versions of a structure previously present in a common ancestor for both animals.

Photograph of a butterfly (Image by moonzigg at www.pixabay.com)

In this sense, we can generalize that the evolution of a similar trait or organ occurs as an adaptive response to perform a common function, which, in the case of the bird and the fly, is flight.

It is pertinent to clarify that some organs can be partially analogous and partially homologous.

The wings of bats and birds, for example, are partially homologous when analyzed in the context of the reptilian ancestor that both animals share (in arrangement of the skeleton of the same forelimb).

However, they are partially analogous from the point of view of the evolution of modifications or adaptations for flight, which arose independently from similar life forms.

Parallelism and convergence

There is another term used by evolutionists that is often difficult to distinguish from evolutionary convergence or the analogy between body structures.

This term is parallelism, which refers to the existence of two or more lineages that have evolved independently in a similar way, so that the "evolved" descendants of each lineage are as similar to each other as their ancestors were.

characteristics

Analogous organs are characterized by:

- Arise by convergent evolution

- Fulfill the same function in distant organisms, phylogenetically speaking (homoplastic)

- Being the product of evolutionary adaptations to similar ways of life

- From the genetic point of view, in many cases, analogous structures or organs are determined by genes that are homologous to each other

- In addition to fulfilling the same function in different species, analogous organs are, many times, structurally and functionally similar, otherwise the homologous organs

Examples of analogous organs

Analogous organs represent, for many experts, overwhelming evidence of the evolutionary processes that occur thanks to adaptations to particular environmental conditions, so their study has allowed us to delve into different evolutionary theories and explanations.

- In animals

Very different species of animals can evolve, acquiring very similar features or organs that fulfill equivalent functions.

Humans and mollusks

Such is the case with eyes, for example, which arose independently in mollusks and hominids.

Although the eye of octopuses, to cite the example, is considerably higher than that of humans, since it does not have a blind spot, in the two groups of animals, both structures perform the same function, despite the fact that the humans and octopuses are evolutionarily very distant.

Another example of analogous organs in animals is that of wings in invertebrates, birds and mammals, which was cited above.

Sharks and dolphins

The case of the fins of dolphins and sharks is another example that is usually used to illustrate the evolutionary phenomenon of analogous organs.

Sharks and dolphins (Source: Charles R. Knight, under the direction of Professor Osborn. Via Wikimedia Commons)

Dolphins belong to the group of mammals and the skeleton of their fins is arranged in an anatomical shape equal to that of the arm of a human being or the wing of a bat, so in this context we refer to it as a homologous organ for the group of mammals.

Sharks, on the other hand, are cartilaginous fish and, despite the superficial similarity between their fins and the fins of a dolphin, which serve similar purposes, these organs in this animal are analogous to those of the dolphin, since they arose from structures Different embryonic cells are anatomically different, but they exercise the same locomotion function.

The mole and the cricket

The mole is a mammal that lives mainly underground, in caves dug by itself. This animal has the distal portions of its forelimbs modified for burrowing.

The mole cricket, an insect that also lives underground, digs its nests thanks to modified appendages on its front legs, which resemble the claws that moles have, so in this sense, both structures are analogous organs.

- In plants

In plants there are also multiple cases of analogies. Classic examples of analogous structures or organs among phylogenetically distant plant groups include spines and fleshy leaves, which have arisen independently in various desert and non-desert plant groups.

A plant with fleshy leaves and thorns (Source: Leonora Enking from West Sussex, England via Wikimedia Commons)

Certain species of aquatic plants have modified some of the leaves that submerge in water, the latter acquiring a morphology very similar to that of the roots of a terrestrial plant and even fulfilling very similar functions.

References

1. Boyden, A. (1943). Homology and analogy: a century after the definitions of »homologue» and »analogue» of Richard Owen. The Quarterly Review of Biology, 18 (3), 228-241.
2. Encyclopaedia Britannica. (2011). Retrieved December 20, 2019, from www.britannica.com/science/analogy-evolution.
3. Gallardo, MH (2011). Evolution: The Course of Life (No. 575 G 162).
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). New York: McGraw-Hill.
5. Nabors, MW (2004). Introduction to botany (No. 580 N117i). Pearson.