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Due to the similarity between the two terms and the fact that they share the same prefix, homology and homoplasy are two concepts related to evolution that are often confused. This article seeks to clarify the differences between both evolutionary terms.
What is homology?
Homology is a term that describes the similarity between phenotypic characteristics of two or more species that are inherited from a common ancestor. This means that a homologous characteristic is a common trait present in two or more species, a trait that appeared in an ancestral species and was maintained over time in all intermediate species that evolved from that ancestor until reaching the species compared in the present.
Examples of homologies
There are many examples of homologies between different species. One of them is the bone structure of the limbs of vertebrates. Despite the fact that these limbs can fulfill very different functions in each species and have radically different appearances, their basic structure, that is, the bone structure that makes them up, is essentially the same. From the human arm, a dog’s paw, and a bird’s wing to a whale’s pectoral fin, all limbs share the same set of bones.
Another example of vertebrate homology is the end of the horse’s foot, which is homologous to the middle finger of the human hand.
What is homoplasy?
Homoplasy, on the other hand, also refers to common features between two or more species; however, in this case the species evolved independently. That is, they are common or functionally similar traits that occur in evolutionarily unrelated species. Homoplasies arise as an evolutionary advantage that allows different species to adapt to similar environments.
Examples of homoplasies
Just as there are a large number of examples of homologies, so is the case with homoplasies. Perhaps one of the most illustrative examples is the eyes. A multitude of different and evolutionarily unconnected species have eyes. They all perform a similar function, which is to be able to see their surroundings, but the particular structure of the eyes in different species can be radically different.
Another example is the wings, which are structures present in birds, chiroptera (bats) and insects. There are many animal species that can fly and they all owe this ability to their wings. However, wings can vary greatly in structure, shape, and composition from one animal species to another. For example, the wings of bats are membranes made of skin and connective tissue that extend between the fingers of the upper extremities. On the other hand, in the case of birds, the wings are formed by the upper extremities covered with feathers, which are responsible for support during flight. These feathers are made of keratin. On the other hand, in the case of insects,
Differences between homology and homoplasy
As we can see, homology and homoplasy are different from several points of view. Each of these differences is detailed below based on different criteria.
They have a different origin.
Homologies between species and homoplasies have a different evolutionary origin. Homologies come from a common ancestor to species that have homologous traits; instead, homoplasies arise independently through evolution.
The causes are different
Homoplasies appear in different species as a consequence of the need to adapt to similar environments. For example, the need to fly in order to forage for food or to migrate during the winter is one of the forces that aided the development of wings in all flying species. The abundance of sunlight on the earth’s surface and its interaction with matter allowed the development of eyes in many species, while the total absence of light on the seabed meant that other species did not need eyes.
Instead, homology is caused by coming from a common species or ancestor.
They arise from different types of evolution
As they consist of features of a common ancestor that were conserved when different species differentiated, homologies arise as a consequence of divergent evolution, that is, thanks to the accumulation of differences between related populations that gives rise to speciation. On the other hand, when dealing with traits that arose from independent evolutionary paths that gave rise to similarities between evolutionarily unrelated species, homoplasy arises from convergent evolution.
Homologous and homoplastic traits differ in relation to their structure.
Homologous traits may have very different appearances, but they have a very similar internal structure, usually made up of the same structural units. This is a consequence of the fact that these are traits that come from the same ancestor but that slowly evolved and changed in different ways, in turn giving rise to different species.
On the other hand, homoplastic traits or homoplasies have functions and appearances that can be very similar, but are structurally very different. As mentioned above, an example of this is the wings, which can be very different in shape, size and structure, but all serve the function of adaptation for sustained flight.
They are genetically different
Being phenotypic characteristics from the same common ancestor, homologous traits are usually very similar genetically from one species to another. For example, the set of genes that code for the bone structure of a human’s hand has a very similar genetic sequence to the genes that code for a whale’s pectoral fin. In fact, this is a way of recognizing and distinguishing when we are in the presence of homologous traits between two species. If the DNA sequence for that trait in both species is very similar, it is likely a case of homology.
This is not so in the case of homoplasy. Evolving naturally and independently in genetically unrelated species, the DNA sequences that code for homoplastic traits are often radically different.
Summary of differences between homology and homoplasy
The following table summarizes the differences between homology and homoplasy explained in the previous section.
| Criterion | homology | homoplasy |
| Definition | Similarities between two or more species that evolved from a common ancestor. | Similarities between two or more species that did not evolve independently. |
| Origin | They come from a common ancestor. | They do not come from a common ancestor. |
| Cause or occurrence | They appear as a result of an evolutionary relationship. | They appear as a consequence of adaptation to similar environmental conditions. |
| type of evolution | They appear by divergent evolution. | They appear by convergent evolution. |
| Structure | The basic structure is similar. | The base structure is not necessarily similar. |
| Function | Homologous features may or may not serve similar functions. | Homoplastic features always serve similar functions. |
| Genetics | Homologous traits have very similar genetic sequences. | Homoplastic traits have different genetic sequences. |
| Example | Vertebrate limbs are examples of homologies stemming from the same common ancestor. | The eyes are examples of homoplasies present in many species that do not share common ancestors. |
References
wings . (nd). EcuRed. https://www.ecured.cu/Alas
Blue whale skeleton. | Whale, Blue whale, Animal skeletons . (nd). pinterest. https://www.pinterest.com/pin/544020829972138948/
Hall, BK (2007, January 1). HOMOLOGY AND HOMOPLASY . Science Direct. https://www.sciencedirect.com/science/article/pii/B9780444515438500216
Lakna, B. (2018, January 26). Difference Between Homology and Homoplasy | Definition, Emergence, Examples and Differences . Pediaa.Com. https://pediaa.com/difference-between-homology-and-homoplasy/
Samanthi, S. (2017, August 9). Difference Between Homoplasty and Homology . DifferenceBetween.Com. https://www.differencebetween.com/difference-between-homoplasy-and-vs-homology/
