WHAT IS A HOMOLOGY IN BIOLOGY? (WITH EXAMPLES) - BIOLOGY - 2025

A homology is a structure, organ or process in two individuals that can be traced back to a common origin. The correspondence does not have to be identical, the structure can be modified in each lineage studied. For example, members of vertebrates are homologous to each other, since the structure can be traced back to the common ancestor of this group.

Homologies represent the basis for comparative biology. It can be studied at different levels, including molecules, genes, cells, organs, behavior, and so on. Therefore, it is a crucial concept in various areas of biology.

Source: Волков Владислав Петрович (Vladlen666); translation by Angelito7, via Wikimedia Commons

Historical perspective

Homology is a concept that has been linked to the classification and study of morphologies throughout history and its roots are found in comparative anatomy. It was already a phenomenon intuited by thinkers like Aristotle, who were familiar with similar structures in different animals.

Belon, in the year 1555, published a work representing a series of comparisons between the skeletons of birds and mammals.

For Geoffroy Saint-Hilaire, there were forms or composition in the structures that could differ in organisms, but there was still a certain constancy in the relationship and connection with adjacent structures. However, Saint-Hilaire described these processes as analogous.

Although the term had its predecessors, historically it is attributed to the zoologist Richard Owen, who defined it as: "the same organ in different animals under each variation of form and function."

Owen believed in the immutability of species, but felt that the correspondence between the structures of organisms needed an explanation. From a pre-Darwinian and anti-evolutionary point of view, Owen focused his concept on “archeotypes” - a kind of scheme or plan that animal groups followed.

What is homology?

Currently, the term homology is defined as two structures, processes or characteristics that share a common ancestor. That is, the structure can be traced back in time to the same characteristic in the common ancestor.

Serial homology

Serial homology is a special case of homology, where there is similarity between successive and repeated parts in the same organism (two species or two individuals are no longer being compared).

Typical examples of serial homologies are the chain of vertebrae in the vertebrate spine, the consecutive branchial arches, and the muscle segments that run along the body.

Molecular homologies

At the molecular level, we can also find homologies. The most obvious is the existence of a common genetic code for all living organisms.

There is no reason why a certain amino acid is related to a specific codon, since it is an arbitrary choice - just as human language is arbitrary. There is no reason why "chair" should be called that, but we do it because we learned it from someone, our ancestor. The same applies to the code.

The most logical reason why all organisms share the genetic code is because the common ancestor of these forms used the same system.

The same is true of a number of metabolic pathways present in a wide range of organisms, such as glycolysis, for example.

Deep homology

The advent of molecular biology and the ability to sequence, gave way to the arrival of a new term: deep homology. These discoveries allowed us to conclude that although two organisms are different in terms of their morphology, they can share a pattern of genetic regulation.

Thus, deep homology brings a new perspective to morphological evolution. The term was used for the first time in an influential article in the prestigious journal Nature entitled: Fossils, genes and the evolution of animal limbs.

Shubin et al., Authors of the article, define it as "the existence of genetic pathways involved in regulation used to construct characteristics in animals that are disparate in terms of morphology and phylogenetically distant". In other words, deep homologies can be found in analogous structures.

The Pax6 gene plays an indispensable role in the generation of vision in mollusks, insects and vertebrates. Hox genes, on the other hand, are important for limb construction in fish and tetrapod limbs. Both are examples of deep homologies.

Source: Washington NL, Haendel MA, Mungall CJ, Ashburner M, Westerfield M, Lewis SE., via Wikimedia Commons

Source: PhiLiP, via Wikimedia Commons

Analogy and homoplasia

When you want to study the similarity between two processes or structure, it can be done in terms of function and appearance, and not just following the criterion of common ancestor.

Thus, there are two related terms: the analogy that describes characteristics with similar functions and may or may not have a common ancestor.

On the other hand, homoplasia refers to structures that simply look alike. Although these terms originated in the 19th century, they gained popularity with the advent of evolutionary ideas.

For example, the wings of butterflies and birds have the same function: flight. Thus, we can conclude that they are analogous, however we cannot trace their origin to a common ancestor with wings. For this reason, they are not homologous structures.

The same goes for the wings of bats and birds. However, the bones that they constitute are homologous to each other, because we can trace a common origin of these lineages that share the pattern of bones of the upper limbs: humerus, cubic, radius, phalanges, etc. Note that the terms are not mutually exclusive.

Homoplasia can be reflected in similar structures, such as the fins of a dolphin and those of a turtle.

Source: John Romanes (1848-1894), via Wikimedia Commons

Importance in evolution

Homology is a key concept in evolutionary biology, since only it

adequately reflects the common ancestry of organisms.

If we wish to reconstruct a phylogeny to establish the kinship, ancestry and descent relationships of two species, and by mistake we use a characteristic that only share form and function, we would reach wrong conclusions.

For example, if we want to determine the relationships between bats, birds and dolphins and mistakenly use the wings as a homologous character, we would come to the conclusion that bats and birds are more related to each other than the bat to the dolphin.

A priori we know that this relationship is not true, because we know that bats and dolphins are mammals and are more related to each other than each group with birds. Therefore, we must use homologous characters, such as the mammary glands, the three small bones of the middle ear, among others.

References

1. Hall, BK (Ed.). (2012). Homology: The hierarchial basis of comparative biology. Academic Press.
2. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
3. Lickliter, R., & Bahrick, LE (2012). The concept of homology as a basis for evaluating developmental mechanisms: exploring selective attention across the life-span. Developmental psychobiology, 55 (1), 76-83.
4. Rosenfield, I., Ziff, E., & Van Loon, B. (2011). DNA: A Graphic Guide to the Molecule that Shook the World. Columbia University Press.
5. Scharff, C., & Petri, J. (2011). Evo-devo, deep homology and FoxP2: implications for the evolution of speech and language. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 366 (1574), 2124-40.
6. Shubin, N., Tabin, C., & Carroll, S. (1997). Fossils, genes and the evolution of animal limbs. Nature, 388 (6643), 639.
7. Shubin, N., Tabin, C., & Carroll, S. (2009). Deep homology and the origins of evolutionary novelty. Nature, 457 (7231), 818.
8. Soler, M. (2002). Evolution: the basis of Biology. South Project.