HOMOLOGOUS AND ANALOGOUS STRUCTURES (WITH EXAMPLES) - BIOLOGY - 2025

The homologous structure are parts of a biological organism that share a common ancestor, while the analog perform similar functions. When comparing two processes or structures, we can assign them as homologues and analogs.

These concepts gained popularity after the emergence of evolutionary theory, and their recognition and distinction are key to the successful reconstruction of the phylogenetic relationships between organic beings.

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

Theoretical bases

In two species, a trait is defined as homologous if it has been derived from a common ancestor. This may have been extensively modified and does not necessarily have the same function.

Regarding analogies, some authors often use the term homoplasia synonymously and interchangeably to refer to similar structures that are present in two or more species and do not share a close common ancestor.

In contrast, in other sources, the term analogy is used to designate the similarity of two or more structures in terms of function, while homoplasia is restricted to evaluating structures similar to each other, morphologically speaking.

Also, a trait can be homologous between two species, but a trait status cannot. The pentadactyl is an excellent example of this fact.

In humans and crocodiles we can distinguish five fingers, however rhinos have structures with three fingers that are not homologous, since this condition has evolved independently.

The application of these terms is not restricted to the morphology of the individual, they can also be used to describe cellular, physiological, molecular characteristics, etc.

How are homologies and analogies diagnosed?

Although the terms homology and analogy are easy to define, they are not easy to diagnose.

Generally, biologists state that certain structures are homologous to each other, if there is correspondence in the position relative to other parts of the body and correspondence in the structure, in case the structure is composite. Embryological studies also play an important role in diagnosis.

Thus, any correspondence that may exist in form or function is not a useful feature for diagnosing homologies.

Why do the analogies exist?

In most cases - but not all - species with similar characteristics inhabit regions or zones with similar conditions and are subjected to comparable selective pressures.

In other words, the species solved a problem in the same way, although not consciously, of course.

This process is called convergent evolution. Some authors prefer to separate convergent evolution from parallels.

Convergent evolution or convergence leads to the formation of superficial similarities that occur through differential developmental pathways. Parallelism, on the other hand, involves similar developmental pathways.

Examples

-Fusiform shape in aquatic animals

In Aristotelian times, the spindle-shaped appearance of a fish and a whale was considered sufficient to group both organisms into the broad and imprecise category of "fish."

However, when we carefully analyze the internal structure of both groups, we can conclude that the resemblance is exclusively external and superficial.

Applying evolutionary thinking, we can assume that, over millions of years, evolutionary forces benefited the increased frequency of aquatic individuals exhibiting this particular shape.

We can also assume that this fusiform morphology conferred some benefit, such as minimizing friction and increasing locomotion capacity in aquatic environments.

There is a very particular case of similarities between two groups of aquatic animals: dolphins and the now extinct ichthyosaurs. If the curious reader were to look for a picture of this last group of sauropsids, they could easily mistake it for dolphins.

-Teeth in anurans

A phenomenon that can lead to the appearance of analogies is the reversion of a character to its ancestral form. In systematics, this event can be confusing, since not all descendant species will present the same characteristics or traits.

There are some species of frogs that, by evolutionary reversion, acquired teeth in the lower jaw. The "normal" condition of frogs is the absence of teeth, although their common ancestor possessed them.

Thus, it would be a mistake to think that the teeth of these peculiar frogs are homologous with respect to the teeth of another animal group, since they did not acquire them from a common ancestor.

-Similar between Australian marsupials and South American mammals

The similarities that exist between both animal groups derive from a common ancestor - a mammal -, but they were acquired differentially and independently in the Australian groups of metatherian mammals and in the South American eutherian mammals.

Cactus

The examples of analogy and homology are not restricted solely to the animal kingdom, these events are diffused throughout the complex and intricate tree of life.

In plants, there are a series of adaptations that allow tolerance to desert environments, such as succulent stems, columnar stems, spines with protective functions and a considerable reduction in the foliar surface (leaves).

However, it is not correct to group all the plants that have these characteristics as cacti since the individuals that carry them did not acquire them from a common ancestor.

In fact, there are three different families of phanerogams: Euphorbiaceae, Cactaceae and Asclepiadaceae, whose representatives convergently acquired adaptations to arid environments.

Consequences of confusing an analogous structure with a homologous one

In evolutionary biology, and in other branches of biology, the concept of homology is fundamental, since it allows us to establish the phylogeny of organic beings - one of the most conspicuous tasks of current biologists.

It must be emphasized that only homologous characteristics adequately reflect the common ancestry of organisms.

Consider that in a certain study we want to elucidate the evolutionary history of three organisms: birds, bats and mice. If we took, for example, the characteristic of wings to reconstruct our phylogeny, we would come to the wrong conclusion.

Why? Because birds and bats have wings and we would assume that they are more related to each other than each to the mouse. However, we know a priori that both mice and bats are mammals, so they are more related to each other than each to the bird.

Then, we must look for homologous characteristics that allow us to correctly elucidate the pattern. For example, the presence of hair or mammary glands.

Applying this new vision we will find the correct pattern of relationships: the bat and the mouse are more related to each other than each one to the bird.

References

1. Arcas, LP (1861). Elements of zoology. Printing of Gabriel Alhambra.
2. Curtis, H., & Schnek, A. (2006). Invitation to Biology. Panamerican Medical Ed.
3. Hall, BK (Ed.). (2012). Homology: The hierarchial basis of comparative biology. Academic Press.
4. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
5. 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.
6. Raven, PH, Evert, RF, & Eichhorn, SE (1992). Plant Biology (Vol. 2). I reversed.
7. Soler, M. (2002). Evolution: the basis of Biology. South Project.