Difference Between Analogy and Homology in Evolution

Evolution is a theory that, based on a large amount of evidence, states that the Earth and life on it have changed . Consequently, on the planet there are organisms developed from others, which managed to respond favorably to environmental conditions.

Among the evidence of evolution are those offered by comparative anatomy, a discipline that studies similarities and differences between organisms, through the analysis, for example, of homologous and analogous structures.

homologous structures

Homologous structures are those that share the same evolutionary origin, but have different functions. Such is the case of the fingers in amphibians, reptiles, and tetrapod (that is, four-legged) mammals. In these animals, even though they are of different classes, five fingers are present in the embryonic stage. These fingers, which can change in number and shape in adult life, have different functions and develop in very different environments.

Another case of homology is that of the extremities between mammals of different types: bat wings, dolphin fins and human arms, to name a few examples, present the same bones located in similar positions, following the same pattern.

Both the fingers of the tetrapods and the extremities of the mentioned mammals are homologous, because they demonstrate the presence of similar structures in different species that is not justified from the functional point of view. According to the theory of evolution, the above evidences the common origin of these animals from an ancestor that had five fingers or that exhibited the same bone structure.

The common ancestor hypothesis can be explained through a mechanism called divergent evolution . It occurs when a population separates from the original and is subjected to different conditions, which is why it develops particular characteristics for its survival. Migration, competition, and DNA mutations can contribute to divergent evolution of species.

analogous structures

Analogous structures are those that have similar functions and occur in different species that do not have a common ancestor that also possesses them. For example, bats, birds, and flying insects have wings that serve the same function, but these animals do not have a common winged ancestor. Bats are mammals and are not related to birds or flying insects. In fact, birds are more related to dinosaurs than insects or mammals. Although bats, birds, and flying insects adapted by developing wings, they do not indicate a close evolutionary relationship.

Analogies are also known as homoplasies , which can occur due to mechanisms of convergence, parallelism, and reversals.

• Convergence analogies occur when different species have similarities that arise from different and distant ancestors. In these cases, similar traits evolve in unrelated species, despite being found in diverse environments, but with similar selection pressures. An example of a convergence analogy is that of hyraxes and marmots, animals that are very similar in appearance and have pronounced incisor teeth. However, hyraxes are the closest living relatives of elephants and are not evolutionarily related to marmots.

• Parallel analogies occur when similarities arise in separate ways in species with the same ancestor. For example, the opossums of Brazil and the koalas of Australia have a common ancestor. However, their similarities, such as the presence of a “bag” called a marsupium in which the young develop, were acquired separately and as a result of environmental characteristics.

• Reversal analogies occur when features that had disappeared reappear. For example, in some species of frogs, individuals develop teeth in the lower jaw, a feature that was common in frog ancestors but not in modern frogs.

In summary, if the structures of related individuals are compared, many similarities are observed. When the organisms that possess them come from a common ancestor, they are called homologous structures; When organisms that do not share a common ancestor have similar structures that serve similar functions, they are known as analogous structures.

Sources

Curtis, H., Barnes, N.S., Schnek, A., Massarini, A. Biology . 7th edition. Editorial Médica Panamericana., Buenos Aires, 2013.