What are the best examples of homologous structures?

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A homologous structure is a similar structure that can be found in very different animals, often pointing towards a common ancestor. When animals look very different on the outside yet have certain structures that appear similar in form or function, they have homologous structures.

To understand why homologous structures play an important role in the study of evolution, let’s take a closer look at some different types of evolution and some examples of homologous structures.

Different Forms Of Evolution

Biologists have long used anatomical comparisons of animals to determine where on the evolutionary tree of life they are, to help them determine which animals may have shared an evolutionary history. When biologists study evolution, they often distinguish between two different forms of evolution: convergent evolution and divergent evolution. Divergent evolution is where an evolutionary lineage splits apart over the course of time, giving rise to many diverse species from a few closely related species. Divergent evolution often occurs when a species migrates to a new environment or environmental changes occur in the area a species lives in.

“Evolution is the secret for the next step.” — Karl Lagerfeld

Species that migrate to a new environment often fill in ecological niches in the new area rather quickly, so divergent evolution can quickly give rise to many different species. A notable example of divergent evolution is the fish known as Characidae. Characidae are fish that evolved from a single common evolutionary lineage yet no constitute many different species like piranhas and tetras. The jaws and teeth of the Characidae evolved to adapt to food supplies within the new environment of the fish. Divergent evolution is also sometimes referred to as adaptive radiation, as the evolutionary trajectory seems to radiate outwards into different species.

In contrast to divergent evolution, convergent evolution is a phenomenon where different species tend to become more similar over time. One of the reasons for convergent evolution is that species who live in similar environments frequently experience similar evolutionary pressures, and therefore adapt with similar changes to fill similar niches. One notable example of convergent evolution is the similarity that exists between North American hummingbirds and Asian fork-tailed sunbirds. The two species of birds come from completely different evolutionary lineages, yet they look extremely similar to one another.

Further Explanation of Homologous Structures

In the study of evolutionary biology, homology refers to the existence of shared ancestry between a pair of structures or genes in a different classification unit or taxa. The term “homologous structures” refers specifically to similar structures found in different species that have a common ancestry or developmental origin. Note that homologous structures don’t have to perform the same function in a species, the only requirement is that they are similar in form and exist in species with common ancestry. As an example, while bats and humans have similar forelimbs, they are used very differently in the two species. Despite this, the skeletal structure of the forelimb is basically the same and both species have the same embryonic origin which could imply a common ancestor.

Cladistics is a specific approach to the biological classification of organisms, involving the grouping of organisms into clades based on their most recent common ancestor. Within cladistics, there are various types of homology. Primary homology refers to the initial hypothesis a researcher makes based upon anatomical connections – homologous structures. Secondary homology is used in parsimony analysis, where an organism’s character state is considered to be homologous if it arises only once on a specific tree.

“It is not the strongest of the species that survives, nor the most intelligent, but the one most responsive to change.” — Charles Darwin

Developmental biology can be useful in identifying homologous structures created from the same tissue during the process of embryogenesis, the process that creates the embryo that develops into an animal. As an example, while adult snakes do not possess legs, during their embryonic stage they have limb-buds of the sort that often become hind legs in other animals. This implies that the ancestors of snakes had legs, a theory that is confirmed by fossil evidence.

DNA sequencing can also assist in the identification of homologous structures. Similarities between DNA sequences and proteins can be used to find common ancestries. Two particular segments of DNA may have shared an ancestry if their DNA points to either a speciation event or a duplication event. If DNA sequencing shows that two species are closely related to one another and they have similar skeletal structures as well, it provides more evidence for the claims that the structures are homologous in nature.

Unlike analogous structures that may not necessarily represent similar evolutionary paths, homologous structures represent similar evolutionary paths as a prerequisite of the fact that two species with homologous structures share a common ancestry. So while analogous structures may have evolved in different circumstances, homologous structures are likely the product of the same evolutionary pressures on the same lineage.

Examples Of Homologous Structures

Photo: MabelAmber via Pixabcay, CC0

Many mammals have tails, which are one of the best examples of homologous structures. The tails of rats, cats, monkeys and many other mammals are extensions of the torso, being made out of vertebrae capable of flexing. Tails are used for balance in many animals, and to ward off insects. While humans don’t have tails, we do have a tailbone. The tailbone is called the coccyx, and it is created out of “rudimentary vertebrae”, and may have once been a fully formed tail. This is an example of a homologous structure and evidence of our common ancestry with other mammals.

Photo: HowardWilks via Pixabay, CC0

The long necks of giraffes are also examples of homologous structures. Giraffes necks have seven cervical vertebrae, and together they are approximately eight feet in length and weigh over 600 pounds. Humans have cervical vertebrae as well, though they are obviously much smaller and shorter than the cervical vertebrae found in giraffes. Yet the bones in the human neck and giraffe neck are still seven cervical vertebrae. Once more this is evidence that giraffes and humans share a common ancestor.

Homologous structures are similar structures that evolved from a common ancestor.

Learning Objectives

• Describe the connection between evolution and the appearance of homologous structures

Key Points

• Homology is a relationship defined between structures or DNA derived from a common ancestor and illustrates descent from a common ancestor.
• Analogous structures are physically (but not genetically) similar structures that were not present the last common ancestor.
• Homology can also be partial; new structures can evolve through the combination or parts of developmental pathways.
• Analogy may also be referred to as homoplasy, which is further divided into parallelism, reversal, and convergence.

Key Terms

• homology: A correspondence of structures in two life forms with a common evolutionary origin, such as flippers and hands.
• analogy: The relationship between characteristics that are apparently similar but did not develop from the same structure
• homoplasy: A correspondence between the parts or organs of different species acquired as the result of parallel evolution or convergence.

Homology is the relationship between structures or DNA derived from the most recent common ancestor. A common example of homologous structures in evolutionary biology are the wings of bats and the arms of primates. Although these two structures do not look similar or have the same function, genetically, they come from the same structure of the last common ancestor. Homologous traits of organisms are therefore explained by descent from a common ancestor.

It’s important to note that defining two structures as homologous depends on what ancestor is being described as the common ancestor. If we go all the way back to the beginning of life, all structures are homologous!

Figure \(\PageIndex{1}\): Homology in the forelimbs of vertebrates: The principle of homology illustrated by the adaptive radiation of the forelimb of mammals. All conform to the basic pentadactyl pattern but are modified for different usages. The third metacarpal is shaded throughout; the shoulder is crossed-hatched.

In genetics, homology is measured by comparing protein or DNA sequences. Homologous gene sequences share a high similarity, supporting the hypothesis that they share a common ancestor.

Homology can also be partial: new structures can evolve through the combination of developmental pathways or parts of them. As a result, hybrid or mosaic structures can evolve that exhibit partial homologies. For example, certain compound leaves of flowering plants are partially homologous both to leaves and shoots because they combine some traits of leaves and some of shoots.

Homologous sequences are considered paralogous if they were separated by a gene duplication event; if a gene in an organism is duplicated to occupy two different positions in the same genome, then the two copies are paralogous.

A set of sequences that are paralogous are called paralogs of each other. Paralogs typically have the same or similar function, but sometimes do not. It is considered that due to lack of the original selective pressure upon one copy of the duplicated gene, this copy is free to mutate and acquire new functions.

Figure \(\PageIndex{1}\): Homology vs. analogy: The wings of pterosaurs (1), bats (2), and birds (3) are analogous as wings, but homologous as forelimbs. This is because they are similar characteristically and even functionally, but evolved from different ancestral roots.

Paralogous genes often belong to the same species, but not always. For example, the hemoglobin gene of humans and the myoglobin gene of chimpanzees are considered paralogs. This is a common problem in bioinformatics; when genomes of different species have been sequenced and homologous genes have been found, one can not immediately conclude that these genes have the same or similar function, as they could be paralogs whose function has diverged.

The opposite of homologous structures are analogous structures, which are physically similar structures between two taxa that evolved separately (rather than being present in the last common ancestor). Bat wings and bird wings evolved independently and are considered analogous structures. Genetically, a bat wing and a bird wing have very little in common; the last common ancestor of bats and birds did not have wings like either bats or birds. Wings evolved independently in each lineage after diverging from ancestors with forelimbs that were not used as wings (terrestrial mammals and theropod dinosaurs, respectively).

It is important to distinguish between different hierarchical levels of homology in order to make informative biological comparisons. In the above example, the bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods.

Analogy is different than homology. Although analogous characteristics are superficially similar, they are not homologous because they are phylogenetically independent. The wings of a maple seed and the wings of an albatross are analogous but not homologous (they both allow the organism to travel on the wind, but they didn’t both develop from the same structure). Analogy is commonly also referred to as homoplasy.