What is Homology?

Homology is most widely understood as the resemblances among different members of a biological group, such as two species of birds. However, biologists take it much more seriously and use it to describe similarity in form due to shared ancestry. Homology arose from the similar shapes between invertebrate eyes and vertebrate eyes. It has also been used extensively in paleontological analysis to determine relationships between organisms by comparing anatomical features.

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The term “homology” can be used in two very different ways. The first use is the more common meaning used by biologists to compare structures of groups of organisms based on shared ancestry and common descent. The second usage is often found in paleontology, which refers to similarities between closely related fossils due to common descent.

Basis of homology among chromosomes

The homology of chromosomes is based on the centromere position. The centromere is found at one end of the chromosome in the human and most animal species. Gene survivorship is very important in homology. This is not always the case in plants, fungi, and some animals with multiple copies of their chromosomes. The relative positions of centromeres in these species can also be used to establish homology between chromosomes.

Define analogy and differentiate between homology and analogy

An analogy is defined as one of the similarities between two or more things that are not identical. The commonality of the two things being compared is due to both being derived from a common ancestor. For example, the wings of flying insects and birds are analogous as they were derived from a shared ancestor who had wings but resembles each other because they were derived from different ancestors.

Homology is similar in form due to common ancestry and/or common descent. For example, the wings of insects and birds are homologous because they were derived from a shared ancestor with wings. The term analogy is occasionally used interchangeably with homology.

The clearest examples of homology observed by naturalists include:

• The forelimbs of mammals (hands).
• The wings of bats and birds (arms).
• The flippers of whales (hand/arm-like appendages).

In each case, the forelimbs serve as levers for movement. They utilize the same types of muscles, bones, nerves, and tendons. These are all homologous structures.

How does homology provide evidence for evolution?

Homologous structures are extremely valuable in evolutionary biology as they provide clear evidence of shared ancestry. The existence of homologous structures is strong evidence for evolution as a mechanism to explain the observed similarities. Homology is very different from artificial selection in so many ways but they are both topics in biology.

The existence of a forelimb shared by all land animals can be explained in several ways but is most parsimoniously explained by descent from a common ancestor equipped with a forelimb.

The mechanical structures of the eye (camera) and the compound eye are homologous because they were derived from a common ancestor. However, the similarity in form does not mean that they also have the same function or anatomical structure. The compound eye of insects has numerous lenses, called ommatidia, which operate independently from each other. In contrast, the eyes of vertebrates have only a single lens, but all light receptors gather information and transmit it to a single ganglion cell.

Different types of homology and their significance to the evolution

Homology can be classified into a number of different types that can be considered more or less significant to the evidence for evolution. They include:

Iterative Homology

This form of homology is where structures are found that are similar in form but can not be found anywhere in the animal kingdom. This indicates a transition from one form to another. For example, the pelvis of vertebrates (e.g., whales) is extremely different from the pelvic girdle of insects such as beetles and grasshoppers (e.g., newts). The similarities in form between these two types of animals are called iterative homology.

Ontogenetic Homology

This homology refers to structures that arise from similar embryonic structures in the developing embryo. For example, the adult forelimb of a frog and a human are very different in shape and function, but both develop from a single limb bud in their early development.

Di-polymorphic Homology

This form of homology refers to structures derived from a single common ancestor with two different modes of function. For example, vertebrate limbs can be adapted for movement (e.g., the forelimb) or grasping (e.g., the hindlimb). The various functions of these structures differentiate them from one another by using different skeletal parts and muscle attachments.

Supra-specific Homology

This homology refers to structures derived from a single ancestor but has a functional role outside the body. For example, the wings of birds and beetles are not used for flight or other purposes but are used instead for buoyancy in the water.

Examples of homology

Examples of homology include:

1. A fly’s eye is similar to a vertebrate’s. They both have an optical component and a nervous component. However, the role and anatomical structure of the optical component are very different in the two eyes.

2. The wings of bats and birds are homologous as they were derived from an ancestral flying animal, but they serve very different purposes as they evolved separately in two distinct groups.

3. The wings of insects and birds are homologous as they were derived from a shared ancestor and resemble each other in form, but their function is different.

4. The flippers of whales and dolphins are homologous because they were derived from a common ancestor that possessed flippers. Still, they serve very different functions as they evolved separately in two distinct groups.

Benefits /uses of homology

The following are benefits of homology:

1. Homology is a mechanism to explain the observation of shared ancestry and thus common descent.

2. Homology provides evidence of genetic and developmental mechanisms responsible for creating these similarities.

3. Homologous structures provide evidence for conserved gene regulatory networks that have been present very early in life.

4. Homologous structures can be used to infer phylogenetic relationships, especially when combined with data from genomics and molecular genetics.

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