[Biology Class Notes] on Difference Between Homologous and Analogous Structures Pdf for EXAM

Structures with similar anatomy, morphology, embryology, and genetics, but different functions are called homologous structures. Structures with similar, but different anatomical structures with the same function are called Analogous structures. In this article, we will talk about the Homologous and Analogous structures, their facts, and differences. 

Homologous and Analogous Structures

Homologous and analogous structures are often difficult to understand. It is really important to understand the importance in comprehending the similarities and differences between the various organisms. Scientists make use of animals for biological processes and drug treatments without extensive and potentially dangerous experimentation on humans with their knowledge. 

The structures which have the same anatomy, morphology, embryology and genetics but are dissimilar in their functions are called homologous structures. Structures that are externally similar but are still dissimilar doing the same function are known as analogous structures. 

Homologous Structures

Homologous structures can be defined as the organs or skeletal elements of animals and organisms that, by virtue of their similarity, belong to a common ancestor. These structures do not necessarily have to look exactly the same, or have the same function.

The word homologous is derived from the Greek words ‘homos’, which means same, and ‘logos,’ which means relation. As a result, it literally means “sharing the same relation.” Homologous structures are structures found in related organisms that were passed down from a common ancestor. In the descendants, these structures may or may not have the same function. The homologous structure evolved as a result of adaptation to various environments.

Analogous Structures

Analogous structures are similar structures that evolved independently into two living organisms for the same purpose. The term analogous structures comes from the root word Analogy, which means where two different things are the basis of their similarities.

Analogous structures are structures that perform the same function but are found in creatures with different ancestral origins and represent different evolutionary lines. 

Furthermore, they do not share a place. Despite this, they can evolve body parts or organs that perform the same function but in different ways. The analogous structure evolved as a result of different species’ adaptation to similar environments.

The wings of a bird, a bat, or an insect, for example, have similar structures.

Difference Between Homologous vs Analogous Structures

Example of Homologous Structures 

Some of the examples of Homologous Structures are the four limbs of tetrapods. Mice, crocodiles, birds and other animals and birds with four limbs form perfect examples of Homologous structures. As the ancestors of these tetrapods evolved in the universe with four limbs, so as their descendants have inherited the same feature. 

However, not all characters can be called homologies. For example, birds and bats both have wings and if compared to mice and crocodiles, they do not have. Does that anyway mean that birds and bats are more closely related to one another than to mice and crocodiles? No. When we examine bird wings and bat wings closely, we see that there are much more differences.

Bat wings consist of the cover of skin that is stretched between the bones of the fingers and arm. Bird wings consist of feathers which are extended all along the arm. The dissimilarities of these structures suggest that wings of birds and wings of bats were not inherited from common ancestors. 

Example of Analogous Structures

Because of their separate evolutionary origins, birds and bats are analogous. Analogies are the result of convergent evolution. Surprisingly, though bird and bat wings are analogous as wings, as forelimbs they are homologous. Birds and bats are known to have not inherited their wings from a common ancestor who were with wings, but they did inherit forelimbs from a common ancestor with forelimbs.

Pictorial Depiction of Homologous and Analogous Structures

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Points to Remember

• Homologous structures exist in organisms that have a common ancestor, so they are monophyletic. 

• Analogous structures are found in organisms that do not have a common ancestor.

• Analogous structures always have similar or identical functions, while homologous structures are not always the case.

Conclusion 

To conclude, anatomical structures in animals or plants frequently different in their functions due to DNA mutations or epigenetic regulation that results in homologous structures in future offspring if the change is favorable for the survival of that organism. Alternatively, changes in gene expression of two completely unrelated species can eventually result in very similar anatomy and physiology in certain tissues. Such structures are analogous, or homoplastic, as they are derived from differing ancestors, but have very similar functions. The closeness of species can be determined to a particular ancestor by looking at the mutations of the DNA level that are typically compared between organisms, where those with fewer changes in the DNA sequences are considered to be more closely related. Importantly, these changes in DNA sequences were passed onto offspring if they were favorable enough to allow the organism to survive long enough to be able to reproduce, a concept known as natural selection.