What is the Difference Between Homologous Structures and Vestigial Structures

The main difference between homologous structures and vestigial structures is that homologous structures are the similar anatomical structures inherited from a common ancestor whereas vestigial structures are the anatomical structures which have reduced their size as they are no longer used. 

Homologous structures and vestigial structures are two types of anatomical structures described based on their evolutionary history. The limbs of mammals is an example of homologous structures while two vestigial structures include human tail bone, whale pelvis, etc.

Key Areas Covered

1. What are Homologous Structures
     – Definition, Features, Examples
2. What are Vestigial Structures
     – Definition, Features, Examples
3. What are the Similarities Between Homologous Structures and Vestigial Structures
     – Outline of Common Features
4. What is the Difference Between Homologous Structures and Vestigial Structures
     – Comparison of Key Differences

Key Terms

Appendix, Common Ancestor, Homologous Structures, Limb of Mammals, Vestigial Structures

What are Homologous Structures

Homologous structures are the structures that occur in related animals with similar anatomy and function. As these structures occur in related animals, they have evolved from a common ancestor. Therefore, homologous structures are the characters shared by related animals evolved from a common ancestor. For example, the limb of humans shows homology to the leg of the cat, the wing of the bat, the wing of the bird, and to the flipper of the whale. All these structures comprise a large upper arm bone, two bones in the lower arm; one is large and the other is small, a collection of bones in the wrist area, leading to the fingers or phalanges. However, the main function of these structures is to aid in locomotion. However, the form of locomotion may alter based on the environment.

Figure 1: The Limb Structures of Related Animals

In addition to anatomical structures, gene sequences and proteins also show homology in related animals. For instance, analogous structures are the opposite type of structures to homologous structures based on their origin. It means that though analogous structures have similar structure and function, they have a different origin; hence, they occur in evolutionary-unrelated animals. Analogous structures arise as similar adaptations to the environment.

What are Vestigial Structures

Vestigial structures are the anatomical structures that have reduced their size during the evolutionary pathway. The reason behind this is that these structures are no longer used by the animal. However, these structures have evolved from a common ancestor and they occur in related animals. However, due to the uselessness of this structure to a particular animal, vestigial structures have reduced in size. Nevertheless, these anatomical structures may function well in other types of animals evolved from the common ancestor.  

Figure 2: Appendix in the Large Intestine in Humans

Some vestigial structures in humans are the appendix, tail bone or coccyx, etc. For example, in herbivorous animals, appendix or cecum has a function in digesting cellulose. But, humans do not digest cellulose; therefore, there is no use of it. However, the appendix in humans has an immune function.

Similarities Between Homologous Structures and Vestigial Structures

• Homologous structures and vestigial structures are two types of anatomical structures that provide evidence of evolution in animals.
• Comparative anatomy is the field studying the similarities and differences between the structures of different species.
• Also, both types of structures show evolution as a result of adaptation to the environment.
• However, both types of structures have a common ancestry as they have evolved as homologous structures.

Difference Between Homologous Structures and Vestigial Structures

Definition

Homologous structures refer to organs or skeletal elements of animals that, by virtue of their similarity, suggest their connection to a common ancestor while vestigial structures refer to the structures in an animal that has lost all or most of its original function in the course of evolution. Thus, this is the main difference between homologous structures and vestigial structures.

Significance

Moreover, homologous structures are the similar anatomical structures found in the evolutionary-related animals while vestigial structures are the anatomical structures that have reduced their size as they are no longer been used. Hence, this is another difference between homologous structures and vestigial structures.

Function

Also, one other difference between homologous structures and vestigial structures is that the homologous structures perform a similar function while vestigial structures have no important function.

Examples

Some homologous structures are the limb of mammals, organs of the body, bones, etc. while some vestigial structures include human tail bone and appendix whale pelvis, etc.

Conclusion

Homologous structures are the similar anatomical structures of evolutionary-related animals. Most often, these structures perform a similar function. Significantly, homologous structures are derived from a common ancestor. In comparison, vestigial structures are the anatomical structures that have reduced their size as they are no longer used by the animal. This type of structures also has a common ancestry. But, they have no important function in the animal. Therefore, the main difference between homologous structures and vestigial structures is their functional relationships.

References:

1. Scoville, Heather. “Homologous Structures Explain Animals’ Places in Evolution.” ThoughtCo, ThoughtCo, 7 Sept. 2018, Available Here.
2. Scoville, Heather. “Vestigial Structures.” ThoughtCo, ThoughtCo, 26 Jan. 2019, Available Here.

Image Courtesy:

1. “Homology vertebrates-en” By Волков Владислав Петрович – Own work (CC BY-SA 4.0) via Commons Wikimedia  
2. “Blausen 0604 LargeIntestine2” By BruceBlaus. Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia