Difference Between Homologous and Analogous Structures

Main Difference – Homologous vs Analogous Structures

Homologous and analogous structures are the two types of structures developed in different species during their evolution from past organisms. Homologous structures are developed during divergent evolution of species whereas analogous structures are developed during convergent evolution. The main difference between homologous structures and analogous structures is that homologous structures are developed from a common ancestor whereas analogous structures belong to unrelated species in spite of having a similar function.

Key Areas Covered

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

Key Terms: Adaptation, Analogous Structures, Common Ancestor, Convergent Evolution, Divergent Evolution, Homologous Structures, Homoplasy, Natural Selection

What are Homologous Structures

Homologous structures are part of the body of a species that are anatomically similar to the comparative part of another species. The homologous structures suggest that diverse species are derived from a common ancestor over time. Therefore, the anatomy of the homologous structures can be used as a fact to develop phylogenetic trees of life. The function of homologous structures can be different. Related species consist of more similar homologous structures in their internal structure.

Figure 1: Skeletons of mammal arms
120 – Salamanders, 121 – Sea turtle, 122 – Crocodile, 123 – Bird, 124 – Bat, 125 – Whale, 126 – Mole, 127 – Human

 The limb structures of mammals is an example of homologous structures. The human arm is very similar to the flipper of whales, the leg of cats, and the wing of bats in anatomical structure. The structure of the limb is composed of a large upper arm bone, two lower arm bones along with a collection of smaller bones in the wrist area. Though the structure of the limb is similar, their functions are different such as walking, flying, and swimming. Homologous structures are developed by divergent evolution as an adaptation to different environmental conditions.

What are Analogous Structures

The similar structures found in unrelated species are referred to as analogous structures. Different species have similar structures, i.e., analogous structures, due to same selection pressure of the environment. Since the natural selection acts on the same way on every species within the same environment, similar kinds of adaptations are more favorable. Accordingly, analogous structures are derived as a result of convergent evolution. During convergent evolution, body parts can emerge, disappear, or rearranged, depending on the function. The species that are affected by convergent evolution are referred to as homoplasy.

Figure 2: The structure of human (left) and octopus (right) eye

Since analogous structures do not share the same evolutionary path, they cannot be used to build phylogenetic relationships. The human eye is very similar to the octopus eye. However, humans and octopus are not closely related. The wings of the bat, birds, and insects are adapted to fly. The body shape and body color of sharks and dolphins are very similar. The analogous structures are developed to perform the same function in unrelated species. The structure of human and octopus eye is shown in figure 2. 

Similarities Between Homologous and Analogous Structures

• Homologous and analogous structures are developed by evolution.
• Both homologous and analogous structures are derived as adaptations to the environment.

Difference Between Homologous and Analogous Structures

Definition

Homologous Structures: Homologous structures are the structures developed from a common ancestor 

Analogous Structures: Analogous structures are structures of different species with similar functions.

Anatomy

Homologous Structures: Homologous structures are similar in anatomy.

Analogous Structures: Analogous structures are dissimilar in anatomy.

Function

Homologous Structures: Homologous structures are dissimilar in function.

Analogous Structures: Analogous structures are similar in function.

Developed in

Homologous Structures: Homologous structures are developed in related species.

Analogous Structures: Analogous structures are developed in unrelated species.

Common Ancestor

Homologous Structures: Homologous structures are developed from a common ancestor.

Analogous Structures: Analogous structures are not developed from a common ancestor.

Developmental Pattern

Homologous Structures: Homologous structures share a similar developmental pattern.

Analogous Structures: The developmental pattern of the analogous structures are dissimilar to each other.

Significance

Homologous Structures: Homologous structures are developed as an adaptation to different environments.

Analogous Structures: Analogous structures are developed as an adaptation to a similar environment.

Evolutionary Pattern

Homologous Structures: Homologous structures are developed by divergent evolution.

Analogous Structures: Analogous structures are developed by convergent evolution.

Inferring Evolutionary Relationships

Homologous Structures: Homologous structures can be used to infer evolutionary relationships among species.

Analogous Structures: Analogous structures cannot be used to infer evolutionary relationships among species.

Conclusion

Homologous and analogous structures are two types of structures found in species as an adaption to the environment. Homologous structures are derived as a result of divergent evolution. The organisms which have homologous structures are evolved from a common ancestor. Analogous structures are derived as a result of convergent evolution. These structures are developed in unrelated species due to the similar selection pressure of the environment. The main difference between homologous and analogous structures is the origin of each species.

Reference:

1. Scoville, Heather. “What Are Homologous Structures?” ThoughtCo. N.p., n.d. Web. Available here. 19 July 2017.
2. “Analogous Structures: Definition & Examples.” Study.com. N.p., n.d. Web. Available here. 19 July 2017. 

Image Courtesy:

1.”Arm skeleton comparative NF 0102.5-2” By Wilhelm Leche – (Public Domain) via Commons Wikimedia
2. “Evolution eye” (CC BY-SA 3.0) via Commons Wikimedia