Homologous and Analogous Structures

Homologous Structures

Definition

Homologous structures are the morphological features of organisms that evolved from a common ancestor. This means that closely related species share homologous traits of a similar structure but may or may not have the same or different functions.

Degree of Relativity

Organisms that share a common ancestor form this, which is called a monophyletic group, when they are classified based on phylogenetic relationships. This close relationship is often seen because development patterns during embryology are similar among organisms with homologous structures. For example, the embryo formed from all vertebrate animals resembles appearance.

Examples from the Animal Kingdom

There are various examples of animals that have homologous structures. The most common examples are the limbs of animals. A common pattern is the same between all representatives, but that has become converted and adapted for different functions. 

The same bones, namely the radius, humour, and ulna, can be seen in the arms of bats, birds, and humans. For example, although a bat and a bird have wings while a human does not, the bones are modified for flight in some vertebrate animals, while in others; they are used for simple movements like walking and running.

Examples from the Plant Kingdom

In some types of plants, the leaves are examples of homologous structures. In some of the plants, the leaves appear from the same basic pattern and ancestor, but while some altered to store water like cactus, others were altered to trap insects, for example, Venus flytrap and pitcher plant.

Analogous structures

Definition

The analogous structures evolved independently to have similar or the same functions. Hence, they may appear on the surface to have a similar structure, but this varies from one organism to another.

Degree of relativity

The organisms with analogous structures are not commonly closely related and do not thus share a common ancestor. So, a phylogenetic tree is not monophyletic. The analogous structure evolves separately in each organism to serve the same or similar purpose present in some different animals as well. However, the structures are not homologous, so they more likely adapt organisms for the same role, like flight. 

The analogous structures could be a result of convergent evolution. The example of the streamlined body shape of fish and squid provides ease of swimming. These convergent evolutions can commonly be seen within the organisms in completely different parts of the world, which show the same morphological adaptations according to the environmental conditions.

Examples from the Animal Kingdom

The analogous structures that are very noticeable in the animal kingdom include the wings of a few insects and birds’ wings. The bird wings are made up of hollow bones that evolved from primitive chordate animals. The insect wings evolved separately from the gill structures of the primitive aquatic insects.

Examples from the Plant Kingdom

The African euphorbias have succulent leaves for water storage, similar to those of cacti found in the New World. However, the Altered storage leaves of these plants represent the convergent evolution, having arisen separately from their different ancestral plants. Hence, however, the leaves in euphorbias and cacti have the same function, although they have different ancestral origins.

Difference between Homologous structures and Analogous structures

The differences between Homologous structures and Analogous structures are discussed one by one briefly below:

1. The structures which evolve in living organisms and share a common ancestor are known as homologous structures. The analogous structures are those that have evolved separately in different living organisms, yet they may or may not function similarly or identically.
2. The developmental pattern Within creatures with homologous traits is extremely similar, as can be observed when looking at the embryos of various organisms. In creatures with Analogous traits, the developmental pattern is often very uncommon.
3. Most of the organisms with homologous structures are generally very closely related and have a common ancestor. The organisms with analogous structures are not at all closely related and do not have a common ancestor.
4. The divergent evolution is responsible for the homologous structure, while the convergent evolution is responsible for the comparable structure. 
5. The homologous structures have very similar anatomy, but functions differ, whereas the analogous structures have Different anatomy but similar functions. 
6. The homologous structures result from adaptation to various environments, whereas the analogous structure has appeared as a result of different specie’s adaptation according to similar environments. 
7. The limbs of vertebrate animals are homologous structures, and the very same bones are present in each animal, though they are Altered. Insects and bird’s wings are good examples of similar structures with slightly different evolutionary patterns and origins.

Conclusion

So, the structures which evolve in living organisms and share a common ancestor are known as homologous structures. The analogous structures are those that have evolved separately in different living organisms, yet they may or may not function similarly or identically. We discussed the difference between homologous and analogous Structures very deeply with animals and plants examples.