Briefly explain - Evolution by stages.
The occurrence of different stages of evolution in a species is not because of a single DNA change. Evolution of eyes and feathers are classic examples to understand the process of evolution by stages.
Evolution of Eyes
- The eye is an example of a complex homologous organ. It has been created in stages over generations.
- The primitive organisms which existed on the Earth were slow moving and smaller in size. They did not require a specialised organ for observing any object.
- Consider the example of flatworms. Flatworms are small invertebrates.
- The eyes of flatworms are simple and exist in the form of eyespots. Eyespots are light-sensitive cells which can detect light.
- As evolution progressed, comparatively larger and mobile organisms evolved. Most of them were predators and required better vision for predation.
- Hence, from the basic design of eyes, more complex forms evolved.
- Insects, octopuses and invertebrates have eyes. However, the structure of the eyes differs in each of these organisms. This suggests that they have a separate evolutionary origin.
Evolution of Feathers
- Birds make use of their feathers for flying. However, feathers did not evolve for flight. They evolved as a means of providing insulation to the body in cold weather.
- Scientists have discovered fossils of dinosaurs which had feathers for insulation against cold weather. Because the dinosaurs were reptiles, it implies that birds evolved from reptiles.