CBSE · Class 10 · Science · Heredity and EvolutionWhat is meant by the term 'evolution'? Explain the various evidences of evolution with suitable examples to show how living organisms have evolved over geological geological time.

Introduction to Evolution\nEvolution is the sequence of gradual changes that occur in living organisms over millions of years, leading to the formation of new species from pre-existing ones. It represents the unfolding of life forms from simple to complex structures. Evolution is a cornerstone of modern biology and explains biodiversity.

1. Homologous Organs (Morphological and Anatomical Evidence)

• Definition: Homologous organs are those organs that have the same basic structural design and developmental origin but perform different functions in different species.
• Example: The forelimbs of humans, lizards, birds, and whales have a similar skeletal structure consisting of the humerus, radius, and ulna, yet they are used for grasping, walking, flying, and swimming respectively.
• Significance: This indicates common ancestry and divergent evolution.

2. Analogous Organs

• Definition: Analogous organs are those organs that perform similar functions and have similar outward appearance, but have completely different basic structural designs and embryonic origins.
• Example: The wings of a bird and the wings of an insect. Both are used for flying, but insect wings are folds of membrane supported by chitinous veins, whereas bird wings are modified forelimbs supported by bones.
• Significance: This indicates convergent evolution, where different species adapt similarly to common environmental challenges.

3. Fossil Evidence (Paleontological Evidence)

• Definition: Fossils are the preserved remains or impressions of dead plants and animals found in sedimentary rocks.
• Example: The fossil of Archaeopteryx possesses characteristics of both reptiles (teeth, long tail) and birds (feathered wings), providing a clear missing link that proves birds have evolved from reptiles.
• Significance: Fossils help scientists determine the absolute timeline of when various species existed and trace direct lineage.

4. Embryological Evidence

• Definition: Comparison of early developmental stages (embryos) of different vertebrate groups.
• Example: Embryos of fish, amphibians, reptiles, birds, and mammals show striking similarities in their early stages, such as the presence of gill slits, suggesting a common ancestral origin.