What is Evolution? — Definition, Types & NEET Biology

Evolution — Explained for NEET

Biological evolution is the central unifying concept of biology. Charles Darwin proposed the theory of evolution by natural selection in his book "On the Origin of Species" (1859). According to Darwin, organisms with favorable variations are more likely to survive and reproduce — a process called natural selection or "survival of the fittest."

The origin of life is explained by the chemical evolution theory. Oparin and Haldane proposed that life arose from simple inorganic molecules through chemical reactions in the primitive Earth's atmosphere (which lacked free oxygen). Stanley Miller and Harold Urey experimentally demonstrated this in 1953 by creating amino acids from a mixture of CH₄, NH₃, H₂, and water vapor using electric discharge.

Evidence for evolution comes from multiple sources: homologous organs (similar structure, different function — e.g., forelimbs of whale, bat, horse), analogous organs (different structure, similar function — e.g., wings of butterfly and bird), fossils, embryology, and molecular biology (DNA and protein sequence comparisons).

The Hardy-Weinberg principle states that allele frequencies in a population remain constant from generation to generation in the absence of evolutionary forces. The five factors that disturb Hardy-Weinberg equilibrium are: gene flow, genetic drift, mutation, natural selection, and non-random mating.

Speciation — the formation of new species — occurs through geographic isolation (allopatric speciation) or reproductive isolation (sympatric speciation). Adaptive radiation, as seen in Darwin's finches on the Galapagos Islands, is a process where organisms diversify rapidly from an ancestral species into many new forms.

Key Points from NCERT

• Darwin's theory of natural selection: organisms with favorable variations survive and reproduce more successfully
• Oparin-Haldane theory: life originated from inorganic molecules; Miller-Urey experiment supported chemical evolution
• Homologous organs indicate common ancestry (divergent evolution); analogous organs indicate convergent evolution
• Hardy-Weinberg equilibrium is disturbed by gene flow, genetic drift, mutation, natural selection, and non-random mating
• Adaptive radiation (e.g., Darwin's finches) shows rapid diversification from a common ancestor into different ecological niches