CBSE Class 12 Biology – Evolution

NCERT-aligned chapter resource | Biology Code 044

1. Origin and Early Evolution of Life

Evolution describes cumulative changes in populations across generations. The chapter connects early ideas about the origin of life with evidence for biological evolution.

2. Evidence for Evolution

Comparative anatomy, fossils, embryology and molecular evidence support evolutionary relationships. Homologous organs indicate common ancestry despite differences in function, whereas analogous organs perform similar functions despite different evolutionary origins.

3. Darwin’s Natural Selection

Populations produce more offspring than can survive. Individuals vary, and some heritable variations improve survival or reproduction. Over generations, favourable variations become more common through natural selection.

4. Hardy–Weinberg Principle

For a population under ideal conditions, allele frequencies remain constant. The equation p² + 2pq + q² = 1 describes genotype frequencies when p+q=1.

5. Factors Affecting Equilibrium

Gene migration, genetic drift, mutation, recombination and natural selection can change allele frequencies. Genetic drift has a particularly strong effect in small populations.

6. Adaptive Radiation

Adaptive radiation occurs when related organisms diversify from a common ancestor into different ecological niches. Darwin’s finches are a standard example.

7. Human Evolution

Human evolution is interpreted through fossil and comparative evidence. Evolutionary history should be understood as branching change rather than a simple ladder from “primitive” to “advanced”.

Worked Population Problem

If q=0.2 in a Hardy–Weinberg population, p=0.8. The expected heterozygote frequency is 2pq = 2(0.8)(0.2)=0.32, or 32%.

Chapter-Specific MCQs

1. Homologous organs primarily indicate: (A) common ancestry (B) identical function only (C) no relationship (D) mutation only. Answer: A.
2. Which process can change allele frequency by chance? (A) Genetic drift (B) Translation (C) Replication (D) Transcription. Answer: A.
3. If p=0.7, q equals: (A) 0.7 (B) 0.3 (C) 1.7 (D) 0.49. Answer: B.

Competency Questions

1. Explain why analogous organs do not provide evidence of close common ancestry.
2. Calculate genotype frequencies when allele frequencies are given.
3. Explain how a small isolated population can undergo rapid genetic change through drift.