Evolution

Evolution is the process by which different kinds of living organisms develop and diversify from earlier forms over generations. It explains the origin of new species, adaptation, and biodiversity. This chapter explores various theories, evidence, mechanisms, and patterns of evolution.

Origin of Life

1. Early Theories of Origin of Life

• Theory of Special Creation: Life was created by a divine force.

• Theory of Spontaneous Generation: Life originated from non-living matter (disproven).

• Biogenesis: Life arises from pre-existing life, supported by Louis Pasteur’s experiment.

2. Oparin-Haldane Theory (Chemical Origin of Life)

• Life originated from inorganic molecules through chemical evolution.

• Miller and Urey’s Experiment (1953): Simulated early Earth conditions and synthesized amino acids, proving organic molecules could form naturally.

Evidence of Evolution

1. Paleontological Evidence (Fossils)

• Fossils provide a record of ancient life and transitional forms (e.g., Archaeopteryx – link between reptiles and birds).

2. Comparative Anatomy and Morphology

• Homologous Organs: Similar structure, different functions (e.g., forelimbs of vertebrates).

• Analogous Organs: Different structure, similar functions (e.g., wings of birds and insects).

• Vestigial Organs: Functionless remnants of ancestral structures (e.g., human appendix, pelvic bones in whales).

3. Embryological Evidence

• Early embryos of vertebrates resemble each other, supporting common ancestry (proposed by Ernst Haeckel).

4. Molecular Evidence (Biochemical Similarities)

• Similar DNA, RNA, and proteins in different species indicate common ancestry.

• Example: Humans and chimpanzees share 98.5% of DNA.

Theories of Evolution

1. Lamarck’s Theory of Evolution (Use and Disuse)

• Jean-Baptiste Lamarck proposed that organisms evolve by acquiring traits during their lifetime and passing them to offspring.

• Example: Giraffes developed long necks by stretching to reach leaves.

• Disproved as acquired traits do not alter DNA.

2. Darwin’s Theory of Natural Selection

• Proposed by Charles Darwin in On the Origin of Species (1859).

• Key Concepts:

  • Overproduction: Organisms produce more offspring than survive.

  • Variation: Differences exist among individuals.

  • Struggle for Existence: Competition for resources.

  • Survival of the Fittest: Organisms with favorable traits survive.

  • Inheritance of Favorable Traits: Traits get passed to the next generation.

3. Neo-Darwinism (Modern Synthesis Theory)

• Combines Darwin’s natural selection with genetics, mutation, and recombination.

Mechanisms of Evolution

1. Mutation (Hugo de Vries)

• Sudden changes in DNA create variations, leading to evolution.

• Example: Industrial melanism (black moths in polluted areas survived better than white moths).

2. Genetic Drift

• Random changes in allele frequency, especially in small populations.

• Example: Founder Effect – new population forms with reduced genetic diversity.

3. Gene Flow (Migration)

• Movement of genes between populations due to migration.

4. Natural Selection

• Directional Selection: Favors one extreme trait (e.g., giraffes with longer necks).

• Stabilizing Selection: Favors intermediate traits (e.g., birth weight in humans).

• Disruptive Selection: Favors both extreme traits (e.g., large and small beak sizes in birds).

Speciation (Formation of New Species)

1. Types of Speciation

• Allopatric Speciation: Due to geographical barriers (e.g., Darwin’s finches).

• Sympatric Speciation: Without physical separation (e.g., polyploidy in plants).

2. Adaptive Radiation (Divergent Evolution)

• Single species evolves into multiple new species in different environments.

• Example: Darwin’s finches evolved different beak shapes for feeding.

3. Convergent Evolution

• Unrelated species evolve similar adaptations due to similar environments.

• Example: Wings in birds and bats.

Human Evolution

1. Evolutionary History of Humans

• Dryopithecus and Ramapithecus: Early ape-like ancestors.

• Australopithecus: First human-like primates, walked upright.

• Homo habilis: First tool maker.

• Homo erectus: Used fire, had a larger brain.

• Homo sapiens (Modern Humans): Developed language, agriculture, and civilization.

2. Evidence of Human Evolution

• Fossils like Lucy (Australopithecus afarensis).

• DNA similarities with primates.

Importance of Evolution in Biology

• Explains biodiversity and species adaptation.

• Helps in understanding diseases and antibiotic resistance.

• Basis for genetics, ecology, and conservation biology.