Chapter 6: Evolution | Summary

The theory of special creation held that all species were created as they are, diversity was constant, and earth was about 4000 years old. These ideas were strongly challenged in the nineteenth century. Charles Darwin, based on his observations during the voyage of H.M.S. Beagle, concluded that existing life forms share similarities with each other and with past life forms, and that gradual evolution has occurred through natural selection. Alfred Wallace, working in the Malay Archipelago, arrived at similar conclusions independently.

Darwin proposed that populations have built-in variation. Individuals better suited to survive in their natural conditions outbreed less-fit individuals. Fitness, according to Darwin, refers ultimately to reproductive fitness. Natural selection favours those who leave more progeny, leading to gradual changes in population characteristics and the appearance of new forms. The geological history of earth closely correlates with its biological history, confirming that earth is billions, not thousands, of years old.

Multiple lines of evidence support evolution. Palaeontological evidence comes from fossils found in sedimentary rock layers, showing that life forms varied over time and certain forms were restricted to specific geological periods. Embryological evidence was proposed by Ernst Haeckel, noting that embryos of all vertebrates share common features such as vestigial gill slits, though Karl Ernst von Baer later showed embryos never pass through the adult stages of other animals.

Comparative anatomy provides evidence through homologous structures — such as the forelimbs of whales, bats, cheetahs, and humans — which have similar anatomical structure despite performing different functions, indicating divergent evolution from a common ancestor. Analogous structures like wings of butterflies and birds, or the eyes of octopus and mammals, show convergent evolution. Industrial melanism in peppered moths in England demonstrated natural selection in action. Biochemical similarities in proteins and genes across diverse organisms also point to common ancestry.

Adaptive radiation is the process by which different species evolve in a given geographical area starting from a single ancestral stock and radiating out to different habitats. Darwin's finches in the Galapagos Islands are a classic example — from an original seed-eating ancestor, multiple varieties with altered beaks evolved to become insectivorous and vegetarian finches. Australian marsupials represent another example, with many different species evolving from a single ancestral stock on the Australian continent.

When more than one adaptive radiation appears to have occurred in an isolated geographical area representing different habitats, it is called convergent evolution. Placental mammals in Australia evolved into forms that appear strikingly similar to corresponding marsupials (e.g., placental wolf and Tasmanian wolf), illustrating how similar environments can drive similar adaptive outcomes in unrelated lineages.

The essence of Darwinian theory is natural selection. The rate of appearance of new forms is linked to life cycle and lifespan — microbes can evolve new phenotypes within hours or days, while changes in fish or birds take millions of years. Fitness is based on inherited characteristics; adaptive ability has a genetic basis. Branching descent and natural selection are the two key concepts of Darwinian Theory.

Lamarck, a French naturalist who preceded Darwin, proposed that evolution was driven by use and disuse of organs, citing the giraffe's elongated neck as an example of an acquired characteristic passed on to offspring. This idea is no longer accepted. Darwin's observations were influenced by Thomas Malthus's work on populations, and were grounded in factual observations: limited natural resources, stable population sizes, inheritable variation among individuals, and competition for survival.

Hugo de Vries, based on his work on evening primrose, proposed that evolution occurs through mutations — large, sudden differences arising in a population — rather than through the minor, gradual variations Darwin described. He called this saltation (single-step large mutation). Mutations are random and directionless, while Darwinian variations are small and directional.

Population genetics clarified the mechanism further: variation arising from mutation, recombination during gametogenesis, gene flow, or genetic drift results in changed allele frequencies in future generations. Natural selection, coupled with enhanced reproductive success, can lead to stabilisation (more individuals acquire mean character value), directional change (individuals acquire values other than the mean), or disruption (more individuals acquire peripheral values at both ends of the distribution curve).

The Hardy-Weinberg principle states that allele frequencies in a population remain stable and constant from generation to generation under ideal conditions — this is called genetic equilibrium. The gene pool (total genes and alleles in a population) remains constant. If allele frequencies are p and q for alleles A and a, the genotype frequencies are p² (AA), 2pq (Aa), and q² (aa), with p² + 2pq + q² = 1, which is the binomial expansion of (p+q)².

Five factors disturb Hardy-Weinberg equilibrium: gene migration or gene flow, genetic drift, mutation, genetic recombination, and natural selection. When a section of a population migrates and the same change occurs by chance, it is called genetic drift. If the new population's allele frequencies diverge so greatly that it becomes a different species, the original drifted population becomes founders and the effect is called the founder effect.

Life on Earth has evolved through distinct eras. The first cellular forms appeared about 2000 mya. By 500 mya, invertebrates were active. Jawless fish evolved around 350 mya. Plants were the first organisms to invade land. Fish with stout fins (lobefins) moved onto land around 350 mya, and the Coelacanth, once thought extinct, was found in South Africa in 1938. Lobefins evolved into amphibians, then reptiles (which lay thick-shelled eggs). Reptiles diversified into dinosaurs and other forms, dominating for 200 million years. About 65 mya, dinosaurs disappeared suddenly.

The first mammals were small shrew-like creatures, viviparous and more intelligent than reptiles. Mammals took over earth after reptiles declined. Continental drift played a key role: when South America joined North America, South American mammals were displaced by North American fauna, while pouched mammals in Australia survived due to isolation and lack of competition. Some mammals secondarily adapted to aquatic life (whales, dolphins, seals, sea cows). The most successful evolutionary story is that of modern humans with language and self-consciousness.

About 15 mya, primates called Dryopithecus (more ape-like) and Ramapithecus (more man-like) existed. Fossils from Ethiopia and Tanzania indicate that about 3–4 mya, hominid primates walked upright in eastern Africa, not taller than 4 feet. Two mya, Australopithecines lived in East African grasslands, hunted with stone weapons, and ate fruit. Homo habilis, the first human-like hominid, had a brain capacity of 650–800cc. Homo erectus (about 1.5 mya) had a larger brain of ~900cc and ate meat.

The Neanderthal man (brain size 1400cc) lived in near-east and central Asia between 100,000–40,000 years ago, used hides for protection, and buried their dead. Homo sapiens arose in Africa, spread across continents, and developed distinct races. Modern Homo sapiens appeared during the ice age (75,000–10,000 years ago). Pre-historic cave art developed around 18,000 years ago; Bhimbetka rock shelter in Madhya Pradesh is an Indian example. Agriculture began around 10,000 years ago, leading to permanent human settlements.

Chapter 6 presents evolution as a unifying concept in biology, tracing life from chemical origins on early earth to the spectacular diversity seen today. The chapter establishes that chemical evolution preceded biological evolution, that natural selection is the primary mechanism driving change in populations, and that evidence from fossils, comparative anatomy, embryology, and biochemistry collectively supports the theory of evolution. The Hardy-Weinberg principle provides a mathematical framework for understanding genetic equilibrium and the forces that disrupt it, including gene flow, genetic drift, mutation, recombination, and natural selection.

For board examinations, students must be able to explain Miller's experiment, compare Lamarckism and Darwinism, describe homology versus analogy with examples, apply the Hardy-Weinberg equation, trace human evolution from Dryopithecus to Homo sapiens, and analyse examples of adaptive radiation. These topics appear consistently in 2-mark, 3-mark, and 5-mark questions, making a thorough understanding of this chapter essential for scoring well in CBSE Class 12 Biology.