Evolution

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XII, Chapter 6.

Why does your arm have the same bones as a whale's flipper?

A whale swims, a bat flies, a cheetah sprints and you type, yet all four forelimbs are built from the same plan: one upper bone, two lower bones, wrist bones, palm bones and finger bones. A designer making each limb for its job would not repeat the plan. Evolution would, if all four animals inherited it from a common ancestor and modified it. That is the central claim of this chapter: living things are related, and they change over time.

Where did it all begin?

The universe. The universe is about 13.8 billion years old. The Big Bang theory says it began with a single huge explosion, after which it expanded and cooled. Hydrogen and helium gathered under gravity into galaxies.

The Earth. The Earth formed about 4.5 billion years ago. Its early atmosphere had water vapour, methane (CH₄), carbon dioxide and ammonia (NH₃), and no free oxygen. UV rays split water into hydrogen and oxygen. Hydrogen, being light, escaped, and the oxygen combined with ammonia and methane to form water and CO₂. An ozone layer formed, and as the surface cooled, rains filled the oceans. Life appeared about 4 billion years ago, roughly 500 million years after Earth formed.

Ideas about how life arose.

Oparin of Russia and Haldane of England proposed that the first forms of life came from pre-existing non-living organic molecules such as RNA and proteins. These formed in a high temperature, volcanic, reducing atmosphere of CH₄ and NH₃, and chemical evolution preceded biological evolution.

Miller's experiment (1953). S. L. Miller built a closed flask with CH₄, H₂, NH₃ and water vapour at 800°C and passed an electric discharge through it. He found amino acids had formed. Similar experiments produced sugars, nitrogen bases, pigments and fats. Analysis of meteorite content showed similar compounds forming in space. The first life was non-cellular, made of giant molecules of RNA, protein and polysaccharides, about 3 billion years ago. The first cellular life, single cells in water, followed about 2000 million years ago.

Beyond the textbook. For the exam: life began about 4 billion years ago, and Miller used CH₄, H₂, NH₃ and water vapour. In reality: the oldest widely accepted fossil evidence of life is roughly 3.5 billion years old, and how the first cell formed remains open. Miller's experiment shows that the building blocks can form easily without life, not that life itself was made in a flask.

Four theories of how species change

Before the 19th century, the theory of special creation held that all living things were created as they are now, that diversity does not change, and that Earth is only about 4000 years old. Evidence led scientists to reject all three claims.

Charles Darwin sailed on HMS Beagle and studied the life of South America and the Galapagos. He concluded that living forms resemble past forms and that gradual change takes place by natural selection. Alfred Russel Wallace, working in the Malay Archipelago, reached the same idea independently.

How Darwin's idea works. Darwin read Thomas Malthus, who pointed out that populations grow geometrically while resources do not. So individuals compete. Those whose heritable variations let them use resources better leave more offspring. This is fitness in the Darwinian sense: reproductive success. Over many generations, the favourable variations accumulate, and new forms arise. Branching descent means that species share common ancestors that lived at different points in history.

Pause and predict: a giraffe stretches its neck daily to reach leaves. According to Darwin, why do giraffes today have long necks?

Check your answer: because giraffes with slightly longer necks, born that way, got more food and left more offspring. Stretching during one's life does not change the genes passed on, which is why Lamarck's explanation fails.

The evidence for evolution

Fossils. Fossils in different layers of sedimentary rock show that life forms changed over time and some existed only in particular periods. The age of a fossil is estimated by radioactive dating.

Embryology. Ernst Haeckel (spelt Heckel in some books) proposed that embryos pass through the adult stages of other animals, based on features like the gill slits in a human embryo. Karl Ernst von Baer disagreed, showing that embryos never pass through the adult stages of other animals.

Comparative anatomy.

Pause and predict: sweet potato is a modified root and potato is a modified stem, and both store food. Homologous or analogous?

Check your answer: analogous. The structures have different origins (root and stem) but the same function.

Biochemistry. Similarities in proteins and genes across very different organisms point to shared ancestry.

Artificial selection. If humans can turn wild plants and animals into crops and dog breeds within a few hundred years, nature, acting over millions of years, can do much more.

Natural selection seen in action.

The moth story shows natural selection acting on variation that was already there. No variant was wiped out: both forms stayed in the population, only their frequency changed. Evolution is not a goal directed process but a stochastic one, driven by chance mutation and chance events.

Beyond the textbook. For the exam: Haeckel's idea was disproved by von Baer. In reality: Haeckel's slogan, that development repeats evolutionary history, is rejected. But early vertebrate embryos do look alike in some features, and that resemblance is real evidence of common ancestry. Likewise, in Britain the pale moths made a comeback after clean air laws reduced soot, which supports the explanation given for melanism.

Adaptive radiation and convergent evolution

Adaptive radiation is the evolution of many species in one geographical area from a common ancestor, radiating into different habitats. Two classic cases:

When two radiations occur separately in different regions, they can produce look-alikes. Placental mammals and Australian marsupials developed similar forms: the placental wolf and the Tasmanian wolf, the mole and the marsupial mole, the flying squirrel and the flying phalanger. This is convergent evolution at the level of whole groups.

The Hardy Weinberg principle: a ruler for evolution

The Hardy Weinberg principle says allele frequencies in a population stay constant from generation to generation, a state called genetic equilibrium. For two alleles A and a, with frequencies p and q:

p + q = 1 and p² + 2pq + q² = 1

Here p² is the frequency of AA, 2pq of Aa, and q² of aa. The second equation is the binomial expansion of (p + q)².

Worked example. In a population, 9 percent of individuals show a recessive trait (aa). Then q² = 0.09, so q = 0.3 and p = 0.7. Carriers (Aa) make up 2pq = 2 × 0.7 × 0.3 = 0.42, or 42 percent. In 10,000 people, that is 4200 carriers. The usual error is to stop at q or to treat q² as the carrier frequency.

Pause and predict: if q² = 0.04, what fraction of the population are carriers?

Check your answer: q = 0.2, p = 0.8, so 2pq = 2 × 0.8 × 0.2 = 0.32. Carriers are far more common than affected people.

The equilibrium is the baseline. If frequencies change, evolution has taken place. Five factors can change them:

Three modes of natural selection act on a bell shaped distribution of a trait.

Beyond the textbook. For the exam: a population in Hardy Weinberg equilibrium is not evolving. In reality: the equation is a null model. It holds only if there is no mutation, migration or selection and if the population is large and mates at random. No real population satisfies all five. Its value is as a comparison: if observed frequencies differ from the expected ones, something is acting on the population.

A timeline of life on Earth

Life moves onto land. Plants were the first organisms to invade land. Fish with strong fins could move onto land and back to water. In 1938 a fish caught off South Africa turned out to be a coelacanth, a lobefin once thought extinct. Lobefins gave rise to the first amphibians, the ancestors of today's frogs and salamanders. Amphibians gave rise to reptiles, whose thick shelled eggs do not dry out in the sun like amphibian eggs. Reptiles of many types flourished: dinosaurs on land (Tyrannosaurus rex was about 20 feet tall) and some, like ichthyosaurs, in the sea. Then the dinosaurs disappeared. Crocodiles, lizards, snakes and turtles survive, and birds are living relatives of the dinosaurs.

The first mammals were small, shrew like and viviparous, and they spread after reptiles declined. Continental drift explains two patterns. When South America joined North America, the northern animals overran many of its mammals. Australia stayed isolated, so its pouched mammals survived.

Plant evolution in order: early land plants (Rhynia type and Psilophyton) → seed ferns → conifers and other gymnosperms → angiosperms.

Beyond the textbook. For the exam: seaweeds and some plants by about 320 million years ago. In reality: the fossil record places the first land plants earlier, at roughly 450 to 470 million years ago. Palaeontologists also recognise five major mass extinctions in the history of life, of which the one that ended the dinosaurs is the best known.

The evolution of humans

Humans did not descend from living chimpanzees or gorillas. We share common ancestors with them.

Brain size rises steadily: 650 to 800, then 900, then 1400 cubic centimetres.

Beyond the textbook. For the exam: Neanderthal man is a stage before Homo sapiens. In reality: Neanderthals were a separate branch, not our direct ancestors. DNA studies show that modern people outside Africa carry about 1 to 2 percent Neanderthal DNA, so the two groups met and interbred.

Exam traps

60-second revision

Where this chapter connects

• Previous: Molecular Basis of Inheritance shows how mutation changes DNA, which supplies the variation selection works on.
• Next: Human Health and Disease shows evolution in action, where microbes evolve resistance to drugs.
• Class XI link: Biological Classification and Animal Kingdom classify the lineages whose history is traced here.