Evolution: NEET notes

1. Early earth and early ideas

NCERT §6.1

• Evolutionary biology studies the history of life forms on earth. To follow it we need the backdrop: how the universe, the stars and the earth themselves came about.
• Starlight left its source millions of years ago, so looking at stars is looking into the past. Distances between stars are measured in light years.
• The universe is almost 13.8 billion years old. The Big Bang theory explains its origin as a single, enormous explosion; the universe then expanded and cooled, hydrogen and helium formed, and gases condensed under gravity into galaxies.
• The earth, in the solar system of the Milky Way galaxy, is thought to have formed about 4.5 billion years ago. Early earth had no atmosphere.
• Water vapour, methane, carbon dioxide and ammonia came out of the molten mass and covered the surface. UV rays split water into hydrogen and oxygen; the light H₂ escaped, and oxygen combined with ammonia and methane to give water, CO₂ and other compounds. The ozone layer formed.
• As the earth cooled, water vapour fell as rain and filled the hollows to make oceans. Life appeared about 500 million years after the earth formed, that is, almost 4 billion years ago.
• Panspermia: some scientists hold that life came from outer space. Early Greek thinkers imagined units of life called spores moving between planets, and some astronomers still like the idea.
• Spontaneous generation held that life arises from decaying matter such as straw and mud. Louis Pasteur showed that life comes only from pre-existing life: in pre-sterilised flasks killed yeast gave rise to nothing, while a flask left open to air gave new organisms. The theory was dismissed for good.

2. Chemical evolution and the first life

NCERT §6.1

• Pasteur's result did not explain where the very first life came from. Oparin (Russia) and Haldane (England) proposed that it came from pre-existing non-living organic molecules such as RNA and protein.
• Their key idea: life was preceded by chemical evolution, the making of diverse organic molecules from inorganic ingredients.
• Conditions on early earth: high temperature, volcanic storms and a reducing atmosphere with CH₄, NH₃ and similar gases.
• In 1953 S.L. Miller, an American scientist, recreated these conditions on a laboratory scale: electric discharge in a closed flask holding CH₄, H₂, NH₃ and water vapour at 800 °C. Amino acids formed.
• Similar experiments by others gave sugars, nitrogen bases, pigments and fats. Meteorites carry similar compounds, suggesting the same chemistry happens elsewhere in space. On this limited evidence, chemical evolution is more or less accepted.
• How the first self-replicating metabolic capsule arose is unknown. The first non-cellular life forms may have appeared about 3 billion years ago as giant molecules (RNA, protein, polysaccharides) that perhaps reproduced themselves.
• Cellular life probably came much later, around 2000 million years ago, most likely as single cells; every life form then lived in water.
• This account, in which the first life arose slowly from non-living molecules through evolutionary forces, is accepted by most scientists.

3. Special creation and Darwin's theory

NCERT §6.2

• Special creation, from conventional religious literature, has three parts: every species was created as it is today; diversity has always been the same and will stay so; and the earth is about 4000 years old.
• All three ideas were strongly challenged in the nineteenth century.
• Charles Darwin sailed round the world on the sail ship H.M.S. Beagle. From what he saw he concluded that living forms resemble one another, and also forms that lived millions of years ago, to different degrees.
• Many past forms no longer exist: extinctions happened, and new forms arose at different times. Life forms have evolved gradually.
• Every population carries variation. Individuals whose traits suit the natural conditions (climate, food, physical factors) survive and outbreed the less-endowed ones.
• Darwin's fitness means reproductive fitness, in the end and only that: the fitter leave more offspring, so nature selects them. He called this natural selection and saw it as a mechanism of evolution.
• Alfred Wallace, a naturalist working in the Malay Archipelago, reached similar conclusions at about the same time.
• All living forms share similarities and common ancestors, which lived in different epochs, periods and eras. Earth's geological history closely matches its biological history, so the earth is billions of years old, not thousands.

4. Fossils and embryos as evidence

NCERT §6.3

• Fossils are remains of the hard parts of life forms, found in rocks. A cross-section of the crust shows sediments laid one over another through earth's history.
• Rock layers of different ages hold fossils of different life forms, those that probably died while that layer was forming. Some resemble modern organisms; others are extinct, such as dinosaurs.
• Fossils in successive layers show the geological period each form lived in: life forms changed over time, and some are confined to certain time spans. So new forms arose at different times. This is paleontological evidence.
• The age of a fossil is worked out by radioactive dating.
• The dinosaur family tree links extinct dinosaurs to their living counterparts, crocodiles and birds.
• Embryological support was proposed by Ernst Haeckel: some features appear in the embryos of all vertebrates but not in adults. For example, every vertebrate embryo, human included, develops a row of vestigial gill slits just behind the head, but they work as gills only in adult fish.
• Karl Ernst von Baer studied embryos closely and rejected the idea: an embryo never goes through the grown-up stage of some other animal.

5. Homology and analogy

NCERT §6.3

• Comparative anatomy and morphology show likenesses and differences among present and past organisms, and tell whether they share ancestors.
• The forelimbs of whales, bats, cheetah and human (all mammals) do different jobs but share one bone plan: humerus, radius, ulna, carpals, metacarpals and phalanges.
• One structure developing in different directions for different needs is divergent evolution, and such structures are homologous. Homology points to common ancestry.
• More homologous examples: vertebrate hearts and vertebrate brains; in plants, the thorn of Bougainvillea and the tendril of Cucurbita.
• Analogy is the opposite case: structures that are not anatomically similar but do the same job, such as the wings of a butterfly and of a bird.
• Analogous structures come from convergent evolution, different structures evolving towards one function. A similar habitat selects similar adaptive features in unrelated groups.
• More analogous examples: the eye of an octopus and of a mammal; the flippers of penguins and dolphins; sweet potato (a root modification) and potato (a stem modification).
• Biochemical likeness, similar proteins and genes doing the same function in diverse organisms, points to shared ancestry in the same way as structural likeness.
• Artificial selection: humans bred plants and animals for agriculture, horticulture, sport and security, making breeds (such as dogs) that differ yet stay one group. If people did this in hundreds of years, nature could have done it over millions.

6. Natural selection seen in action

NCERT §6.3

• In England, moths collected in the 1850s, before industrialisation, were mostly white-winged; dark-winged (melanised) moths were fewer.
• A collection from the same area in 1920, after industrialisation, had more dark-winged moths: the proportion had reversed.
• Explanation: predators spot a moth that contrasts with its background. Before industrialisation, trunks were covered by thick, almost white lichen, so white moths survived and dark ones were picked off.
• After industrialisation, smoke and soot darkened the trunks and lichens could not grow; now the white moths were eaten and the melanised ones survived. Moths that could camouflage themselves survived.
• Lichens do not grow where the air is polluted, so they can be used as indicators of industrial pollution.
• Support: in rural areas without industry, melanic moths stayed few. In a mixed population, the better-adapted survive and increase, but no variant is completely wiped out.
• Heavy use of herbicides and pesticides has selected resistant varieties within a short time. The same holds for microbes treated with antibiotics and for eukaryotic cells treated with drugs: resistance appears in months or years, not centuries.
• These are examples of evolution by anthropogenic action. They also show evolution is not directed in a deterministic way: it is a stochastic process resting on chance events in nature and chance mutations in organisms.

7. Adaptive radiation

NCERT §6.4

• On his voyage Darwin visited the Galapagos Islands and saw an amazing diversity of creatures, above all small black birds later called Darwin's finches.
• Many varieties of finch lived on the same island. Darwin conjectured that all evolved on the island itself.
• From the original seed-eating form, others with altered beaks arose, letting them become insectivorous and vegetarian finches.
• Adaptive radiation: the evolution of different species in one geographical area, starting from one point and radiating out into other habitats. Darwin's finches are the textbook example.
• Australian marsupials are another: many different marsupials evolved from one ancestral stock, all within the Australian island continent.
• When more than one adaptive radiation seems to have taken place in an isolated area (in different habitats), it can be called convergent evolution.
• Placental mammals in Australia also radiated into forms that each look similar to a matching marsupial, for example the placental wolf and the Tasmanian wolf (a marsupial).

8. Biological evolution and Darwin's reasoning

NCERT §6.5

• Evolution by natural selection, in the true sense, would have begun when cellular life forms with different metabolic abilities appeared.
• The essence of Darwin's theory is natural selection. How fast new forms appear depends on the life cycle or life span.
• Bacteria divide fast and become millions within hours. A colony A with built-in variation in using a feed component meets a changed medium; only the part B that can survive grows, outgrows the rest and appears as a new species within days. In fish or fowl the same would take millions of years.
• Under the new conditions B is fitter than A, and nature picks the fitter. Because this fitness depends on characters that are inherited, selection and evolution need a genetic basis.
• The ability to adapt is itself inherited. Fitness is what results when an organism can adapt and so gets selected.
• Darwin's theory rests on two key ideas: branching descent and natural selection.
• Before Darwin, the French naturalist Lamarck said evolution was driven by use and disuse of organs: giraffes stretching for high leaves lengthened their necks and passed on this acquired character. Nobody accepts this now.
• Darwin may have been influenced by Thomas Malthus's work on populations. Natural selection rests on facts: resources are limited, population sizes stay stable apart from seasonal changes, members vary (no two are alike), and most variations are inherited.
• Populations could grow exponentially if all reproduced to the maximum, yet real sizes are limited, so there is competition for resources. Darwin's insight: heritable variations that make resource use better let their bearers reproduce more, so over generations the population's characters change and new forms appear.

9. Mutation and the Hardy-Weinberg principle

NCERT §6.6, §6.7

• Where does variation come from, and how does speciation occur? Mendel had described inherited 'factors', but Darwin either ignored this or said nothing.
• In the first decade of the twentieth century, Hugo de Vries, working on evening primrose, brought out the idea of mutation: a large difference arising suddenly in a population.
• De Vries held that mutation, not the small heritable variations Darwin described, causes evolution. Mutations are random and directionless; Darwin's variations are small and directional.
• For Darwin evolution was gradual; de Vries thought mutation caused speciation in a single large step, which he called saltation. Later, population genetics clarified the picture.
• Hardy-Weinberg principle: allele frequencies in a population are stable and constant from generation to generation. The gene pool (all the genes and their alleles in a population) stays constant; this is genetic equilibrium.
• The allele frequencies add up to 1. In a diploid, p is the frequency of allele A and q that of allele a, so p + q = 1.
• The chance that A appears on both chromosomes is p × p, so AA individuals have frequency p²; aa has q² and Aa has 2pq.
• So p² + 2pq + q² = 1, the binomial expansion of (p + q)².
• If the measured frequencies differ from these expected values, the difference and its direction show the extent of evolutionary change. A change in allele frequencies, that is a disturbance of Hardy-Weinberg equilibrium, is read as evolution.

10. What disturbs the equilibrium

NCERT §6.7

• Hardy-Weinberg equilibrium is upset by five factors: natural selection, mutation, genetic recombination, gene migration (gene flow) and genetic drift.
• Gene migration: when part of a population moves to another place and population, gene frequencies change in both. The new population gains alleles that the old one loses. Migration repeated many times is gene flow.
• Genetic drift: the same change in frequencies happening by chance.
• Sometimes the drifted sample differs so much that it becomes a different species. The drifted population becomes the founders, and this is the founder effect.
• Experiments on microbes show that when an advantageous mutation already present is selected, new phenotypes appear; within a few generations this can lead to speciation.
• Variation from mutation, from recombination during gametogenesis, from gene flow or from genetic drift changes the frequencies of genes and alleles in later generations. Coupled with greater reproductive success, natural selection makes the population look different.
• Stabilising selection: more individuals come to have the mean character value; the curve narrows around the mean.
• Directional selection: more individuals come to have a value other than the mean; the peak shifts to one side.
• Disruptive selection: more individuals come to have the extreme values at both ends of the curve; two peaks form.

11. A brief account of evolution

NCERT §6.8

• The first cellular life appeared about 2000 million years ago (mya). How non-cellular aggregates of giant molecules became cells with membranes is not known.
• Some early cells could release O₂, probably by a reaction like the light reaction of photosynthesis, where water is split using captured solar energy. Single-celled forms slowly became multicellular.
• Invertebrates had formed and were active by about 500 mya. Jawless fish probably arose around 350 mya, and seaweeds with a few plants were probably present around 320 mya.
• Plants were the first organisms to invade land, and were widespread there when animals followed.
• About 350 mya, fish with sturdy fins could walk onto land and return to water. These lobefins gave rise to the first amphibians. In 1938 a Coelacanth, a lobefin believed extinct, was caught in South Africa.
• No specimens of the first amphibians survive, but they were ancestors of today's frogs and salamanders. Amphibians gave rise to reptiles, whose thick-shelled eggs do not dry up in the sun; turtles, tortoises and crocodiles are their modern descendants.
• For the next 200 million years or so reptiles of many shapes and sizes dominated. Giant ferns (pteridophytes) fell and slowly formed coal deposits. Some land reptiles returned to water as fish-like reptiles probably around 200 mya (for example Ichthyosaurs).
• The land reptiles were the dinosaurs; the biggest, Tyrannosaurus rex, stood about 20 feet tall with huge dagger-like teeth. About 65 mya they suddenly disappeared; the true reason is unknown (climate change, or evolution into birds, or something in between). Small reptiles of that era still live.
• The first mammals were shrew-like, with small fossils. Mammals were viviparous, protecting the unborn young inside the mother, and better at sensing and avoiding danger; when reptiles declined, mammals took over.
• Continental drift: when South America joined North America, its horse-, hippopotamus-, bear- and rabbit-like mammals were overridden by North American fauna, while Australia's pouched mammals survived for lack of competition. Some mammals, such as whales, dolphins, seals and sea cows, live wholly in water.

12. Origin and evolution of man

NCERT §6.9

• Around 15 mya lived two primates, Dryopithecus and Ramapithecus: hairy, and walking the way gorillas and chimpanzees do. Ramapithecus was closer to man, Dryopithecus closer to apes.
• A few fossils of man-like bones from Ethiopia and Tanzania show hominid features, suggesting that about 3-4 mya man-like primates walked upright in eastern Africa. They were probably no taller than 4 feet.
• About 2 mya Australopithecines probably lived in the grasslands of East Africa. They hunted with stone weapons but mainly ate fruit.
• Homo habilis, the first human-like being (hominid), had a brain capacity of 650-800 cc and probably did not eat meat.
• Homo erectus, known from fossils found in Java in 1891, lived about 1.5 mya, had a large brain of about 900 cc, and probably ate meat.
• Neanderthal man, brain about 1400 cc, lived in the near east and central Asia between 1,00,000 and 40,000 years ago. They protected their bodies with hides and buried their dead.
• Homo sapiens arose in Africa and spread across continents, developing into distinct races. Modern Homo sapiens arose during the ice age, between 75,000 and 10,000 years ago.
• Prehistoric cave art developed about 18,000 years ago; such paintings can be seen at the Bhimbetka rock shelter in Raisen district, Madhya Pradesh.
• Agriculture began about 10,000 years ago, and human settlements started. A skull comparison shows the baby chimpanzee's skull is more like an adult human's than an adult chimpanzee's.

Must-know facts

1. The universe is almost 13.8 billion years old; the earth formed about 4.5 billion years ago; life appeared about 4 billion years ago.
2. Louis Pasteur disproved spontaneous generation with killed yeast in pre-sterilised flasks: life comes only from pre-existing life.
3. Oparin and Haldane: life came from non-living organic molecules, preceded by chemical evolution.
4. Miller (1953): electric discharge in CH₄, H₂, NH₃ and water vapour in a closed flask gave amino acids.
5. First non-cellular life about 3 billion years ago; first cellular life about 2000 mya.
6. Darwin sailed on H.M.S. Beagle; Alfred Wallace reached similar conclusions in the Malay Archipelago.
7. Darwinian fitness means reproductive fitness only.
8. Haeckel proposed the embryological evidence; Karl Ernst von Baer disproved it.
9. Homologous (divergent evolution): forelimbs of whale, bat, cheetah and human; vertebrate hearts and brains; thorn of Bougainvillea and tendril of Cucurbita.
10. Analogous (convergent evolution): wings of butterfly and bird; eye of octopus and mammal; flippers of penguin and dolphin; sweet potato and potato.
11. Industrial melanism in England: 1850s mostly white-winged moths; 1920 mostly dark-winged. Lichens indicate pollution.
12. Herbicide, pesticide, antibiotic and drug resistance are evolution by anthropogenic action.
13. Adaptive radiation: Darwin's finches of the Galapagos; Australian marsupials. Placental wolf and Tasmanian wolf show convergent evolution.
14. Darwinism rests on branching descent and natural selection; Lamarck proposed use and disuse (giraffe neck).
15. Hugo de Vries (evening primrose): mutation, saltation; mutations are random and directionless.
16. Hardy-Weinberg: p + q = 1 and p² + 2pq + q² = 1; AA = p², Aa = 2pq, aa = q².
17. Five factors disturb H-W equilibrium: gene migration/gene flow, genetic drift, mutation, genetic recombination, natural selection.
18. Selection types: stabilising (mean favoured), directional (one side), disruptive (both ends).
19. Coelacanth, a lobefin thought extinct, was caught in South Africa in 1938; dinosaurs vanished about 65 mya.
20. Brain sizes: Homo habilis 650-800 cc, Homo erectus about 900 cc, Neanderthal man 1400 cc.

Common traps

Key terms

Panspermia

The idea that units of life (spores) reached earth from outer space.

Spontaneous generation

The disproved idea that life arises from decaying non-living matter.

Chemical evolution

Formation of diverse organic molecules from inorganic ones, before life appeared.

Special creation

The belief that species were created as they are, diversity never changes and earth is about 4000 years old.

Natural selection

Nature favouring individuals whose heritable traits let them leave more offspring.

Fitness

In Darwin's sense, reproductive fitness: the ability to leave more progeny.

Fossil

Remains of the hard parts of an organism preserved in rock.

Paleontological evidence

Evidence for evolution from fossils in rock layers of different ages.

Homologous organs

Organs with the same basic structure and origin but different functions.

Analogous organs

Organs with different structure but similar function.

Divergent evolution

One ancestral structure developing in different directions for different needs.

Convergent evolution

Different structures evolving towards the same function, often in similar habitats.

Industrial melanism

The rise of dark-winged moths where soot darkened tree trunks.

Adaptive radiation

Many species evolving from one ancestral stock in one area, spreading into different habitats.

Saltation

De Vries's single-step large mutation causing speciation.

Gene pool

All the genes and their alleles in a population.

Genetic equilibrium

Allele frequencies staying constant across generations (Hardy-Weinberg equilibrium).

Genetic drift

Change in allele frequencies by chance.

Founder effect

A drifted population, much changed in allele frequency, founding a new group that may become a new species.

Gene flow

Repeated gene migration between populations.