Evolution

How life began is the starting point of evolution. The universe and Earth are explained by the Big Bang theory. Early ideas of spontaneous generation (life arising from non-living matter like mud) were disproved by Louis Pasteur with his famous swan-neck flask experiment, which showed life comes only from pre-existing life. Panspermia proposed that life (spores) came from outer space.

The widely accepted scientific idea is chemical evolution, proposed independently by Oparin and Haldane. They suggested that the first forms of life arose from non-living organic molecules in the conditions of the early reducing atmosphere (rich in CH₄, NH₃, H₂ and water vapour, but no free oxygen). This was tested by the classic Miller–Urey experiment (1953): they passed electric sparks (mimicking lightning) through a flask of CH₄, NH₃, H₂ and water vapour, and after about a week amino acids and other organic molecules had formed — strong support for chemical evolution. The first organisms are thought to have been chemoheterotrophs in water.

Once life existed, how did the diversity of organisms arise? Lamarck proposed the inheritance of acquired characters (use and disuse): organs used more become better developed and this is passed on — his classic example was the long neck of the giraffe from stretching. This idea is now rejected because acquired (non-genetic) traits are not inherited.

Charles Darwin gave the cornerstone theory of natural selection in his book On the Origin of Species. Its core ideas are: organisms produce more offspring than can survive; individuals show heritable variation; those with favourable variations survive and reproduce more ('survival of the fittest'), so favourable traits accumulate over generations. (Wallace reached similar conclusions independently.) Later, Hugo de Vries proposed the mutation theory, that evolution proceeds by sudden large changes (saltation), based on his work on the evening primrose (Oenothera). For NEET, fix Pasteur (disproved spontaneous generation), Oparin–Haldane + Miller–Urey (chemical evolution), and the three evolution theories with their key proponents and ideas.

Several independent lines of evidence support evolution. The most direct is palaeontological evidence (fossils). Fossils are the remains or impressions of past organisms, usually found in sedimentary rock layers; the deeper the layer, the older the fossil. They show that life forms have changed over time and reveal extinct intermediate forms. A classic example is the evolution of the horse (from the small Eohippus to the modern Equus), and Archaeopteryx (a fossil connecting reptiles and birds).

Comparative anatomy gives two key kinds of evidence. Homologous organs have the same basic structure and origin but may do different jobs — for example, the forelimbs of a human, whale, bat and cheetah have the same bone plan but are used for grasping, swimming, flying and running. Homology indicates a common ancestor and is the result of divergent evolution.

Analogous organs, by contrast, have different structures but the same function — for example, the wings of an insect and a bird, or the eye of an octopus and a mammal. Analogy does not indicate common ancestry; it results from convergent evolution, where unrelated organisms independently evolve similar features for similar needs. Closely related to homology are vestigial organs — reduced, functionless remnants of organs that were useful in ancestors, such as the vermiform appendix and wisdom teeth in humans.

Further support comes from embryological evidence (early embryos of different vertebrates look strikingly similar) and powerful modern molecular evidence — the similarities in DNA and proteins across organisms, where more closely related species share more similar sequences. There is also direct evidence in real time, such as the development of antibiotic and pesticide resistance in bacteria and insects. For NEET, the must-knows are the homologous (divergent, common ancestry) vs analogous (convergent, similar function) distinction with their examples, plus fossils as direct evidence and the idea of molecular/embryological support.

The modern understanding combines Darwin's natural selection with genetics. Evolution acts on heritable variation in a population. Variation arises mainly from mutation and from recombination during sexual reproduction. Natural selection then acts on this variation, favouring individuals better suited to the environment, so that allele frequencies in the population change over generations — that change is evolution.

To describe when a population is not evolving, the Hardy–Weinberg principle states that in a large, randomly mating population the allele and genotype frequencies stay constant across generations — the population is in genetic equilibrium. It is written as p² + 2pq + q² = 1 (with p + q = 1), where p and q are the frequencies of two alleles, p² and q² the homozygotes and 2pq the heterozygotes.

This equilibrium holds only under ideal conditions; in reality, five factors disturb it and therefore cause evolution: gene migration (gene flow), genetic drift (random change in allele frequency, important in small populations — including the founder effect and bottleneck effect), mutation, genetic recombination and natural selection. If allele frequencies are changing, the Hardy–Weinberg equilibrium is broken and evolution is happening.

Natural selection can act in three ways, depending on which individuals it favours. Stabilising selection favours the average/intermediate phenotype and removes extremes (reducing variation). Directional selection favours one extreme, shifting the population toward it. Disruptive selection favours both extremes over the average (and can split a population). A famous real example is industrial melanism in the peppered moth (Biston betularia) of England: after the Industrial Revolution darkened tree trunks with soot, the dark moths were better camouflaged and survived while light ones were eaten — a textbook case of natural selection observed in nature. For NEET, fix sources of variation, the Hardy–Weinberg statement and equation, the five disturbing factors (including drift/founder/bottleneck), the three types of selection, and industrial melanism.

When a single ancestral species spreads to different habitats and diversifies into many forms suited to those habitats, the process is called adaptive radiation. The classic examples are Darwin's finches of the Galapagos Islands — from one ancestral finch, many species evolved with different beak shapes suited to different diets — and the Australian marsupials, which radiated into many forms filling roles taken by placental mammals elsewhere. When two unrelated groups undergo adaptive radiation and come to resemble each other, it shows convergent evolution.

The story of our own species, human evolution, is a frequently asked sequence. Humans and apes share common ape-like ancestors. Dryopithecus and Ramapithecus were ape-like primates (Ramapithecus was more man-like). The line leading to humans then passed through Australopithecus — early hominids that lived in East Africa, walked upright and probably ate fruit.

The genus Homo follows. Homo habilis ("handy man", about 2 million years ago) was the first to make and use stone tools and had a larger brain. Homo erectus (about 1.5 million years ago) had a still larger brain and is associated with the use of fire and better tools. Homo neanderthalensis (Neanderthals) lived in Europe and Asia, made tools and buried their dead.

Finally, modern humans, Homo sapiens, arose in Africa and spread across the world, developing language, art and agriculture. The broad trend across this sequence is increasing brain size and tool-making ability, and increasingly upright posture. So the simplified line is: Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens. For NEET, fix the definition of adaptive radiation with Darwin's finches/Australian marsupials, and the human-evolution sequence with each species' landmark (habilis = first tools; erectus = fire; sapiens = modern human).