Evolution

Evolution is the slow, gradual change in living organisms over very long periods of time, by which new species arise from earlier ones. It explains the enormous variety of life on Earth and how all organisms are related.

How did life begin? The widely accepted scientific idea is chemical evolution: the first simple life arose from non-living chemicals in the conditions of the early Earth. The Oparin–Haldane hypothesis proposed that simple molecules combined to form more complex organic molecules in the early oceans. The Miller–Urey experiment supported this by showing that, when a mixture of gases thought to be present on early Earth was given energy (sparks, mimicking lightning), simple organic molecules like amino acids were formed.

Several lines of evidence support evolution:

• Fossils — the preserved remains of ancient organisms in rocks; they show how life forms have changed over time and reveal extinct species.
• Homologous organs — organs with the same basic structure but different functions (e.g. the forelimbs of a human, whale and bat), showing common ancestry (divergent evolution).
• Analogous organs — organs with different structure but the same function (e.g. the wings of a bird and an insect), showing similar adaptation to similar needs (convergent evolution).
• Embryological and molecular evidence — similarities in early embryos and in DNA/proteins across species.

The most important explanation of how evolution happens is Charles Darwin's theory of Natural Selection, put forward in his book On the Origin of Species (1859). Its main ideas are:

• Organisms produce far more offspring than can survive, so there is a struggle for existence (competition for food, space, etc.).
• Individuals of a species show variations; some variations are useful in the struggle.
• Individuals with useful (favourable) variations are more likely to survive and reproduce — this is survival of the fittest, or natural selection.
• Over many generations, the favourable variations accumulate, gradually changing the species and producing new ones.

An earlier idea by Lamarck (inheritance of acquired characters — e.g. the giraffe's neck lengthening from stretching) is now rejected, because changes acquired during life are not passed to offspring.

Today we understand the raw materials and forces of evolution more fully. Variations arise mainly from mutations (changes in DNA) and from the reshuffling of genes during sexual reproduction (recombination). Natural selection then acts on this variation. Other factors include genetic drift (random change in small populations) and migration. A simple example seen in real time is the way bacteria become resistant to antibiotics, or insects to pesticides: a few resistant individuals survive and multiply — natural selection in action.

Humans, too, are a product of evolution. Modern humans and the modern apes (like the chimpanzee and gorilla) share a common ancestor that lived millions of years ago — humans did not evolve from today's monkeys, but humans and apes evolved along separate lines from that shared ancestor.

The broad story of human evolution, pieced together from fossils, includes several stages:

• Dryopithecus and Ramapithecus — ape-like ancestors; Ramapithecus was more man-like.
• Australopithecus — early human-like forms in Africa that walked upright (bipedal) but had small brains.
• Homo habilis — the "handy man," who could make and use simple stone tools; brain larger than Australopithecus.
• Homo erectus — walked fully upright, had a larger brain, and is thought to have used fire.
• Homo sapiens — modern humans, with a large, well-developed brain, language and culture.

The key trends across human evolution were: walking upright on two legs (freeing the hands), an increase in brain size, the making of better tools, and the development of language and culture. These changes gave humans their unique abilities. Human evolution is a powerful illustration of the same natural processes — variation and selection over long time — that have shaped all life on Earth.

Darwin explained natural selection but did not know about genes. The modern synthetic theory of evolution combines Darwin's natural selection with modern genetics. It views evolution as a change in the frequency of alleles (gene forms) in a population over generations, brought about by five main factors: mutation and recombination (which create variation), natural selection, genetic drift (random change in small populations) and gene flow (migration) — the movement of alleles between populations as individuals move in or out.

To describe a population that is not evolving, scientists use the Hardy-Weinberg principle. It states that in a large, randomly mating population with no mutation, no selection, no genetic drift and no gene flow, the allele frequencies stay constant from generation to generation (a state called genetic equilibrium). It is written as p + q = 1 and p² + 2pq + q² = 1, where p and q are the frequencies of the two alleles. If the real frequencies change, it means one of the five evolutionary forces is acting — in other words, evolution is happening. So Hardy-Weinberg is a useful baseline for detecting evolution.

Adaptive radiation is the process by which a single ancestral species diversifies into many different forms, each adapted to a particular way of life (niche). The classic example is Darwin's finches on the Galapagos Islands, which evolved different beak shapes for different foods from one ancestor. The marsupials of Australia are another example. Adaptive radiation shows how evolution fills the variety of available habitats.