EVOLUTION

Evolutionary biology explains the origin of life, evidence for common ancestry, processes that change populations, and the broad history of life including human evolution.

Life is considered a unique event in the history of the universe. The earth formed about 4.5 billion years ago and the earliest life forms appeared about 4 billion years ago.

• Oparin and Haldane proposed that the first life arose from non-living organic molecules under early-earth conditions.
• Miller experimentally created amino acids from methane, ammonia, hydrogen and water vapour exposed to electric discharge.
• The first organisms were probably anaerobic heterotrophs. Photosynthesis later released oxygen, which changed the atmosphere and allowed aerobic life to evolve.
• RNA may have been the first genetic material because it can both store information and catalyse reactions.

Lamarck proposed inheritance of acquired characters. Darwin explained evolution through natural selection acting on heritable variation. Individuals with favourable variations leave more offspring and gradually become more common.

• Fossils reveal extinct forms and their succession through geological time.
• Homologous organs have a common origin and basic plan but may have different functions; they show divergent evolution.
• Analogous organs have similar functions but different origins; they show convergent evolution.
• Molecular evidence, including similarities in DNA and proteins, supports common ancestry.

Adaptive radiation is the evolution of different species from a common ancestor in one geographical area, with each species adapting to a different ecological niche.

• Darwin's finches on the Galapagos Islands evolved different beak shapes from an ancestral seed-eating finch.
• Australian marsupials show adaptive radiation. Similarities between marsupials and placental mammals are examples of convergent evolution.
• Evolution is not directed by need; it is the result of selection acting on inherited variation in a population.

Evolution means a change in allele frequencies in a population's gene pool. Mutations and recombination create variation; natural selection, genetic drift and gene flow alter frequencies.

• Natural selection may stabilise an intermediate phenotype, shift a population toward one extreme, or favour both extremes.
• Genetic drift is random change in allele frequency, especially important in small populations. The founder effect is genetic drift after a small group establishes a new population.
• Gene flow occurs when migration adds or removes alleles from a population.

In a large, randomly mating population with no evolutionary forces, allele frequencies remain constant: p² + 2pq + q² = 1, where p and q are allele frequencies and p + q = 1.

Gene migration, genetic drift, mutation, recombination and natural selection disturb this equilibrium. A departure from Hardy-Weinberg equilibrium indicates that evolution is occurring.

Life diversified from invertebrates to fishes, amphibians, reptiles, birds and mammals. Dinosaurs dominated the earth in the Jurassic period and later became extinct; mammals diversified after their extinction.

• Dryopithecus was more ape-like and Ramapithecus more man-like; both lived about 15 million years ago.
• Australopithecines lived in East African grasslands. Homo habilis had brain capacity of 650-800 cc; Homo erectus appeared about 1.5 million years ago and had a brain around 900 cc.
• Neanderthal man lived about 100,000-40,000 years ago and had brain size about 1400 cc. Modern Homo sapiens evolved in Africa and spread across continents.

• Chemical evolution preceded biological evolution; RNA likely came before DNA.
• Fossils, comparative anatomy and molecular evidence support evolution.
• Natural selection acts on inherited variation and can change allele frequencies.
• Hardy-Weinberg equilibrium is a baseline; its disturbance indicates evolution.
• Human evolution is a branching history from common ancestors, not a linear ladder.