Evolution

The One Thing

Evolution is the population-level change in heritable characteristics and allele frequencies over generations. It is driven by genetic variation and evolutionary forces, while evidence from fossils, comparative anatomy, embryology, molecular biology, biogeography and observed natural selection supports both common ancestry and biological diversity.

Reactions, Processes and Experiments

Key Terms

• Origin of life: The process by which the first living organisms arose from non-living matter on the primitive Earth.
• Chemical evolution: The proposed formation of simple inorganic substances, organic molecules, complex polymers and primitive living systems through gradual chemical changes.
• Oparin-Haldane hypothesis: The idea that life originated in the primitive ocean from organic molecules formed under reducing atmospheric conditions and energy from sources such as lightning and ultraviolet radiation.
• Miller-Urey experiment: An experiment simulating early-Earth conditions that produced organic molecules such as amino acids.
• Panspermia: The hypothesis that life or spores came to Earth from outer space; it does not explain the original origin of life.
• Biological evolution: The change in inherited traits and allele frequencies of populations over successive generations.
• Natural selection: The process in which individuals with advantageous heritable variations survive and reproduce more successfully in a particular environment.
• Variation: Differences among individuals of the same species arising from mutation, recombination and other genetic processes.
• Mutation: A sudden, stable and heritable change in genetic material that can introduce new alleles.
• Genetic drift: Random change in allele frequencies, especially in small populations, which may cause alleles to become fixed or lost.
• Gene flow: Movement of alleles between populations through migration and reproduction.
• Genetic equilibrium: A condition in which allele and genotype frequencies remain constant from generation to generation when evolutionary forces are absent.
• Hardy-Weinberg principle: The principle that allele frequencies remain constant in an ideal population if there is no mutation, migration, selection, genetic drift or non-random mating.
• Gene pool: The complete set of genes and alleles present in a population at a particular time.
• Adaptive radiation: The evolution of different species from a common ancestor as they become adapted to different habitats or ecological niches.
• Homologous organs: Organs with the same basic structural plan and common evolutionary origin but different functions, such as the forelimbs of humans, whales and bats.
• Analogous organs: Organs with different structural origins but similar functions, such as the wings of birds and insects.
• Vestigial organs: Reduced structures with little or no important function in an organism but which were functional in ancestors, such as the human appendix.
• Fossils: Preserved remains, impressions or traces of ancient organisms that provide evidence about past life and evolutionary history.
• Archaeopteryx: A fossil organism showing both reptilian and avian features, providing evidence for the evolutionary connection between reptiles and birds.
• Biogeography: The study of the geographical distribution of organisms, which helps explain relationships, isolation and evolutionary history.
• Speciation: The formation of new species, commonly through accumulation of genetic differences and reproductive isolation.
• Reproductive isolation: The inability of populations to exchange genes successfully because of geographical, ecological, behavioural, temporal or genetic barriers.
• Divergent evolution: Evolution in which related organisms become increasingly different because they adapt to different environments.
• Convergent evolution: Evolution in which unrelated organisms develop similar adaptations because they face similar environmental conditions.
• Industrial melanism: The increase of dark-coloured forms of peppered moths in polluted areas because they were better camouflaged from predators.
• Fitness: The relative ability of an organism to survive and leave fertile offspring in a particular environment.

Easily Confused

• Origin of life vs biological evolution: Origin of life concerns the emergence of the first living organisms from non-living matter; biological evolution concerns changes in inherited characteristics after life exists.
• Chemical evolution vs biological evolution: Chemical evolution describes the formation of organic molecules and primitive living systems; biological evolution describes population-level genetic change over generations.
• Panspermia vs chemical evolution: Panspermia proposes that life came from outer space, whereas chemical evolution proposes that life developed through gradual chemical processes on early Earth; panspermia does not explain life’s original origin.
• Mutation vs natural selection: Mutations arise randomly with respect to an organism’s needs; natural selection non-randomly favours advantageous heritable variations.
• Natural selection vs evolution: Natural selection is an evolutionary mechanism; evolution is the resulting change in heritable characteristics or allele frequencies in populations.
• Genetic drift vs natural selection: Genetic drift changes allele frequencies by chance, especially in small populations; natural selection changes them through differential survival and reproduction.
• Founder effect vs bottleneck effect: The founder effect begins with a small group establishing a new population; the bottleneck effect follows a sharp reduction in an existing population.
• Homologous organs vs analogous organs: Homologous organs have common structural origins but different functions and support divergent evolution; analogous organs have different origins but similar functions and support convergent evolution.
• Divergent evolution vs convergent evolution: Divergent evolution makes related organisms increasingly different; convergent evolution makes unrelated organisms increasingly similar in function or adaptation.
• Lamarckism vs the modern synthetic theory: Lamarckism proposed inheritance of acquired characters; the modern synthetic theory explains evolution through natural selection, Mendelian genetics and population-genetic processes.
• Individual change vs population evolution: An individual does not evolve during its lifetime; evolution occurs when allele frequencies change across generations in a population.
• Fitness vs perfection: Fitness is relative reproductive success in a particular environment; evolution is not directed toward perfection.

What Gets Asked

• Explain the proposed sequence of chemical evolution: questions may require the sequence from simple inorganic molecules through organic molecules, complex polymers, self-replicating molecules, protocells and primitive cells. Omitting or reordering these specific stages loses marks.
• Describe the Miller-Urey experiment and its significance: include the early-atmosphere gases, electric spark simulating lightning, and formation of amino acids and other organic compounds. Do not present it as proof that the first cell was created.
• Use evidence to distinguish evolutionary relationships: homologous organs, embryological similarities, molecular similarities, fossils and biogeography support common ancestry, whereas analogous organs indicate convergent evolution. The wings of birds and insects must not be classified as homologous.
• Apply the Hardy-Weinberg equations: use p + q = 1 and p² + 2pq + q² = 1, identifying p², 2pq and q² correctly. Remember that the principle applies only when mutation, migration, selection, genetic drift and non-random mating are absent.
• Explain population change using evolutionary forces: mutation, recombination, gene flow, genetic drift and natural selection alter genetic composition. A common error is to state that an individual organism evolves during its lifetime.
• Compare adaptive radiation, industrial melanism and speciation: Darwin’s finches and Australian marsupials illustrate adaptive radiation; industrial melanism illustrates natural selection acting on pre-existing variation; speciation requires genetic divergence and reproductive isolation.