Evolution

The One Thing

Evolution explains the origin and diversity of life through gradual heritable change over geological time. Chemical evolution accounts for the emergence of early life, while variation, natural selection, and other genetic mechanisms explain the diversification of populations and the evolution of humans.

Reactions, Processes and Experiments

Key Terms

• Origin of life: The emergence of the first living organisms from non-living matter through a long sequence of chemical and biological changes.
• Chemical evolution: The proposed formation of simple organic molecules from inorganic substances on the primitive Earth, followed by the formation of complex molecules.
• Primitive atmosphere: The early Earth's atmosphere, believed to have contained methane, ammonia, hydrogen, and water vapour, with little or no free oxygen.
• Abiogenesis: The hypothesis that life originated from non-living matter under suitable environmental conditions.
• Biogenesis: The principle that living organisms arise from pre-existing living organisms.
• Miller-Urey experiment: An experiment that simulated early-Earth conditions and produced simple organic molecules, including amino acids, from inorganic gases and electrical energy.
• Evolution: The heritable change in populations over successive generations, often resulting in adaptation and the formation of new species.
• Variation: Differences among individuals of the same species; variations may be genetic, environmental, continuous, or discontinuous.
• Fossils: Preserved remains, impressions, or traces of ancient organisms found mainly in sedimentary rocks.
• Fossil record: The historical sequence of fossils showing the appearance, modification, and extinction of organisms through geological time.
• Homologous organs: Organs with the same basic structural plan but different functions, indicating common ancestry; examples include the forelimbs of humans, whales, bats, and horses.
• Analogous organs: Organs with different structural origins but similar functions, such as the wings of birds and insects, indicating convergent evolution.
• Vestigial organs: Reduced structures with little or no important function in a present organism but which were functional in ancestors, such as the human vermiform appendix.
• Embryological evidence: Similarities in early developmental stages of different organisms that suggest common ancestry.
• Molecular evidence: Similarities in DNA, RNA, proteins, and the genetic code that reveal evolutionary relationships.
• Biogeography: The study of the geographical distribution of organisms; isolated regions often contain distinctive species reflecting evolutionary history.
• Darwinism: Darwin's theory that evolution occurs mainly through natural selection acting on heritable variations.
• Natural selection: The process by which organisms with favourable heritable traits survive and reproduce more successfully than others.
• Struggle for existence: Competition among organisms for limited resources such as food, space, shelter, and mates.
• Survival of the fittest: The greater reproductive success of organisms better adapted to their environment; fitness refers to reproductive success, not merely physical strength.
• Adaptation: A heritable feature or characteristic that improves an organism's survival and reproduction in a particular environment.
• Artificial selection: The selective breeding of organisms by humans to obtain desired traits, demonstrating how selection can produce change over generations.
• Speciation: The formation of new species, commonly through the accumulation of genetic differences and reproductive isolation.
• Genetic drift: A random change in allele frequencies, especially significant in small populations.
• Gene flow: The movement of alleles between populations through migration and interbreeding.
• Mutation: A sudden heritable change in genetic material that can introduce new alleles and contribute to evolution.
• Hardy-Weinberg principle: A principle stating that allele frequencies remain constant in an ideal population when evolutionary forces are absent.
• Human evolution: The evolutionary history of humans from ape-like ancestors, involving increasing bipedalism, brain development, tool use, language, and complex culture.
• Bipedalism: Walking upright on two legs, a major adaptation in human evolution.
• Hominins: Humans and their more closely related extinct ancestors after divergence from the lineage leading to modern apes.

Easily Confused

• Abiogenesis and biogenesis: Abiogenesis proposes that life originated from non-living matter, whereas biogenesis states that living organisms arise from pre-existing living organisms.
• Homologous and analogous organs: Homologous organs have a similar structural plan but different functions and indicate divergent evolution; analogous organs have different structural origins but similar functions and indicate convergent evolution.
• Natural selection and artificial selection: Natural selection results from environmental pressures, whereas artificial selection is directed by human selective breeding.
• Variation and adaptation: Variation is a difference among individuals; an adaptation is a heritable feature that improves survival and reproduction in a particular environment.
• Survival of the fittest and physical strength: Fitness means reproductive success, not necessarily greater physical strength.
• Natural selection and evolution: Natural selection acts on individual phenotypes, whereas evolutionary change is measured as changes in allele frequencies within a population.
• Genetic drift and gene flow: Genetic drift changes allele frequencies randomly, especially in small populations; gene flow transfers alleles between populations through migration and interbreeding.
• Humans and modern monkeys: Humans did not evolve directly from present-day monkeys; humans and modern apes share common ancestors.
• Evolutionary sequence and evolutionary ladder: The major human evolutionary forms represent a branching sequence rather than a single straight ladder.

What Gets Asked

• Explain the origin of life using chemical evolution: Questions may require the sequence from inorganic molecules to simple organic molecules, complex molecules, self-replicating systems, membrane-bound protocells, and primitive cells. Omitting the sequence or replacing the named Miller-Urey experiment with a generic decomposition experiment loses specificity.
• Describe the Miller-Urey experiment: State that it was conducted in 1953 by Stanley Miller and Harold Urey, used methane, ammonia, hydrogen, water vapour, and electrical sparks, and produced simple organic molecules including amino acids.
• Distinguish evidence for evolution: Fossils provide direct historical evidence, whereas homologous organs, vestigial organs, embryology, molecular similarities, and biogeography provide comparative evidence. Homologous structures must not be confused with analogous structures.
• State Darwin's mechanism of evolution: Include overproduction, limited resources, struggle for existence, heritable variation, differential survival and reproduction, and accumulation of favourable variations. Darwin published this explanation in 1859, while Alfred Russel Wallace independently developed a similar idea.
• Use the Hardy-Weinberg equations and conditions: Apply p + q = 1 and p^2 + 2pq + q^2 = 1, identifying p and q as allele frequencies. The required conditions are a large population, random mating, no mutation, no migration, and no natural selection.
• Describe human evolution: Identify Australopithecus, Homo habilis, Homo erectus, Homo neanderthalensis, and Homo sapiens, together with bipedalism, increasing brain capacity, tool use, language, cooperation, and culture. Do not state that modern humans evolved directly from present-day monkeys.