Organic Evolution: Theories and Mechanisms
Foundations of organic evolution covering its definition and core premises, seven methods used to study evolutionary change including palaeontology, molecular biology, embryology, taxonomy, biogeography, cytogenetics, and immunology, the three major theoretical phases from Lamarckism through Darwinism to the Synthetic Theory, the mechanism of evolution through changes in gene frequency driven by migration, isolation, hybridization, recombination, genetic drift, natural selection, and mutation, Ernst Mayr's comparison of Darwinism and Lamarckism, the distinction between macroevolution and microevolution, and the two competing theories of macroevolution: punctuated equilibrium with systemic and homeotic mutations versus gradualism supported by plant hybridization evidence
Topics
Organic Evolution and Its Study
What organic evolution means, its foundational premises, seven methods used to study it, the three theoretical phases, and the mechanism of gene frequency change
Lamarckism: The Theory of Inherited Acquired Characters
Lamarck's four laws of evolution including elan vital, environmental pressure, use and disuse of organs, and inheritance of acquired characters, along with key criticisms, experimental evidence for and against, and modern epigenetic support
Darwin's Theory of Natural Selection
Charles Darwin's theory of evolution by natural selection, the five-year HMS Beagle voyage that shaped his thinking, the five key influences on his theory, and the five postulates of natural selection including overproduction, struggle for existence, variation, survival of the fittest, and origin of new species
Darwin's Other Theories and Their Criticism
Darwin's theories beyond natural selection, including artificial selection at conscious and unconscious levels, sexual selection and male competition for mates, common descent linking related organisms to shared ancestors, the pangenesis hypothesis and its rejection by Galton and Mendelism, eight major criticisms of Darwinism, and a summary flowchart of how natural selection drives the origin of new species
Darwinism vs Lamarckism, and Macro vs Micro Evolution
Ernst Mayr's 1982 comparison of Darwinism and Lamarckism covering their shared ground in gradualness, use and disuse, and environmental role, alongside key differences in common descent, origin of diversity, vertical vs horizontal evolution, and scientific evidence, followed by the distinction between microevolution (within-species change, Goldschmidt 1940, sickle cell example) and macroevolution (above-species change, fossil record, Homo erectus to Homo sapiens), and why the two scales are complementary
Factors Driving Microevolution
The five forces that drive microevolution: mutation as the raw material of change, natural selection in its three modes (stabilizing, directional, disruptive) supported by molecular genetics, trait distribution, and single population studies, gene flow across population boundaries, genetic drift in small populations with examples like six-fingered dwarfism in the Amish and retinitis pigmentosa in Tristan da Cunha, and the roles of isolation and hybridization in shaping genetic divergence, plus evidence that microevolution is still ongoing in modern humans
Macroevolution: Punctuated Equilibrium and Gradualism
Two competing theories of macroevolution covering the punctuated equilibrium model where morphological change happens in rapid bursts during speciation events followed by long periods of stability, Goldschmidt's systemic mutation hypothesis involving complete chromosomal repatterning with the ape-to-human chromosome 2 fusion example, homeotic mutations that produce dramatic phenotypic effects by acting early in embryonic development illustrated by the Drosophila bithorax mutant, and the gradualist model proposing that small mutations accumulate steadily over time with supporting evidence from fertile interspecific and intergeneric plant hybrids
The Synthetic Theory of Evolution (Neo-Darwinism)
How the Synthetic Theory brought together Darwin's natural selection with modern genetics and population dynamics to form a unified explanation of evolution, covering the three main contributor groups (population dynamics, genetics and mutation, population genetics), Fisher's classic 1930 work, and the three core factors driving evolutionary change: mutation as the ultimate source of genetic variation, natural selection operating in stabilizing, directional, and disruptive modes, and reproductive isolation through pre-zygotic barriers (ecological, temporal, behavioural, structural, gametic) and post-zygotic barriers (zygotic mortality, hybrid inviability, hybrid sterility, hybrid breakdown)
Synthetic Theory: Migration, Drift, Recombination, and Critique
The remaining evolutionary factors in the Synthetic Theory beyond mutation, selection, and isolation: how migration reshuffles gene pools across populations, how genetic drift causes unpredictable frequency shifts in small groups, how genetic recombination creates fresh trait combinations through crossing over, and how hybridization boosts heterozygosity, followed by the three main criticisms of the Synthetic Theory and a four-point comparison showing how it differs from Darwin's original framework
Is Evolution Still Happening? Evidence That Change Never Stops
Why evolution has not stopped and is in fact accelerating in modern humans: theoretical proof through the Hardy-Weinberg principle showing at least inbreeding or hybridization plus natural selection are always active, and practical evidence from rising mutation rates due to industrialization and nuclear radiation, declining inbreeding and genetic drift from urbanization, increasing hybridization from global mobility, and the shrinking human jaw with the delayed and disappearing third molar
Adaptive Radiation: How One Ancestor Becomes Many Species
How a single ancestral species diversifies into many distinct forms occupying different ecological niches, the concept coined by H.F. Osborn in 1902 and explained by Darwin and Buettner Janusch, the four identifying features (CPRT: common ancestry, phenotype-environment correlation, trait utility, rapid speciation), the three conditions that trigger it (new habitat, isolation, niche availability), and classic examples including Darwin's finches on the Galapagos and mammalian radiation during the Palaeocene
Convergence, Divergence, Parallelism, and the Homology-Analogy Distinction
How unrelated species develop similar traits through convergent evolution, how related species diverge into different forms, how parallel evolution produces similar traits in related lineages independently, and the crucial distinction between homology (shared origin), analogy (shared function), homoplasy (shared appearance), and serial homology (comparing structures within one organism)
Cope's Rule and Dollo's Law: Size Trends and Irreversibility in Evolution
Two foundational rules of palaeontology: Cope's Rule on the evolutionary tendency of organisms to increase in body size over time, with its exceptions and limitations, and Dollo's Law on the irreversibility of evolution, explaining why structures that change during evolution do not revert to their earlier forms
Gause's Rule and Mosaic Evolution: Competition and Uneven Change
Gause's Rule of competitive exclusion showing that two species competing for identical resources cannot coexist at the same trophic level, demonstrated through Paramecium experiments, and mosaic evolution as the process by which different body parts evolve at different rates and times, illustrated by the bipedalism-before-brain-expansion pattern in human evolution and the sequential development of the primate brain
Unicentric vs Multicentric Origins and Social Darwinism
The debate between the unicentric (Out of Africa) and multicentric theories of Homo sapiens origin, key evidence from mitochondrial DNA studies by Kenn et al. (1987) and gene marker analysis by Neil and Livshitz (1989) supporting African origins, Krammer's (1991) mandible evidence from Java and Simmons and Smith's (1991) fossil review supporting multiple centres, the significance of the oldest Homo erectus fossils in Asia, and Social Darwinism as a misapplication of natural selection to justify imperialism, racism, and inequality in the late 1800s before its discrediting by the end of World War II
Natural Selection Acting on Variation
How natural selection drives evolution by differentially filtering variation, Darwin's five-step mechanism from differential reproduction to organic evolution, the three forms of selection (stabilizing, directional, disruptive) with the African swallowtail butterfly and Batesian mimicry example, magnitude of selection through density dependent, frequency dependent, and group selection including baboon troupe leadership, the ecological basis of selection contrasting r-selection and K-selection with the rural-urban social perspective, and the conclusion that natural selection works alongside mutation, isolation, and genetic drift
Genetic Drift: The Sewall Wright Effect
How random, unpredictable events change gene frequencies in small isolated populations through genetic drift (the Sewall Wright Effect), the two mechanisms of bottleneck effect and founder effect with real-world examples from the Old Order Amish (Ellis van Creveld syndrome) and Tristan da Cunha (retinitis pigmentosa), the role of drift in species divergence and the success of disadvantageous genes, the interplay of selection and drift in ABO blood group distribution as argued by Livingstone, and the connection between drift and population expansion through the Out of Africa model