# Biology Biology is the science of [[Living_systems|living systems]] — matter organized so that it metabolizes, self-maintains, reproduces, and evolves. Its objects span about nine orders of magnitude in length, from a 1 nm protein to a 30 m whale, yet a handful of unifying principles hold across all of them: cell theory, [[Evolution|evolution]] by natural selection, heredity through nucleic acids, energy flow through [[Energy_transformation|transformation]] chains, and regulation by [[Feedback|feedback]]. Biology is also the original systems science: [[Homeostasis|homeostasis]], [[Self-organization|self-organization]], [[Emergence|emergence]], and [[Adaptation|adaptation]] were biological facts long before they were engineering vocabulary, and [[Ludwig_von_Bertalanffy|Bertalanffy]], a theoretical biologist, generalized them into [[Systems_theory|general systems theory]]. ## Life as chemistry that copies itself Cell theory (Schleiden and Schwann, 1838–39; Virchow's "every cell from a cell," 1855) fixed biology's unit of account. The [[Chemistry|chemistry]] inside is narrow and universal: life runs on [[Carbon|carbon]] scaffolds in liquid [[Water|water]], using [[Hydrogen_bond|hydrogen bonds]] weak enough (~10–30 kJ/mol) to make and break at body temperature — the trick behind protein folding and DNA base pairing alike. The 1953 double-helix structure explained heredity mechanically: complementary strands make templated [[Self-replication|self-replication]] possible, with proofreading polymerases holding error rates near 10⁻⁹ per base per division. The genetic code maps 64 codons to 20 amino acids in essentially every organism, powerful evidence of common descent. Genome sequencing (human draft 2001, "complete" telomere-to-telomere 2022) turned the inventory over to [[Genomics|genomics]] and [[Bioinformatics|bioinformatics]]; the surprise was how few protein-coding genes a human carries — roughly 20,000 — pushing explanatory weight onto regulation and network context, the home turf of [[Systems_biology|systems biology]]. ## Energy flow and the entropy bill Organisms are open thermodynamic systems: they hold internal [[Entropy|entropy]] low by importing free energy and exporting disorder, exactly the arrangement [[Open_system_(systems_theory)|open-system]] theory formalizes. The energy ledger starts at photosynthesis, where the [[Oxygen-evolving_complex|oxygen-evolving complex]] strips electrons from [[Water|water]] and releases the [[Oxygen|O₂]] that transformed Earth's atmosphere after the Great Oxidation Event (~2.4 billion years ago). Respiration runs the ledger's other side, charging the universal energy currency ATP; a working human turns over roughly their body mass in ATP daily. Transport physics matters: [[Diffusion|diffusion]] suffices only below ~100 μm, which is why bigger animals evolved circulation and why [[Hemoglobin|hemoglobin]] — a four-subunit protein whose cooperative O₂ binding is a textbook [[Nonlinear_system|nonlinearity]] — exists. At [[Ecosystem|ecosystem]] scale, [[Systems_ecology|systems ecologists]] such as [[Howard_T._Odum|H. T. Odum]] traced the same currency through food webs, where only ~10% of energy survives each trophic transfer. ## Evolution: the algorithm that needs no designer Darwin (1859) supplied biology's central mechanism: heritable variation plus differential reproduction yields cumulative design without a designer. Mendel (1866) found heredity's discrete carriers; the modern synthesis (1930s–40s, with [[Sewall_Wright|Sewall Wright]], Fisher, and Haldane) fused the two into mathematical population genetics, making evolution a branch of [[Probability_theory|probability theory]] and [[Population_dynamics|population dynamics]]. The theory's extensions are game-theoretic: Hamilton's rule (1964) — help kin when rb > c — grounds [[Kin_selection|kin selection]]; Maynard Smith's [[Evolutionarily_stable_strategy|evolutionarily stable strategies]] (1973) imported [[Evolutionary_game_theory|game theory]] wholesale. Evolution is also an exportable [[Algorithm|algorithm]]: [[Genetic_algorithm|genetic algorithms]] and [[Evolutionary_computation|evolutionary computation]] optimize by mutation and selection, and [[Evolutionary_developmental_biology|evo-devo]] explains how conserved regulatory toolkits (Hox genes) make body plans [[Evolvability|evolvable]] — small genetic changes, coherent morphological ones. ## Regulation: feedback keeps you alive Claude Bernard's *milieu intérieur* and Cannon's [[Homeostasis|homeostasis]] (1932) described physiology as a lattice of [[Negative_feedback|negative-feedback]] controllers: core temperature held near 37 °C, blood glucose near 5 mM by the insulin–glucagon pair, blood pH near 7.4. Molecular biology found the same [[Control_theory|control logic]] in gene regulation — Jacob and Monod's *lac* operon (1961) is a transcriptional switch — and [[Biocybernetics|biocybernetics]] made the parallel with engineered controllers explicit. [[Positive_feedback|Positive feedback]] appears where commitment beats stability: the sodium-channel spike of a [[Nervous_system|nerve impulse]], blood clotting, apoptosis. Even single-celled life computes collectively: [[Quorum_sensing|quorum sensing]] lets bacteria count themselves before committing to bioluminescence or virulence, a molecular vote implemented in signaling chemistry and studied today with the tools of [[Information_theory|information theory]]. ## Scales and emergent organization Biology's hierarchy — molecule, cell, tissue, organism, population, [[Ecosystem|ecosystem]], biosphere — is a stack of [[Emergence|emergence]]: each level obeys the ones below yet exhibits dynamics invisible there. Turing's 1952 [[Reaction–diffusion_system|reaction–diffusion]] mechanism shows how molecular kinetics plus [[Diffusion|diffusion]] yield spots and stripes ([[Pattern_formation|pattern formation]] without a blueprint); [[Swarm_behaviour|flocking and swarming]] arise from local rules with no leader; Lotka–Volterra equations (1920s) generate predator–prey [[Oscillation|oscillations]] as a property of the coupling, not of either species. [[Theoretical_ecology|Theoretical]] and [[Spatial_ecology|spatial ecology]] formalize how [[Carrying_capacity|carrying capacity]], dispersal, and disturbance shape communities, and [[Mathematical_and_theoretical_biology|mathematical biology]] supplies the [[Dynamical_system|dynamical-systems]] language — attractors, bifurcations, stability — in which modern biology states its laws. ## Biology as a systems science The 21st-century field increasingly studies wiring diagrams rather than parts lists: [[Systems_biology|systems biology]] and [[Metabolic_network_modelling|metabolic network modeling]] treat the cell as a [[Complex_adaptive_system|complex adaptive system]]; [[Modelling_biological_systems|whole-system models]] simulate it; [[Neuroscience|neuroscience]] does the same for the brain. [[Humberto_Maturana|Maturana]] and [[Francisco_Varela|Varela's]] [[Autopoiesis|autopoiesis]] (1972) proposed self-production as life's defining organization, [[Biosemiotics|biosemiotics]] reads organisms as sign-processing systems, and [[Artificial_life|artificial life]] tests which of biology's regularities are substrate-independent. The applied stack — [[Medicine|medicine]], [[Genetic_engineering|genetic engineering]] (CRISPR-Cas9, 2012), [[Tissue_engineering|tissue engineering]], [[Biological_engineering|biological engineering]] — now edits the systems it once only described, which is why the honest caveat matters: after ~3.8 billion years of iteration, most of the network biology remains unmapped. **On the spine:** [[Living_systems]] · [[Evolution]] · [[Systems_biology]] · [[Ecology]] · [[Homeostasis]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Biology) : [Wikitube](https://en.wikitube.io/wiki/Biology) ## Previous hub tags Hubs: `Systems`. Portals: [[PORTAL_Systems]], [[PORTAL_Emergence]], [[PORTAL_Dynamical_system]], [[PORTAL_Systems_theory]], [[PORTAL_Feedback]]. --- *Repopulated 2026-08-12 · redlink fill · 0 deletions.*