Why This Matters
In Philosophy of Biology, you're not just learning what scientists believe about life—you're interrogating why these theories count as knowledge, how they relate to each other, and what they reveal about the nature of biological explanation. The major theories covered here represent different levels of biological organization (molecular, cellular, organismal, population) and different types of explanatory frameworks (mechanistic causation, historical explanation, statistical modeling). Understanding how these theories interconnect—and where they create philosophical tensions—is essential for tackling exam questions about reduction, emergence, and the structure of biological science.
Don't just memorize definitions. For each theory, know what kind of explanation it offers, what philosophical questions it raises, and how it relates to other theories on this list. Can cellular processes be fully explained by molecular mechanisms? Is natural selection the only driver of evolutionary change? These are the conceptual battles you're being tested on.
Foundational Frameworks: What Is Life?
These theories establish the basic ontology of biology—what kinds of things exist and what properties define living systems. They answer the fundamental question: what makes something alive?
Cell Theory
- All living organisms are composed of cells—establishing the cell as biology's fundamental unit of organization and the boundary between life and non-life
- All cells arise from pre-existing cells (omnis cellula e cellula)—this continuity principle eliminates spontaneous generation and grounds biological explanation in material causation
- Philosophical significance: raises questions about emergence—can cellular properties be fully reduced to chemistry, or does organization itself create new causal powers?
Homeostasis
- Organisms maintain stable internal conditions despite environmental fluctuations—this self-regulation is often cited as a defining feature of life
- Regulatory mechanisms involve feedback loops that balance variables like temperature, pH, and glucose levels
- Teleological implications: homeostasis invites functional explanation ("the kidney exists in order to filter blood"), raising debates about whether biology requires purpose-talk that physics doesn't
Compare: Cell Theory vs. Homeostasis—both attempt to define life's essential features, but Cell Theory emphasizes structure (what life is made of) while Homeostasis emphasizes function (what life does). FRQ tip: if asked about criteria for life, these two theories offer complementary but distinct answers.
These theories explain how biological information is stored, expressed, and passed between generations. They raise central philosophical questions about genetic determinism, reductionism, and the relationship between genotype and phenotype.
Gene Theory
- Traits are inherited through genes—discrete segments of DNA that encode information passed from parents to offspring
- Genetic information influences development and behavior, but the gene-to-trait relationship is rarely simple; pleiotropy (one gene, many traits) and polygeny (many genes, one trait) complicate the picture
- Philosophical tension: supports reductionist explanations of heredity while simultaneously revealing that "the gene" is a contested concept with multiple definitions
Central Dogma of Molecular Biology
- Information flows from DNA → RNA → protein through transcription and translation—this unidirectional flow was proposed by Francis Crick in 1958
- "Dogma" is a misnomer—exceptions exist (reverse transcriptase, prions), but the general pattern holds for most gene expression
- Reductionist cornerstone: suggests that understanding molecular mechanisms is sufficient to explain biological phenomena, though critics argue this ignores contextual factors like cellular environment
Germ Theory of Disease
- Specific microorganisms cause specific diseases—establishing a mechanistic, causal framework that replaced miasma theory and humoral medicine
- Revolutionized medicine by enabling targeted interventions: antiseptic surgery, vaccines, antibiotics
- Philosophical model: exemplifies causal specificity in biology—one pathogen, one disease—though modern understanding recognizes microbiome complexity complicates this picture
Compare: Gene Theory vs. Central Dogma—Gene Theory addresses inheritance (vertical transmission across generations) while the Central Dogma addresses expression (how genetic information becomes functional within an organism). Both are reductionist, but they explain different biological processes.
Evolutionary Mechanisms: How Life Changes
These theories explain the processes driving biological change over time. They represent different—sometimes competing—answers to the question: what causes evolution?
Theory of Evolution by Natural Selection
- Organisms better adapted to their environment survive and reproduce more successfully—Darwin's mechanism requires heritable variation, competition for resources, and differential fitness
- Explains both diversity and adaptation—speciation arises from accumulated changes; apparent "design" results from selection, not intention
- Central philosophical achievement: provides a non-teleological explanation for apparent purpose in nature, though debates continue about whether selection explanations are genuinely causal or merely statistical
Modern Synthesis
- Integrates Darwinian selection with Mendelian genetics—developed in the 1930s-40s by Dobzhansky, Mayr, Simpson, and others
- Genetic variation within populations is the raw material for natural selection; evolution is defined as change in allele frequencies
- Bridges micro and macro: connects small-scale genetic changes to large-scale evolutionary patterns, though critics (Extended Evolutionary Synthesis proponents) argue it neglects development, plasticity, and niche construction
Neutral Theory of Molecular Evolution
- Most molecular evolution results from random genetic drift, not natural selection—proposed by Motoo Kimura in 1968
- Neutral mutations neither help nor harm organisms; their fate is determined by chance, especially in small populations
- Philosophical challenge: undermines adaptationist assumptions by showing that much genetic variation is selectively invisible—not everything in biology is "for" something
Hardy-Weinberg Equilibrium
- Allele frequencies remain constant in an idealized, non-evolving population—requires no mutation, random mating, infinite population size, no migration, and no selection
- Functions as a null model—deviations from equilibrium indicate that evolution is occurring and help identify which forces are responsible
- Philosophical status: like ideal gases in physics, Hardy-Weinberg represents an idealization—useful precisely because real populations never meet its conditions
Compare: Natural Selection vs. Neutral Theory—both explain evolutionary change, but they emphasize different mechanisms. Selection-driven evolution produces adaptation; drift-driven evolution produces random divergence. Exam tip: if asked whether all evolution is adaptive, the Neutral Theory is your counterexample.
Origins and Complexity: How Life Builds Itself
These theories address how complex biological structures and systems originated—raising questions about emergence, levels of organization, and historical explanation.
Endosymbiotic Theory
- Mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells—proposed by Lynn Margulis in the 1960s
- Evidence includes double membranes, independent DNA, bacterial-like ribosomes, and reproduction by binary fission
- Philosophical significance: demonstrates that cooperation and integration, not just competition, drive major evolutionary transitions; challenges strictly gene-centric views of evolution
Compare: Endosymbiotic Theory vs. Modern Synthesis—the Modern Synthesis emphasizes gradual change through mutation and selection, while Endosymbiosis represents a saltational event where entirely new organizational levels emerged through symbiotic merger. This tension highlights ongoing debates about evolutionary tempo and mode.
Quick Reference Table
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| Defining characteristics of life | Cell Theory, Homeostasis |
| Reductionist/mechanistic explanation | Central Dogma, Germ Theory, Gene Theory |
| Evolutionary mechanisms | Natural Selection, Neutral Theory, Hardy-Weinberg |
| Levels of selection debate | Modern Synthesis, Endosymbiotic Theory |
| Role of chance in biology | Neutral Theory, Hardy-Weinberg |
| Historical vs. functional explanation | Evolution by Natural Selection, Endosymbiotic Theory |
| Idealization in biological models | Hardy-Weinberg Equilibrium |
| Emergence and complexity | Cell Theory, Endosymbiotic Theory, Homeostasis |
Self-Check Questions
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Which two theories both address evolutionary change but attribute it to fundamentally different causes? What philosophical implications follow from accepting both as partially correct?
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Cell Theory and the Central Dogma both support reductionist approaches to biology. How do they differ in what they reduce biological phenomena to, and what objections might an anti-reductionist raise against each?
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Compare Hardy-Weinberg Equilibrium with the Germ Theory of Disease in terms of their explanatory structure. One is an idealization, the other posits specific causal agents—how do these different approaches function in biological reasoning?
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If an FRQ asks you to evaluate whether all biological explanation is ultimately mechanistic, which theories on this list would support that claim, and which might challenge it? Explain your reasoning.
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The Endosymbiotic Theory and the Modern Synthesis offer different pictures of how evolutionary novelty arises. How might a philosopher of biology use this contrast to argue that evolution operates at multiple levels of organization?