Major Evolutionary Theories

Why This Matters

Evolutionary theories aren't just historical curiosities—they're the conceptual toolkit you need to explain how and why life changes over time. Your exam will test whether you understand the mechanisms driving evolutionary change: natural selection, genetic drift, mutation, gene flow, and non-traditional inheritance patterns. Each theory in this guide represents a different answer to the question "What causes evolution?"—and you'll need to know when each explanation applies.

These theories also reveal how scientific thinking itself evolves. You're being tested on your ability to distinguish between gradualism and punctuated change, between selection-driven and drift-driven evolution, and between vertical and horizontal inheritance. Don't just memorize names and dates—know what problem each theory solves and what evidence supports it. That's what separates a 3 from a 5.

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Classical Foundations: Selection-Based Evolution

These theories establish natural selection as the primary mechanism of adaptive evolution. Organisms with traits that enhance survival and reproduction pass those traits to offspring at higher rates, shifting population characteristics over generations.

Darwin's Theory of Evolution by Natural Selection

• Variation, inheritance, and differential reproduction—these three conditions must exist for natural selection to operate
• Fitness means reproductive success, not strength or speed—an organism's fitness is measured by how many viable offspring it produces
• Descent with modification explains both adaptation to environments and the branching pattern of life's diversity

Modern Synthesis (Neo-Darwinism)

• Integrates Mendelian genetics with natural selection—finally explaining how traits are inherited and how variation arises
• Mutations provide the raw material for evolution; recombination shuffles existing alleles into new combinations
• Population genetics framework allows quantitative predictions about allele frequency changes at both microevolutionary and macroevolutionary scales

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Challenging Gradualism: Alternative Tempos and Mechanisms

Not all evolutionary change is slow and steady. These theories propose that evolution can occur in bursts, through random processes, or via mechanisms beyond classical selection.

Punctuated Equilibrium

• Stasis interrupted by rapid change—species remain stable for millions of years, then evolve quickly during speciation events
• Fossil record patterns support this model, showing sudden appearances of new forms rather than smooth transitions
• Speciation as the driver—major evolutionary changes concentrate in small, isolated populations undergoing speciation

Neutral Theory of Molecular Evolution

• Genetic drift, not selection, drives most molecular evolution—many mutations have no effect on fitness and spread by chance
• Molecular clock concept emerges from neutral theory, allowing scientists to estimate divergence times between species
• Challenges adaptationist thinking—not every genetic difference between species represents an adaptation

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Development and Gene Regulation

These theories emphasize that what genes do matters as much as what genes exist. Changes in developmental timing and gene expression can produce dramatic evolutionary innovations.

Evo-Devo (Evolutionary Developmental Biology)

• Regulatory gene changes can produce major morphological differences with minimal genetic change—same toolkit genes, different deployment
• Hox genes and other developmental regulators are highly conserved across animals, revealing deep evolutionary connections
• Heterochrony (changes in developmental timing) and heterotopy (changes in gene expression location) explain many evolutionary novelties

Epigenetic Inheritance

• Environmental factors alter gene expression without changing DNA sequence—through methylation, histone modification, and other mechanisms
• Transgenerational effects mean acquired characteristics can sometimes be inherited, challenging strict Weismannian barriers
• Phenotypic plasticity allows organisms to respond to environmental variation within a single generation

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Beyond Vertical Inheritance: Lateral and Symbiotic Evolution

Classical evolution assumes genes pass from parent to offspring. These theories reveal that genetic information can move sideways between organisms or arise from mergers between species.

Horizontal Gene Transfer

• Genes move between unrelated organisms—especially common in bacteria through transformation, transduction, and conjugation
• Antibiotic resistance spread is a prime example, with resistance genes jumping between bacterial species
• Challenges tree-like phylogenies—prokaryotic evolution looks more like a web or network than a branching tree

Endosymbiotic Theory

• Mitochondria and chloroplasts were once free-living bacteria—engulfed by ancestral eukaryotic cells in ancient symbiotic events
• Double membranes and circular DNA in these organelles provide key evidence for their bacterial origins
• Eukaryotic complexity arose through merger, not just mutation—a fundamentally different evolutionary pathway

Symbiogenesis

• New species arise through symbiotic mergers—not just competition, but cooperation drives major evolutionary transitions
• Lichens (fungi + algae/cyanobacteria) exemplify how symbiosis creates functionally new organisms
• Challenges "nature red in tooth and claw"—collaboration is as evolutionarily significant as competition

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Levels of Selection: Beyond the Individual

Classical theory focuses on individual fitness, but selection may operate at multiple levels simultaneously.

Group Selection Theory

• Selection can favor groups over individuals—groups with cooperative members may outcompete groups of selfish individuals
• Altruistic behaviors that reduce individual fitness but benefit the group become explainable under this framework
• Controversial but revived—multilevel selection theory provides a more nuanced modern version of this idea

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Quick Reference Table

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Self-Check Questions

1. Which two theories both challenge the idea that evolution is always gradual, and how do their explanations differ?

2. If you observed that two distantly related bacterial species share an identical antibiotic resistance gene, which theory best explains this—and why wouldn't classical vertical inheritance account for it?

3. Compare and contrast Evo-Devo and Epigenetic Inheritance: what do they share in their focus, and how do their timescales and mechanisms differ?

4. An FRQ asks you to explain how mitochondria provide evidence for a non-traditional mode of evolution. Which theory would you use, and what three pieces of evidence would you cite?

5. Why might a population geneticist use Neutral Theory to estimate when two species diverged, even if they believe natural selection shaped most visible traits?