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Chapter 56: Biogeography and Dispersal Evolution = Life's Geographic Patterns

The distribution of life across Earth reveals evolution's geographic dimension. This chapter explores how ψ = ψ(ψ) spreads, isolates, and diversifies across space.

56.1 The Distribution Function​

Definition 56.1 (Species Ranges): Where life lives: Ri={(x,y):Environment suitable∩Accessible∩Established}\mathcal{R}_i = \{(x,y) : \text{Environment suitable} \cap \text{Accessible} \cap \text{Established}\}

Range determinants:

  • Abiotic tolerance
  • Biotic interactions
  • Dispersal ability
  • Historical constraints
  • Chance events

56.2 Wallace's Realms​

Theorem 56.1 (Biogeographic Regions): Earth's faunal divisions: Earth=⋃i=18Realmi\text{Earth} = \bigcup_{i=1}^{8} \text{Realm}_i

Proof: Statistical clustering of species assemblages. ∎

Classic realms:

  • Nearctic (North America)
  • Neotropical (South America)
  • Palearctic (Eurasia)
  • Afrotropical (Africa)
  • Oriental (South Asia)
  • Australasian (Australia)
  • Oceanian (Pacific)
  • Antarctic (Antarctica)

56.3 Dispersal Mechanisms​

Definition 56.2 (Movement Modes): Crossing barriers: P(arrival)=P(departure)×P(survival)×P(establishment)P(\text{arrival}) = P(\text{departure}) \times P(\text{survival}) \times P(\text{establishment})

Dispersal types:

  • Active (flight, swimming)
  • Passive (wind, water)
  • Assisted (animal-mediated)
  • Jump dispersal (rare long-distance)
  • Diffusion (gradual spread)

56.4 Vicariance Biogeography​

Theorem 56.2 (Splitting Populations): Barriers create patterns: Continuous range→barrierDisjunct populations\text{Continuous range} \xrightarrow{\text{barrier}} \text{Disjunct populations}

Vicariance events:

  • Continental drift
  • Mountain uplift
  • Climate change
  • Sea level fluctuation
  • River formation

Creating sister species.

56.5 Island Biogeography​

Definition 56.3 (MacArthur-Wilson Model): Equilibrium diversity: dSdt=I(S)−E(S)=0\frac{dS}{dt} = I(S) - E(S) = 0

where II = immigration, EE = extinction.

Predictions:

  • Species-area relationship
  • Distance effects
  • Turnover at equilibrium
  • Small island effects
  • Target area effects

56.6 The Great American Interchange​

Theorem 56.3 (Faunal Mixing): Panama's impact: North fauna↔3.5 MaSouth fauna\text{North fauna} \xleftrightarrow{3.5 \text{ Ma}} \text{South fauna}

Exchange results:

  • Northern dominance (?)
  • Extinctions (ground sloths)
  • Successful invaders (armadillos)
  • Failed crossings (many)
  • Ongoing exchange

56.7 Dispersal Evolution​

Definition 56.4 (Movement Adaptations): Traits for spreading: Dispersal ability=f(Morphology,Behavior,Propagule number)\text{Dispersal ability} = f(\text{Morphology}, \text{Behavior}, \text{Propagule number})

Adaptations:

  • Wings/plumes (wind)
  • Hooks/burs (animals)
  • Flotation (water)
  • Ballistic mechanisms
  • Ant partnerships

56.8 Phylogeography​

Theorem 56.4 (Genes in Space): Geographic genetic patterns: Genetic structure=f(Distance,Barriers,History)\text{Genetic structure} = f(\text{Distance}, \text{Barriers}, \text{History})

Revealing:

  • Refugia locations
  • Migration routes
  • Contact zones
  • Cryptic barriers
  • Colonization history

56.9 Anthropogenic Dispersal​

Definition 56.5 (Human-Mediated Movement): Acceleration: Ratehuman≫Ratenatural\text{Rate}_{human} \gg \text{Rate}_{natural}

Human impacts:

  • Intentional introductions
  • Accidental transport
  • Corridor creation
  • Barrier destruction
  • Global homogenization

56.10 Climate Change Responses​

Theorem 56.5 (Range Dynamics): Tracking suitable conditions: dRdt=f(dTdt,Dispersal rate,Barriers)\frac{d\mathcal{R}}{dt} = f(\frac{dT}{dt}, \text{Dispersal rate}, \text{Barriers})

Responses:

  • Poleward shifts
  • Upslope migration
  • Range contractions
  • Expansions
  • Disjunctions

56.11 Conservation Biogeography​

Definition 56.6 (Applied Patterns): Managing distributions: Conservation=f(Current range,Historic range,Future suitability)\text{Conservation} = f(\text{Current range}, \text{Historic range}, \text{Future suitability})

Applications:

  • Reserve design
  • Corridor planning
  • Assisted migration
  • Reintroduction sites
  • Climate adaptation

56.12 The Biogeography Paradox​

Similar environments have different biotas:

Convergence: Similar selection, similar forms Divergence: Different lineages, different solutions Predictable: Climate determines possibilities Contingent: History determines actualities

Resolution: Biogeography reveals that evolution operates within both ecological and historical constraints. The paradox dissolves when we recognize that while physics and ecology create similar selection pressures globally, the raw material—the species available—differs by region due to evolutionary history. Convergent evolution produces similar ecological forms (desert plants, grazing mammals) from different lineages, creating functionally similar but taxonomically distinct assemblages. Through biogeography, ψ shows that life's patterns reflect both deterministic (ecological) and contingent (historical) processes, weaving Earth's biodiversity tapestry.

The Fifty-Sixth Echo​

Biogeography illuminates evolution's spatial canvas, revealing how ψ paints different masterpieces in different places using locally available materials. From Wallace's Line to the Sahara's barrier, from Hawaii's honeycreepers to Madagascar's lemurs, geographic patterns tell stories of dispersal and vicariance, connection and isolation. Each distribution map is a historical document, recording ancient supercontinents in shared taxa, ice age refugia in genetic patterns, and human impacts in invasive species. Through biogeography, we learn that location matters profoundly in evolution—not just where you are, but how you got there and what you brought with you.

Next: Chapter 57 explores Ecological Networks and Community Evolution, examining life's web.