2.6 Vicariance and geodispersal

Vicariance and geodispersal are key processes shaping global biodiversity. Vicariance occurs when populations are separated by physical barriers, leading to speciation. Geodispersal involves range expansion as barriers disappear, allowing species to spread to new areas.

These processes are driven by geological events like continental drift, sea level changes, and mountain formation. Understanding vicariance and geodispersal helps explain current species distributions, evolutionary patterns, and how biodiversity may respond to future environmental changes.

Definition of vicariance

• Vicariance describes the geographic separation of populations due to physical barriers leading to speciation
• Plays a crucial role in shaping global biodiversity patterns and species distributions
• Fundamental concept in world biogeography explaining how similar species occur in different parts of the world

Types of vicariant events

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• Tectonic events split continents and create new physical barriers (formation of the Isthmus of Panama)
• Climate changes cause habitat fragmentation and isolate populations (glacial periods)
• Mountain range formation divides previously continuous habitats (uplift of the Andes)
• Sea level fluctuations create or remove land bridges between regions (Bering land bridge)

Importance in biogeography

• Explains disjunct distributions of related species across different continents
• Provides a mechanism for allopatric speciation and evolution of endemic species
• Helps reconstruct historical biogeography and past continental configurations
• Influences current biodiversity patterns and species richness in different regions

Geodispersal concept

• Refers to the expansion of species ranges due to the removal of geographic barriers
• Contrasts with vicariance by focusing on the joining of previously separated areas
• Significant in understanding species distributions and biogeographic patterns over geological time

Mechanisms of geodispersal

• Formation of land bridges connects previously isolated landmasses (Isthmus of Panama)
• Sea level drops expose continental shelves and create new dispersal routes
• Tectonic collisions join separate landmasses and their biotas (India-Asia collision)
• Climate changes create corridors through previously inhospitable regions (greening of the Sahara)

Geodispersal vs vicariance

• Geodispersal results in range expansion while vicariance leads to range fragmentation
• Both processes can occur cyclically as barriers form and disappear over geological time
• Geodispersal often increases species richness in an area while vicariance may lead to speciation
• Understanding the interplay between these processes helps explain complex biogeographic patterns

Geological processes

• Fundamental drivers of both vicariance and geodispersal events in world biogeography
• Operate over long time scales shaping the Earth's surface and influencing species distributions

Plate tectonics and vicariance

• Continental drift separates landmasses and creates vicariant events (breakup of Gondwana)
• Seafloor spreading forms new oceanic barriers between populations
• Subduction and mountain building create topographic barriers (Andes formation)
• Island arc formation leads to stepping-stone dispersal and subsequent isolation

Sea level changes

• Glacial-interglacial cycles cause global sea level fluctuations
• Low sea levels expose land bridges facilitating geodispersal (Bering land bridge)
• High sea levels isolate populations on islands or high elevation areas
• Influences coastal habitat availability and marine species distributions

Biological implications

• Vicariance and geodispersal significantly impact evolutionary processes and biodiversity patterns
• Understanding these processes helps explain current species distributions and predict future changes

Allopatric speciation

• Geographic isolation leads to independent evolution of separated populations
• Genetic drift and adaptation to local conditions drive divergence
• Reproductive isolation develops over time preventing gene flow if populations reconnect
• Results in sister species on different sides of a barrier (Darwin's finches on Galápagos Islands)

Genetic divergence

• Vicariance events initiate genetic differentiation between isolated populations
• Mutation rates and selection pressures influence the speed of divergence
• Genetic markers used to estimate divergence times and reconstruct biogeographic history
• Phylogeographic studies reveal population structure and historical demographic changes

Case studies

• Specific examples illustrate the principles of vicariance and geodispersal in world biogeography
• Provide evidence for the impact of geological and climatic events on species distributions

Gondwanan vicariance

• Breakup of the supercontinent Gondwana led to vicariant speciation events
• Ratite birds (ostriches, emus, kiwis) evolved on different southern continents
• Marsupial mammals diversified in Australia and South America
• Plant families show disjunct distributions across former Gondwanan landmasses (Nothofagus trees)

Marine organism dispersal

• Ocean currents facilitate long-distance dispersal of marine species
• Periodic land bridge formation allows exchange between ocean basins (Great American Biotic Interchange)
• Vicariance occurs when ocean basins become isolated (closure of the Tethys Sea)
• Larval dispersal patterns influence genetic connectivity of marine populations

Methods of analysis

• Various analytical techniques help researchers study vicariance and geodispersal patterns
• Combining multiple methods provides a more comprehensive understanding of biogeographic history

Phylogenetic approaches

• Construct evolutionary trees to infer relationships between species
• Compare phylogenies with geological events to identify potential vicariance or dispersal events
• Biogeographic methods like Dispersal-Vicariance Analysis (DIVA) optimize ancestral distributions
• Parsimony and maximum likelihood methods used to reconstruct biogeographic scenarios

Molecular clock techniques

• Estimate divergence times between lineages using genetic data
• Calibrate molecular clocks with fossil evidence or geological events
• Help distinguish between ancient vicariance and more recent long-distance dispersal
• Relaxed clock models account for rate variation across lineages and through time

Vicariance biogeography

• Theoretical framework emphasizing the role of geological events in shaping species distributions
• Developed as an alternative to dispersalist explanations for biogeographic patterns

Historical development

• Emerged in the 1970s with the acceptance of plate tectonic theory
• Pioneered by researchers like Léon Croizat and Gareth Nelson
• Emphasized pattern-based approaches to biogeography
• Led to the development of cladistic biogeography methods

Criticisms and limitations

• Overemphasis on vicariance at the expense of dispersal explanations
• Difficulty in distinguishing between vicariance and geodispersal in some cases
• Challenges in dating divergence events accurately
• Neglect of ecological factors influencing species distributions

Dispersal-vicariance analysis

• Analytical method combining elements of both dispersalist and vicariance approaches
• Aims to reconstruct ancestral distributions and biogeographic events

DIVA method

• Optimizes ancestral distributions on phylogenetic trees
• Assigns costs to different biogeographic events (vicariance, dispersal, extinction)
• Finds the most parsimonious explanation for current distributions
• Implemented in software packages like RASP (Reconstruct Ancestral State in Phylogenies)

Applications in research

• Used to study historical biogeography of various plant and animal groups
• Helps identify major biogeographic events and dispersal routes
• Combines with molecular dating to test hypotheses about timing of events
• Informs conservation strategies by revealing historical connectivity between populations

Biogeographic patterns

• Observable distribution patterns of species and higher taxa across the globe
• Result from complex interactions of historical and ecological processes

Disjunct distributions

• Discontinuous ranges of closely related taxa separated by large geographic distances
• Often explained by vicariance events or long-distance dispersal
• Amphi-Atlantic distributions in plants suggest former land connections
• Circum-Antarctic distributions in marine organisms reflect ancient vicariance and dispersal

Endemism and vicariance

• High levels of endemism often associated with long-term isolation due to vicariance
• Biodiversity hotspots frequently result from vicariant events (Madagascar, New Caledonia)
• Relict species represent remnants of formerly widespread groups isolated by vicariance
• Island archipelagos showcase endemism patterns related to vicariance and dispersal (Hawaiian honeycreepers)

Conservation implications

• Understanding vicariance and geodispersal processes informs conservation strategies
• Historical biogeography provides context for current biodiversity patterns and future changes

Habitat fragmentation

• Anthropogenic fragmentation mimics natural vicariance processes
• Disrupts gene flow between populations leading to genetic isolation
• May accelerate speciation in some cases but often threatens population viability
• Conservation corridors attempt to mitigate fragmentation effects

Climate change effects

• Alters species distributions and creates new opportunities for dispersal or isolation
• May lead to novel combinations of species as ranges shift
• Threatens species adapted to specific climatic conditions or with limited dispersal abilities
• Understanding past responses to climate change helps predict future biodiversity patterns