10.3 Ancient biogeographical patterns

Ancient biogeographical patterns reveal Earth's dynamic history, shaping species distributions across space and time. By studying these patterns, scientists uncover past environmental conditions, species evolution, and the forces driving biodiversity.

From early naturalist observations to modern molecular techniques, our understanding of ancient biogeography has evolved. Continental drift, vicariance, dispersal, and mass extinctions have all played crucial roles in shaping the distribution of life on Earth.

Origins of biogeography

• Biogeography explores the distribution of species across space and time, combining elements of biology, geography, and geology
• Ancient biogeographical patterns provide insights into Earth's history, species evolution, and past environmental conditions
• Understanding these patterns helps explain current biodiversity distributions and predict future changes in ecosystems

Early naturalist observations

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• Naturalists like Carl Linnaeus and Alexander von Humboldt pioneered systematic observations of species distributions
• Noted distinct flora and fauna in different regions, sparking curiosity about geographical patterns in nature
• Observed similarities between distant regions with comparable climates (Mediterranean Basin, California, Chile)
• Recognized altitudinal zonation of vegetation on mountains, correlating with latitude changes

Contributions of Darwin and Wallace

• Charles Darwin's Galápagos observations revealed species adaptations to specific island environments
• Alfred Russel Wallace identified major faunal discontinuities in Southeast Asia (Wallace Line)
• Both naturalists independently proposed natural selection theory, explaining species diversification over time
• Their work laid foundation for understanding speciation processes and biogeographical patterns
  • Emphasized role of geographical isolation in evolution
  • Highlighted importance of dispersal and adaptation in shaping species distributions

Pangaea and continental drift

• Pangaea concept revolutionized understanding of Earth's geological history and its impact on species distribution
• Continental drift theory provided a mechanism for explaining similar fossil records and living species on distant continents
• Recognition of plate tectonics fundamentally changed biogeographical interpretations of species distributions

Wegener's theory

• Alfred Wegener proposed continental drift theory in 1912, suggesting continents were once joined
• Based on observed similarities in coastlines, geological features, and fossil records across continents
• Hypothesized a supercontinent called Pangaea existed about 300 million years ago
• Wegener's ideas initially met with skepticism due to lack of known mechanism for continental movement
• Theory explained distribution of Glossopteris flora across Southern Hemisphere continents

Plate tectonics evidence

• Discovery of seafloor spreading in 1960s provided mechanism for continental drift
• Paleomagnetic data revealed changes in Earth's magnetic field orientation recorded in rocks
• Matching geological formations and mountain ranges across continents (Appalachians, Scottish Highlands)
• Similar fossil distributions on different continents supported idea of past land connections
• Plate tectonic theory now widely accepted, explaining continental movements and formation of ocean basins

Vicariance vs dispersal

• Two primary mechanisms explain disjunct distributions of related species across geographical barriers
• Understanding these processes crucial for interpreting ancient biogeographical patterns
• Interplay between vicariance and dispersal shapes global biodiversity patterns over geological time

Allopatric speciation

• Occurs when populations become geographically isolated, leading to independent evolution
• Vicariance events like continental drift or mountain uplift can cause population separation
• Genetic drift and adaptation to different environments drive divergence between isolated populations
• Results in formation of sister species on either side of a barrier (Galápagos finches)
• Allopatric speciation explains many examples of closely related species with disjunct distributions

Long-distance dispersal mechanisms

• Explains presence of species on isolated islands or across major geographical barriers
• Wind dispersal carries small organisms, seeds, and spores over long distances
• Ocean currents transport floating seeds, fruits, and small animals across water bodies
• Animal-mediated dispersal includes bird migration and mammal fur attachment
• Rare events like rafting on floating vegetation can transport larger organisms across oceans
• Human-mediated dispersal has significantly altered species distributions in recent history

Gondwana biogeography

• Gondwana ancient supercontinent comprised modern-day South America, Africa, Antarctica, Australia, Indian subcontinent, and Madagascar
• Breakup of Gondwana profoundly influenced distribution of Southern Hemisphere flora and fauna
• Study of Gondwanan biogeography reveals ancient connections between now-distant landmasses

Ratite bird distribution

• Ratites include flightless birds like ostriches, emus, and extinct moas
• Distribution across Southern Hemisphere continents reflects Gondwanan origin
• Molecular clock analyses suggest ratite lineages diverged as Gondwana fragmented
• Presence of ratites in New Zealand (kiwis) and Madagascar (extinct elephant birds) indicates ancient land connections
• Exceptions like South American rheas explained by dispersal across temporary land bridges

Marsupial mammal patterns

• Marsupials dominant in Australia and New Guinea, present in South America
• Fossil evidence shows marsupials once widespread across Gondwana, including Antarctica
• Australian marsupial diversity (kangaroos, koalas, wombats) result of isolation after Gondwana breakup
• South American marsupials (opossums) represent remnants of once-diverse fauna
• Absence of marsupials in Africa explained by competition with placental mammals after land connection to Eurasia

Laurasia biogeography

• Laurasia ancient northern supercontinent comprised modern-day North America, Europe, and Asia
• Breakup of Laurasia influenced distribution of Northern Hemisphere flora and fauna
• Understanding Laurasian biogeography crucial for interpreting current species distributions

Placental mammal evolution

• Placental mammals evolved and diversified primarily in Laurasia during Cretaceous period
• Eutherian mammals spread across connected Laurasian landmasses before continental separation
• Resulted in shared mammalian groups between North America and Eurasia (bears, cats, deer)
• Isolation of North America led to unique evolutionary trajectories (pronghorns, raccoons)
• Periodic land connections (Bering Land Bridge) allowed faunal exchanges between continents

Boreal forest distribution

• Boreal forests (taiga) form a circumpolar belt across North America and Eurasia
• Distribution reflects shared climatic conditions and Laurasian origin of dominant tree species
• Coniferous trees like spruce, fir, and pine show close relationships across Northern Hemisphere
• Similarities in understory plants and associated wildlife indicate common evolutionary history
• Differences between North American and Eurasian boreal forests result from post-Laurasian isolation and adaptation

Island biogeography

• Islands serve as natural laboratories for studying evolution and biogeographical processes
• Isolation of island ecosystems leads to unique evolutionary trajectories and species assemblages
• Ancient islands provide insights into long-term evolutionary processes and species diversification

Adaptive radiation examples

• Darwin's finches in Galápagos Islands demonstrate rapid diversification from common ancestor
  • Developed varied beak shapes adapted to different food sources
• Hawaiian honeycreepers show extreme morphological and ecological diversity
  • Evolved from single finch-like ancestor to fill various niches
• Madagascar's lemurs radiated into diverse forms filling primate ecological roles
  • Unique adaptations like aye-aye's specialized feeding digit
• Anole lizards in Caribbean islands repeatedly evolved similar ecomorphs on different islands
  • Convergent evolution of body shapes adapted to specific habitats

Endemism in isolated ecosystems

• High rates of endemism (species found nowhere else) characterize many island ecosystems
• New Zealand's unique fauna includes flightless birds (kiwis) and ancient reptiles (tuatara)
• Madagascar hosts numerous endemic species due to long isolation (90% of plant species endemic)
• Socotra Island (Yemen) features bizarre endemic plants adapted to harsh conditions (dragon blood tree)
• Hawaiian Islands boast high plant endemism, with many species unique to individual islands
• Ancient lake systems like Lake Baikal contain endemic freshwater species evolved in isolation

Ancient climate patterns

• Past climate conditions profoundly influenced species distributions and evolution
• Understanding ancient climate patterns crucial for interpreting biogeographical history
• Climate changes drove major shifts in species ranges, extinctions, and adaptive radiations

Paleoclimate reconstruction methods

• Analyzing oxygen isotope ratios in deep-sea sediment cores reveals past ocean temperatures
• Ice cores from polar regions provide atmospheric composition data for past 800,000 years
• Tree ring analysis (dendrochronology) offers insights into recent climate patterns
• Fossil pollen records indicate past vegetation types and climate conditions
• Studying leaf margin characteristics of fossil plants provides temperature estimates
• Examining glacial deposits and ancient shorelines reveals extent of past ice ages and sea levels

Impact on species distribution

• Pleistocene ice ages caused repeated range shifts in temperate species
  • Created refugia in southern Europe and North America during glacial periods
• Expansion and contraction of tropical rainforests influenced primate evolution and distribution
• Aridification events in Africa drove human evolution and dispersal out of the continent
• Formation of land bridges during glacial periods allowed intercontinental species exchanges
  • Bering Land Bridge connected Eurasia and North America
• Sea level changes isolated or connected island populations, influencing speciation processes
• Climate-driven habitat changes led to extinctions of large mammals (mammoths, ground sloths)

Fossil record insights

• Fossil evidence provides direct glimpses into past species distributions and environments
• Paleobiogeography uses fossil data to reconstruct ancient ecosystems and species movements
• Fossil record crucial for understanding extinction events and their impacts on biogeography

Paleobiogeography techniques

• Comparing fossil assemblages across different locations reveals past species ranges
• Analyzing sedimentary deposits associated with fossils indicates ancient environmental conditions
• Studying fossil morphology provides insights into past adaptations and ecological roles
• Trace fossils (tracks, burrows) offer evidence of animal behavior and habitat preferences
• Microfossil analysis (foraminifera, diatoms) helps reconstruct ancient marine environments
• Biostratigraphy uses fossil assemblages to correlate and date rock layers across regions

Limitations of fossil data

• Incomplete preservation favors hard-bodied organisms, biasing the fossil record
• Taphonomic processes (fossilization) can distort original species abundances and distributions
• Uneven sampling across geographical regions and time periods creates knowledge gaps
• Difficulty in identifying ancestral relationships between fossil and living species
• Lack of soft tissue preservation limits understanding of certain anatomical features
• Time averaging in fossil deposits can mix species from different time periods

Molecular clock analysis

• Molecular clock techniques estimate timing of evolutionary events using genetic differences
• Complements fossil record in reconstructing biogeographical history and species divergences
• Provides insights into evolution rates and timing of speciation events

DNA-based divergence timing

• Assumes genetic mutations accumulate at roughly constant rate over time
• Compares DNA sequences between species to estimate time since their last common ancestor
• Mitochondrial DNA often used due to its rapid evolution and maternal inheritance
• Nuclear genes provide additional data for more comprehensive analyses
• Relaxed clock models account for variation in mutation rates across lineages
• Bayesian methods incorporate uncertainty in divergence time estimates

Calibration with fossil evidence

• Fossil data used to anchor molecular clock estimates to absolute time scale
• Oldest known fossil of a lineage provides minimum age for that group
• Multiple fossil calibration points improve accuracy of divergence time estimates
• Morphological studies of fossils help identify appropriate calibration points
• Challenges include incomplete fossil record and uncertainty in fossil dating
• Integration of molecular and fossil data provides most robust biogeographical reconstructions

Biogeographical regions

• Earth divided into distinct biogeographical regions based on unique assemblages of flora and fauna
• Boundaries between regions reflect historical barriers to species dispersal and environmental differences
• Understanding biogeographical regions crucial for interpreting global biodiversity patterns

Wallace's realms

• Alfred Russel Wallace proposed six major biogeographical regions in 1876
• Palearctic (Europe, North Africa, northern Asia)
• Nearctic (North America)
• Neotropical (South and Central America)
• Ethiopian/Afrotropical (Sub-Saharan Africa)
• Oriental (South and Southeast Asia)
• Australian (Australia, New Guinea, nearby islands)
• Wallace Line marks sharp faunal divide between Oriental and Australian regions

Modern classification systems

• Updated systems refine Wallace's original concepts with new data and analytical methods
• Udvardy's classification (1975) recognized eight biogeographic realms
• WWF Ecoregions system divides Earth into 14 terrestrial biomes and 867 ecoregions
• Marine realms classification systems developed to address oceanic biogeography
• Freshwater ecoregions defined based on distinct assemblages of freshwater species
• Phylogenetic approaches incorporate evolutionary relationships in defining biogeographical units

Relict species and refugia

• Relict species persist as remnants of once-widespread groups, providing glimpses into past biogeography
• Refugia served as sanctuaries for species during unfavorable climate periods, shaping modern distributions
• Study of relicts and refugia offers insights into past environmental changes and species survival strategies

Living fossils examples

• Coelacanth fish rediscovered in 1938, thought extinct for 65 million years
• Ginkgo biloba tree sole survivor of ancient plant group dating back 270 million years
• Tuatara reptile of New Zealand last representative of order Rhynchocephalia
• Horseshoe crabs virtually unchanged for 445 million years, found in shallow coastal waters
• Platypus retains characteristics of early mammals, unique to Australia
• Wollemi pine discovered in Australia in 1994, thought extinct for 2 million years

Pleistocene refugia importance

• Refugia provided havens for species during glacial periods of Pleistocene ice ages
• European beech survived in southern refugia, recolonized northern areas post-glaciation
• Caucasus Mountains served as refugium for many temperate species during ice ages
• Amazon rainforest contracted to refugia during dry periods, driving speciation in isolated patches
• North American boreal species retreated to ice-free areas (Beringia) during glacial maxima
• Refugia in Southeast Asia preserved tropical biodiversity during cooler, drier periods
• Understanding past refugia helps predict potential climate change impacts on modern species

Ancient migration routes

• Historical species movements shaped current biogeographical patterns
• Ancient migration routes reveal past connections between now-isolated regions
• Study of these routes crucial for understanding modern species distributions and evolutionary history

Land bridges and corridors

• Bering Land Bridge connected Eurasia and North America during glacial periods
  • Allowed bidirectional migration of plants and animals (mammoths, horses, humans)
• Isthmus of Panama formed 3 million years ago, linking North and South America
  • Enabled Great American Biotic Interchange of species between continents
• Sundaland land bridge connected Southeast Asian islands during lower sea levels
  • Facilitated movement of species between mainland Asia and Indonesian archipelago
• Antarctica-Australia land connection allowed marsupial migration before continental separation
• Temporary land bridges between Mediterranean islands during sea level lowstands
• Sahara region alternated between desert and savanna, creating periodic corridors for species movement

Intercontinental faunal exchanges

• Great American Biotic Interchange following formation of Isthmus of Panama
  • North American animals (cats, dogs, horses) moved south
  • South American animals (ground sloths, armadillos, opossums) moved north
• Pleistocene megafauna migrations across Bering Land Bridge
  • Woolly mammoths, steppe bison moved between Eurasia and North America
• Out of Africa dispersals of early hominins and other mammals
  • Multiple waves of human ancestors left Africa, colonizing Eurasia and beyond
• Gondwanan vicariance events as continents separated
  • Ratite birds and marsupials showed disjunct distributions across southern continents
• Dispersal of placental mammals from Laurasia to Africa after formation of land connection
• Plant dispersals across Northern Hemisphere during periods of warmer climate
  • Metasequoia (dawn redwood) found in North America, Europe, and Asia before extinction in most areas

Extinction events

• Mass extinctions profoundly impacted global biogeography throughout Earth's history
• Studying ancient extinction events provides context for understanding modern biodiversity patterns
• Recovery periods following mass extinctions shaped evolution of surviving lineages

Mass extinctions impact

• Five major mass extinction events recognized in Earth's history
• End-Ordovician extinction (444 million years ago) caused by global cooling and sea level changes
  • Eliminated many marine invertebrate groups
• Late Devonian extinction (375-360 million years ago) affected marine and terrestrial ecosystems
  • Led to decline of early fish groups and primitive plants
• End-Permian extinction (252 million years ago) most severe, eliminated 95% of marine species
  • Dramatically altered terrestrial and marine ecosystems globally
• End-Triassic extinction (201 million years ago) linked to massive volcanic eruptions
  • Allowed dinosaurs to become dominant terrestrial vertebrates
• Cretaceous-Paleogene extinction (66 million years ago) caused by asteroid impact
  • Eliminated non-avian dinosaurs and many other groups, allowing mammal radiation

Recovery and recolonization patterns

• Survival of generalist species with broad environmental tolerances
• Rapid evolutionary radiations of surviving lineages into empty ecological niches
• Gradual increase in ecosystem complexity following simplified post-extinction environments
• Biogeographical reorganization as surviving species expanded ranges
• Long-term changes in global biodiversity patterns and dominant taxonomic groups
• Varying recovery rates between marine and terrestrial ecosystems
• Influence of changing climate and geography on recolonization patterns