biogeography

Biogeography: Study of Geographic Distributions of Organisms

• Focuses on the geographic distributions of organisms with historical and modern implications. It integrates ecology, evolution, and Earth sciences to understand why species are found where they are.

• Notably influenced by Charles Darwin, as demonstrated in his observations while aboard the H.M.S. Beagle.

  • Darwin remarked on certain facts regarding the distribution of organisms and geological connections between past and present inhabitants of South America. His observations of unique species on oceanic islands (like the Galapagos) and the distinct yet related faunas across different geographic regions were fundamental to the development of his theory of evolution by natural selection.

  • (Darwin $1859$, first sentence in On the Origin of Species). This seminal work laid the groundwork for integrating patterns of distribution with evolutionary processes.

Historical vs. Ecological Biogeography

• Historical Biogeography: Refers to the study of past organisms and their distributions, exploring how evolutionary and geological events over long timescales (millions of years) have shaped the current distribution of species. This includes phenomena like continental drift, glaciation cycles, and ancient land bridges, which dictate patterns of speciation and extinction.

• Ecological Biogeography: Focuses on current distributions in relation to immediate environmental conditions (e.g., climate, topography, species interactions, resource availability). It investigates the ecological processes that govern where species can currently live over shorter, ecological timescales.

Biogeographic Realms

Definition of Biogeographic Realms

• Major geographic divisions of biodiversity, characterized by long periods of organismal isolation. These realms represent vast areas of Earth's land surface that share a distinctive assemblage of species, primarily due to long-term geological and climatic isolation. They are characterized by high levels of endemism and unique biodiversity patterns.

• Introduced by Alfred Russel Wallace in $1862$; Wallace's Line exemplifies significant barriers in biogeography, notably separating organisms of Asian origin from those of Australian origin.

Current Biogeographic Realms

• Neotropical

• Nearctic

• Ethiopian

• Palearctic

• Oriental

• Australian

Pleistocene Land Connections in SE Asia

• Wallace’s Line is a significant water barrier separating the Oriental from Australian realms. It represents a deep-water channel that maintained its integrity even during glacial maxima when sea levels dropped significantly, preventing the exchange of terrestrial fauna between the Sunda Shelf (part of Asia, including Borneo, Java, Sumatra) and the Sahul Shelf (part of Australia, including New Guinea and Australia mainland). This persistent water barrier explains the dramatic difference in flora and fauna on either side.

Continental Plates

Importance

• Essential for understanding the historical biogeography of organisms. Plate tectonics, the movement of Earth's lithospheric plates, is the dominant geological force driving long-term changes in landmass configuration. This movement directly influences historical biogeography by creating and breaking land connections, forming mountain ranges, and opening/closing ocean basins, thereby shaping the evolutionary trajectories and distributions of species over millions of years.

Historical Context

• Key events include the breakup of supercontinents:

  • Pangaea (began breaking apart around $175$ Mya)

  • Gondwana (southern portion)

  • Laurasia (northern portion)

Historical Biogeography Mechanisms

Vicariance vs. Dispersal

• Vicariance:

  • A barrier is imposed, resulting in an existing population being split into two or more geographically isolated populations by geological changes (e.g., mountain range uplift, river formation, land bridge submergence, continental drift). Over time, these isolated populations diverge genetically due to different selective pressures and genetic drift, potentially leading to speciation.

• Dispersal:

  • Movement of individuals or populations across barriers or distances from their place of origin to new areas. This can be active (e.g., flight, walking) or passive (e.g., rafting on vegetation, wind dispersal of spores). If successful, dispersal can lead to colonization of new habitats and subsequent diversification, often across pre-existing barriers.

Prediction of Vicariance

• Species relationships can often be inferred through land area relationships and historical split times. In vicariance, the phylogenetic relationships among species should mirror the geological history of the areas they inhabit. For instance, if a continent splits, the speciation events of organisms on those landmasses should coincide with the timing of these geological splits.

• Example Relationships:

  • Species $1$ (Sp.$1$) to Species $5$ (Sp.$5$) show historical split times based on geographical isolation:

  • Eurasia: split $105$ Mya, leading to species divergence relative to other landmasses.

  • North America (NA): split $130$ Mya from other landmasses.

  • Madagascar (MAD): split $100$ Mya from the India-Antarctica landmass.

  • Africa: split $170$ Mya from South America (early Gondwana fragmentation).

Dinosaur Biogeography

Historical Context

• Evidence shows a connection between dinosaur phylogeny and continental splits approximately: Studies of dinosaur phylogeny, particularly large predatory theropods, demonstrate strong correlations between their evolutionary divergences and the sequential breakup of the supercontinent Pangaea. For example, the distribution of certain clades of carnivorous dinosaurs aligns with continental splits approximately $170$ Mya (early Gondwana fragmentation) and $105$ Mya (further separation of Gondwana and Laurasian continents), providing substantial support for the concept of vicariance as a major force in their diversification.

Dispersal of Dinosaurs

• Example of polar dispersal of ceratopsians (horned dinosaurs) from Asia to North America during the Late Cretaceous period, indicating their ability to cross substantial geographical distances, possibly via temporary land bridges or more continuous polar landmasses during warmer periods.

Lungfish Biogeography: Vicariance or Dispersal?

Considerations

• A study by Heinicke et al. ($2009$) indicated a split of approximately $120$ Mya based on molecular evidence, strongly supporting vicariance. The global distribution of lungfish (Dipnoi), found in South America, Africa, and Australia, presents a classic biogeographical puzzle. Their comprehensive molecular phylogenetic study utilized a robust dataset including $6$ nuclear and mitochondrial gene markers and $10$ fossil calibration points to estimate divergence times. Their findings indicated that the deepest splits among extant lungfish lineages occurred approximately $120$ Mya, a timeframe highly consistent with the breakup of Gondwana.

• Specimens analyzed with $6$ genes and $10$ fossil calibrations provided compelling support for vicariance as the primary mechanism explaining the disjunct distribution of lungfish, rather than long-distance dispersal.

Mammalian Vicariance and Dispersal

Placental Mammals

• Molecular clock timetree indicates a split circa $103$ Mya (with a $95$ $-$ $114$ Mya range). This predates the full breakup of Gondwana and the formation of modern continents, implying that some placental mammal ancestors were distributed across supercontinents before vicariance isolated them, while significant diversification also occurred alongside subsequent dispersal events.

Dispersal Mechanisms

• Examples of effective dispersers include organisms possessing traits that allow them to overcome environmental barriers, such as being transported by wind, water (e.g., rafting), and possessing desiccation resistance.

• In the case of Galapagos plants:

  • $60$% are bird-transported seeds, highlighting the critical role of avian vectors in colonizing geographically isolated islands.

  • $31$% moved via wind, leveraging small size or specialized structures.

  • $9$% through water, often involving floating seeds or fruit.

Key Terms

• Flotsam: Floating wreckage, debris, or vegetation (e.g., large mats of driftwood or tangled roots) that passively drifts on ocean currents. It can serve as a natural raft, transporting small animals, plants, and their propagules across vast oceanic distances, often leading to founder events on isolated landmasses.

• Jetsam: Items or materials (often lighter ones) that are cast overboard from a vessel in distress to lighten the load, or simply objects that are discarded. In a broader ecological sense, materials that can be easily picked up and transported by wind or water after being discarded or released from their source, sometimes inadvertently facilitating dispersal of small organisms or propagules, though flotsam is generally more pertinent for direct biological transport over water.

• Corridor: A geographical pathway, such as a continuous stretch of suitable habitat, that allows for relatively unimpeded movement and genetic exchange of multiple species between two larger habitat patches or regions. Corridors facilitate broad-scale migration and colonization.

• Filter: A habitat connection or geographical pathway that selectively permits the passage of some species while hindering or preventing others. Filters are discontinuous or contain environmental conditions that only certain species can tolerate, thus acting as selective barriers to movement.

Galapagos Passion Flower

• A case study showing that these plants experienced $380$ colonization events, indicating high dispersal capabilities and successful establishment on multiple islands.

Flotsam Dispersal

• Evident in various photos and studies; for instance: A compelling modern example occurred in $1995$, when multiple green iguanas (Iguana iguana) were sighted on the shores of Anguilla, a Caribbean island. Genetic analyses and circumstantial evidence strongly suggested these individuals had arrived by riding a mat of uprooted trees and debris (flotsam) washed away from Guadeloupe, an island over $300$ km away, following a hurricane. This event demonstrated that viable populations can be established through such infrequent, long-distance dispersal events.

Great American Biotic Interchange

Corridor/Filter Dispersal

• Land mammals migrated between North and South America approximately $3$ Mya. This significant paleozoogeographic event (GABI) was driven by the formation of the Isthmus of Panama, which transformed what was a marine barrier into a terrestrial corridor between North and South America. This allowed for extensive two-way migration of land mammals, reptiles, amphibians, and plants, fundamentally reshaping the biodiversity of both continents.

• Example Organisms:

  • South American invaders: Sloth (e.g., Megatherium), anteater, armadillo, porcupine, and opossum moved northward.

  • North American invaders: Raccoon, rat, cougar, llama, horse, camel, and bear migrated southward. This interchange led to significant faunal mixing and competitive interactions, with a greater net flow of species from North to South America, though both continents experienced extinctions and new radiations.

The Global Environment

Heat and Air Pressure

• Describes the climatic impact of the equator. The unequal heating of Earth's surface, particularly the greater solar radiation received at the equator compared to the poles, drives global atmospheric circulation. This differential heating creates areas of rising warm, moist air (low pressure) near the equator and sinking cool, dry air (high pressure) at higher latitudes. These pressure gradients, in turn, generate winds and influence precipitation patterns, leading to major climate zones (e.g., tropical rainforests, deserts, temperate forests) that dictate organismal distributions.

Coriolis Effect

• Describes the deflection of movement based on Earth’s rotation.

• Mechanism: As Earth rotates, points at the equator move at a faster linear speed than points closer to the poles. Fluids (like air and water currents) moving across Earth's surface tend to maintain their initial momentum. When these fluids move from areas of high linear speed to low (or vice-versa), they are deflected. In the Northern Hemisphere, this deflection is to the right, leading to counter-clockwise rotation in low-pressure systems (cyclones). In the Southern Hemisphere, the deflection is to the left, resulting in clockwise cyclone rotation.

Summary of Atmospheric Dynamics

• Air currents affected by the Coriolis effect cause significant temperature and pressure differentials, leading to predictable global patterns of wind and ocean currents. These dynamics are critical in shaping climatic patterns such as deserts and rainforests, distributing heat and moisture around the planet, and consequently defining the broad biogeographic patterns observed for terrestrial and aquatic life.

Environmental Impact of Pleistocene Refugia

• Refugia are locations where species persisted during climate shifts. During periods of significant climate change, such as the repeated glacial advances and retreats of the Pleistocene epoch, refugia served as critical 'arks' where populations of species could persist in localized areas with more favorable conditions (e.g., warmer, wetter), even as surrounding regions became inhospitable (e.g., covered by ice sheets or became arid).

• The evolutionary implications include:

  • Allowing for allopatric speciation, as isolated populations within refugia diverged genetically due to restricted gene flow and different selective pressures.

  • New species becoming sympatric and further cycling through these mechanisms helps establish current biodiversity. When climate warmed and species expanded their ranges from multiple refugia, previously isolated populations could become sympatric again, sometimes leading to hybrid zones, competition, or further diversification. This dynamic cycle of fragmentation, isolation, divergence, and subsequent re-contact has been a powerful engine shaping current biodiversity patterns, especially in temperate regions.

Evidence of Pleistocene Refugia

• Fossil pollen and ancient distributions provide evidence for past climatic conditions and biodiversity hotspots prevalent during glacial cycles. Scientific understanding of Pleistocene refugia relies on multiple lines of evidence: paleoecological models reconstruct ancient distributions, and genetic studies comparing populations often reveal patterns of reduced genetic diversity in areas re-colonized from refugia, and deep genetic divergences between populations that survived in different refugial areas.

• Glacial minima (warmer periods) vs. glacial maxima (cooler, drier periods) demonstrate differing refugial capacities and spatial distributions of species. For example, during glacial maxima, forests in Europe contracted into smaller, isolated southern refugia (e.g., Iberian, Italian, Balkan peninsulas), while during glacial minima, these species expanded northward.

Key Findings

• The Pleistocene refugia theory supports the role of historical climate changes in shaping current biodiversity and evolutionary patterns. The repeated cycles of expansion and contraction during the Pleistocene have left an indelible genetic signature on many extant species, demonstrating that climate history is a crucial driver of present-day biogeographic patterns and genetic diversity.