🌾World Biogeography Unit 5 – Speciation and Extinction in Biogeography

Key Concepts and Definitions

• Speciation: The evolutionary process by which new species arise from existing species over time
• Extinction: The permanent loss of a species from Earth, occurs when the last individual of a species dies
• Allopatric speciation: Speciation that occurs when populations become geographically isolated from one another
• Sympatric speciation: Speciation that occurs within the same geographic area without physical barriers
• Reproductive isolation: The inability of two populations to interbreed and produce viable, fertile offspring
• Gene flow: The transfer of genetic material from one population to another through interbreeding
• Genetic drift: Random changes in allele frequencies within a population, particularly in small populations
• Natural selection: The process by which organisms with favorable traits survive and reproduce more successfully than those without these traits

Mechanisms of Speciation

• Prezygotic barriers prevent mating or fertilization between populations, leading to reproductive isolation
  • Habitat isolation: Populations occupy different habitats and do not encounter each other (montane vs. lowland species)
  • Temporal isolation: Populations have different mating seasons or times (diurnal vs. nocturnal species)
  • Behavioral isolation: Populations have different courtship behaviors or mating rituals (bird songs, dance patterns)
• Postzygotic barriers reduce the fitness or viability of hybrid offspring, reinforcing reproductive isolation
  • Hybrid inviability: Hybrid offspring do not survive to reproductive age due to genetic incompatibilities (mule)
  • Hybrid sterility: Hybrid offspring are viable but unable to produce functional gametes (ligers, tigons)
• Ecological speciation occurs when populations adapt to different environmental conditions, leading to divergence
• Polyploidy, the duplication of entire sets of chromosomes, can instantly create reproductive barriers (many plant species)

Types of Speciation

• Allopatric speciation: Populations become geographically isolated, allowing them to diverge independently
  • Vicariance: A population is split by the formation of a physical barrier (continental drift, mountain formation)
  • Peripatric speciation: A small subpopulation becomes isolated at the periphery of the parent species' range
• Sympatric speciation: Populations diverge within the same geographic area without physical barriers
  • Disruptive selection: Extreme phenotypes are favored over intermediate ones, leading to divergence (cichlid fish in African lakes)
  • Polyploidy: Instant reproductive isolation through chromosome duplication (many plant species)
• Parapatric speciation: Populations have partially overlapping ranges with limited gene flow
• Allochronic speciation: Populations diverge due to differences in breeding times or seasons (periodical cicadas)

Factors Influencing Speciation

• Geographic isolation reduces gene flow between populations, allowing them to accumulate genetic differences
• Ecological opportunity, such as the colonization of new habitats or the extinction of competitors, can drive adaptive radiation
• Sexual selection, through mate choice or intrasexual competition, can lead to rapid divergence in reproductive traits
• Hybridization between closely related species can sometimes result in the formation of new hybrid species
• Coevolution between species (predator-prey, host-parasite) can drive reciprocal adaptations and speciation
• Founder effects and genetic bottlenecks in small populations can accelerate divergence through genetic drift

Extinction Processes

• Background extinction: The gradual, continuous loss of species over geological time due to various factors
• Mass extinction: Rapid, widespread loss of species in a relatively short period (end-Permian, end-Cretaceous)
  • The "Big Five" mass extinctions in Earth's history caused by various factors (volcanism, climate change, impacts)
• Pseudoextinction: Lineage extinction through anagenesis (gradual evolution) or cladogenesis (splitting into daughter species)
• Local extinction (extirpation): The disappearance of a species from a particular area, while persisting elsewhere
• Lazarus taxa: Species that disappear from the fossil record, only to reappear later (coelacanth, dawn redwood)
• Living fossils: Species that have remained relatively unchanged over long periods of geological time (horseshoe crabs, ginkgo)

Causes of Extinction

• Habitat loss and fragmentation due to human activities (deforestation, urbanization, agriculture)
• Overexploitation of species for food, medicine, or other resources (passenger pigeon, great auk)
• Invasive species that outcompete or prey upon native species (brown tree snake on Guam, Nile perch in Lake Victoria)
• Climate change, both natural and anthropogenic, can alter habitats and disrupt species' ecological niches
• Pollution and environmental toxins can reduce fertility, survival, and overall fitness of species
• Disease outbreaks, particularly in small or vulnerable populations (chytrid fungus in amphibians, white-nose syndrome in bats)
• Trophic cascades: The loss of keystone species can have ripple effects throughout the ecosystem (sea otters, wolves in Yellowstone)

Biogeographical Patterns

• Latitudinal diversity gradient: Species richness generally increases from poles to equator
  • Rapoport's rule: Species ranges tend to be smaller at lower latitudes and larger at higher latitudes
• Elevational diversity gradient: Species richness often peaks at intermediate elevations in mountainous regions
• Island biogeography: The number of species on an island is determined by its size and isolation
  • Equilibrium theory of island biogeography: Balances immigration and extinction rates (MacArthur & Wilson)
• Endemism: Species that are unique to a particular geographic area (islands, isolated habitats)
• Disjunct distributions: Species or higher taxa that occur in widely separated areas (Gondwanan distribution)
• Bergmann's rule: Within a species or genus, larger-bodied individuals are found in colder environments

Case Studies and Examples

• Darwin's finches: Adaptive radiation of finches in the Galápagos Islands, showcasing speciation through natural selection
• Hawaiian honeycreepers: Diverse assemblage of birds that evolved from a single ancestral species
• African Rift Valley lakes: Rapid speciation of cichlid fish driven by ecological opportunity and sexual selection
• Caribbean Anolis lizards: Convergent evolution of ecomorphs on different islands
• Peppered moth: Industrial melanism as an example of natural selection and rapid evolutionary change
• Passenger pigeon: Once the most abundant bird in North America, driven to extinction by overhunting and habitat loss
• Thylacine (Tasmanian tiger): Marsupial carnivore that went extinct in the 20th century due to hunting and competition with introduced species
• Dodo: Flightless bird endemic to Mauritius, driven to extinction by human exploitation and introduced predators