Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 25.4 The Rise and Fall of Groups of Organisms Reflect Differences in Speciation and Extinction Rates
The diversity of life has fluctuated throughout Earth's history due to variations in rates of speciation (formation of new species) and extinction (loss of species). Major events, such as mass extinctions, adaptive radiations, and continental drift, significantly influenced these changes.
Plate Tectonics and Continental Drift
Earth's continents are on large crustal plates that move slowly over geological time (continental drift).
Plate movements reshape Earth's geography, climate, and habitats, affecting organisms' survival, distribution, and evolution.
Example: Formation of the supercontinent Pangaea (~250 million years ago) led to extinction and speciation by altering habitats and climate dramatically.
Consequences of Continental Drift
Habitat alteration: Pangaea formation caused extinction of shallow-water marine species due to loss of coastal habitat.
Climate change: Continents' movement to different latitudes forced species to adapt, migrate, or go extinct.
Allopatric speciation: Separation of landmasses isolated species, allowing independent evolutionary paths (e.g., marsupials evolved uniquely in Australia due to its isolation).
Mass Extinctions: Major Turning Points
Mass extinctions eliminate large numbers of species globally; five major events ("Big Five") significantly impacted life:
Permian extinction (~252 million years ago): Caused by volcanic activity, global warming, ocean acidification, and oxygen depletion, wiping out ~96% of marine species.
Cretaceous extinction (~66 million years ago): An asteroid impact (Chicxulub crater, Mexico) triggered global climate disruptions, ending the era of dinosaurs (except birds) and over half of marine species.
Sixth Mass Extinction?
Human activities (habitat destruction, climate change) significantly increase extinction rates today—potentially leading to a sixth mass extinction.
Modern extinction rates estimated at 100–1,000 times higher than natural background rates.
Adaptive Radiations: Rapid Diversification After Extinctions
After mass extinctions, surviving organisms diversify quickly, occupying vacant ecological niches (adaptive radiations).
Example: Mammals diversified dramatically after dinosaurs went extinct (Cretaceous extinction ~66 million years ago).
Regional adaptive radiations also occur when organisms colonize isolated environments with limited competition (e.g., Hawaiian Islands).
In a Nutshell
The history of life is shaped by varying speciation and extinction rates influenced by continental drift, mass extinctions, and adaptive radiations. These events profoundly affect biodiversity, shaping the evolutionary trajectory of life on Earth.