01Why species disappear
Extinction is common: over 99% of species that ever lived are extinct. Background extinction is steady; mass extinctions (five major ones, like the asteroid ending the dinosaurs ~66 million years ago) wipe out many groups quickly.
Today, human activity — habitat loss, climate change, invasive species, overharvesting — is raising extinction rates.
02Aftermath
Extinction opens niches, often followed by adaptive radiation (mammals diversifying after dinosaurs). Low genetic diversity makes species more extinction-prone.
03Notebook box
Extinction → empty niches → adaptive radiation.
Worked example: Why did mammals diversify rapidly about 66 million years ago? → The dinosaur mass extinction opened ecological niches.