What process is most responsible for the extinction of most species of plants and animals that have lived on Earth?

• A. gene mutation
• B. environmental changes
• C. selective breeding
• D. decrease in reproduction

Answer: (B)

Environmental changes drive extinction because when the conditions where organisms live—such as climate, vegetation, water availability, and habitat structure—shift, many species can’t adapt quickly enough, migrate, or find enough food, and their populations fade away. Over Earth’s history, rapid or large-scale changes in environment have wiped out whole ecosystems, leaving only species that could cope with the new conditions. This is different from other ideas: gene mutations create new variation and can help evolution, but they don’t by themselves cause most extinctions; selective breeding is a human-directed process and not the natural cause of species loss; a decline in reproduction can threaten a species, but the widespread disappearances people study are usually tied to changing environments that remove or alter homes and resources.