• A scientific theory is a broad explanation for events that is widely accepted as true.
  • To become a theory, a hypothesis must be tested over and over again, and it must be supported by a great deal of evidence.
  • The theory of evolution by natural selection explains how living things are changing today and how modern living things have descended from ancient life forms that no longer exist on Earth.
  • The germ theory of disease is a scientific theory that proposes that microorganisms are the cause of many diseases.
  • The cell theory is another important scientific theory of biology.
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