Use the word bellwether to indicate someone or something that takes the lead in a group or movement.

Literally, a bellwether is a sheep (often wearing a bell) that leads a herd. Unless you herd sheep, you probably don't see many bellwethers that say "Baaa." That's the origin of this word, but there are many other types of bellwethers who lead the way in different areas. Apple was a bellwether in technology. Rosa Parks was a bellwether in civil rights. And, during election season, those "bellwether states" are the ones where the primaries seem to predict the outcome of a party's nomination.

Definitions of bellwether
  1. noun
    someone who assumes leadership of a movement or activity
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    type of:
    a person who rules or guides or inspires others
  2. noun
    sheep that leads the herd often wearing a bell
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    type of:
    male sheep especially a castrated one
  3. noun
    a predictor or indicator of future trends
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    type of:
    forerunner, harbinger, herald, precursor, predecessor
    something that precedes and indicates the approach of something or someone
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