In the 1980s, a Hollywood prop-maker had an idea.  Wouldn't it be funny, Chris Gilman joked, to promote evolution by putting feet on a fish?  His friends thought it was hilarious, and thus a fad was born.

The inspiration for this bumper bling was, of course, the Christian fish, a symbol now ubiquitous in Christian cars & jewelry.  Why a fish?  Supposedly the inspiration comes from the use of the Greek word for fish--ichthus--as a acronym for Jesus Christ, Son of God, Savior:

Iesous CHristos THeou Uios Soter

However, in recent years the Darwin fish has become so ubiquitous in popular culture that some people think that the fish was itself originally a symbol for Darwin's theory of evolution!