Is fear a conditioned response or an adaptive response?
Seligman in 1971 posited with his Preparedness Theory that some stimuli, like snakes, elicited biologically based fear from humans. The fear is passed down our evolutionary mapping for our survival.
In more recent analysis, Ohman and Mineka (2003), study the anatomy of fear. The human limbic system which gave us our evolutionary leap, may show the area of the brain that is evolutionarily designed to fear snakes rather than trained to do so.
references too long to fit in the Reference Section:
Practitioners Guide to Treating Fear and Anxiety In Children and Adolescents: A Cognitive Behavioral Approach, Andrew R. Eisen and Christopher A. Kearney; 1995
The Malicious Serpent: Snakes as a Prototypical Stimulus for an Evolved Module of Fear, Arne Ohman and Susan Mineka; American Psychological Society, 2003