Many people are naturally scared of spiders and snakes. Some evolutionary psychologists like Dr. Paul Ekman claim that some fears like these are coded into our genetics because of the many of our ancestors before us who were killed by them.
I thought the idea was that fear of spiders (a heuristic in an arachnophobic's amygdala that they inherited genetically) was at some point adaptive for their ancestors. That is, people with a fear of spiders are around today because their ancestors had some random genetic mutation that caused them to exercise caution around spiders, which conferred a survival advantage in their particular context. In our context it's maladaptive (evolution is very slow), and so considered an irrational phobia.
Inherited instincts and emotions.