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.