Phobia leads someone to believe that among our ancestors, those who feared animals, darkeness, and heights would likely live long enough to reproduce?

Perhaps: Those that learn to live with animals, develop way to illuminate darkness, and build buildings and planes will also live long!

Irrational fear: A phobia is an irrational fear--we know what we fear is probably not dangerous. Behavioral psychologists view them as learned and treatment is to desensitize or unlearn them. An atavistic trait is something we inherit that can keep us from danger but is not necessary in today's world. This could be a fear of bugs, snakes, heights, etc. Peace and good health.