def METHOD_NAME(individual):
    # Transform the tree expression in a callable function
    func = toolbox.compile(individual)
    # Evaluate the sum of squared difference between the expression
    # and the real function : x**4 + x**3 + x**2 + x
    values = (x/10. for x in range(-10, 10))
    diff_func = lambda x: (func(x)-(x**4 + x**3 + x**2 + x))**2
    diff = sum(map(diff_func, values))
    return diff,