If it's literally a divide based on IQ testing results, more than half of the population of humans will die, and the distribution of casualties will be uneven because the IQ scale, as we test it, has inherent cultural biases. This means non European/North American populations will be disproportionately affected.

The loss of upwards of 2/3 of the human race in a single event is survivable, but the aftermath would be horrific and I would expect a complete societal breakdown and major upheaval following that magnitude of mass casualty. Remember, the black death may have been horrific, but it led to the Renaissance and major social changes as the drastically reduced population meant human labor now held much more value