That is not Darwinian selection, because you are not selecting anything. As you mention yourself, it doesn't matter which half you discard and which half you keep. In fact, you can keep the entire thing. Discarding just makes it more practical.
If all conditions are fine, yeast will grow exponentially, meaning your amount of yeast will double every 2 hours or so. By discarding half of your mix, you simply reduce the volume you are working with. Otherwise you would have to add double the ingredients every time. For example, let's assume by adding fresh ingredients you double your volume. Starting with 250ml you would have 500ml the next day, 1l the day after, 2..4..8.. 16 liters and soon you have a bucket full of yeast.
Handling that volume to avoid overgrowth and insufficient nutrient/oxygen supply is way harder than a small volume. In the lab we grow yeast preferentially on shakers, mixing the solution to ensure all cells get the right amount of nutrients. Otherwise the yeast would die once the nutrients are depleted. So, how do you do that in your yeast bucket? ;) Just stirring it once won't do the job. The yeast would settle and die.
You could keep the halfs in separate smaller containers, but how much space do you have to store these? Here is a video where this issue is explained and what you can do with the discarded half: https://youtu.be/Ye4A2judXck
In the end, what would you do with all that starter? Once you have your sourdough starter ready, you can take half and use it for baking and keep growing the other half. How much bread do you want to bake? Do you need a bucket full of sourdough starter?
Sure, some of the yeast will die and the cells best adapted to the growing conditions will remain. This is a selection process, but it's independent from discarding half of it.