No, still not biology - tradition. I covered this earlier in the thread, but the idea is that the first way that a channeler learns how to channel becomes so engrained to their style that it's extremely difficult to unlearn. That's more than enough to explain the formation of the various matriarchal societies as we see them at the start of the series, before Rand starts breaking their chains. The dichotomy of the Source itself can be disproven any time after Saidin is cleansed, say, though it can wait all the way until the end if needed - which gives plenty of time to layer in foreshadowing.