I'm not sure I understand your question, but eliminating the left tail of a bell curve would change the average but not necessarily extend the right tail.
What exactly happens depends on the model, but I think it would be very difficult to build a model with nonzero heritability that produced a bell curve and where truncating the left tail did not affect the right tail.
Usually bell curves occur from the sum of many small discrete variables. That appears to be true for IQ. Under this model, any form of selection has basically the same effect, at least in the long term. If the old equilibrium had random mating and the next generation is also produced by random mating, then a new bell curve will be produced in ...
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.