Let's work that through. We'll do a few generations, starting with 8 families of each type, with each type on their own separate isolated island to keep things

Let's work that through. We'll do a few generations, starting with 8 families of each type, with each type on their own separate isolated island to keep things simple. We start with 16 people on each island, half male and half female. Going generation by generation: First: 56 kids, or 8 kids Second: 196 kids, or 4 kids Third: 686 kids, or 2 kids Fourth: 2401 kids, or 1 kid Fifth: 8403 kids, or extinction Now, your idea that a child of a large family of 7 is less reproductively fit isn't going to help the low-reproduction families at all. They go extinct; they are thus clearly unfit. Unless the high reproduction families are losing about 5 of every 7 to early death, their population grows exponentially. In reality, there are not separate islands. The resource exhaustion caused by the high-rate families will not just affect them. Everybody is effected. Even if the high-rate families are more affected, it isn't likely to stop them and anyway there is no saving the low-rate families from extinction.