This is a ludicrus assumption. All members do not have to remain viable for evolution to occur in fact all differences in viability account for differences in species.
It is ludicrous however it is the assumption made by punk-eek that a whole isolated group transforms itself into a new more complex species. Indeed for a new transformed individual to be able to reproduce there must be others with the same transformation to enable it to reproduce. So you have a problem here that you need the transformation to arise in more than one individual. Due to genetics and the fact that mutations are not additive among groups but must occur in the same line of individuals carrying the particular mutation (you cannot have a single base mutation in one gene added to by another single base mutation in the same gene from another individual because they will be in two different alleles) such trasformations are well nigh impossible. Since specific mutations occur in individuals, not in entire species, it is ludicrous to say that even a small group in a species will transform itself together into a new more complex species. To say that such transformations occur all the time as evolution proposes is total nonsense.