It is the conidiogenous cell and its arrangement and the number of this phialide will be the criteria for speciate.

Benthic snail larvae are more likely to become isolated and therefore speciate at a higher rate creating more lineages.

Not guarantee, but it is rather helpful. It is not too likely that two population split by a solid geographic barrier would not begin to speciate. To not speciate they would have to not have any beneficial mutations over time that would carry their gene pools apart. Then there is the possibility of founder effect by having the frequency of alleles vary between the two populations in a random manner. So, not guarantees, but even modern ring species vary greatly enough that at the ends of the ring they are different species. ( Google ring species )

The condition in which an organism has complete extra sets of chromosomes is called polyploidy. This condition can cause a number of severe birth defects. Most do not live a full lifespan for their species.