Which assumption is NOT part of the three major assumptions of cladistics?

The assumption regarding species not being able to evolve is incorrect within the context of cladistics. Cladistics is based on the evolutionary relationships among organisms, and one of its fundamental principles is that species do indeed evolve over time. This evolution includes changes in traits and adaptations that occur as lineages diverge from common ancestors.

The first assumption—stating that all organisms descended from a common ancestor—supports the idea of evolutionary relationships, a core tenet of cladistic analysis, as it emphasizes the interconnectedness of life forms. The second assumption acknowledges that there are changes in characteristics over time within these lineages, providing the framework for understanding how new traits can arise and how they affect the relationships among species. The fourth assumption acknowledges that when a lineage splits, it divides into two distinct groups, simplifying the branching patterns that can arise during speciation.

In contrast, the idea that species cannot evolve fundamentally contradicts the principles of evolutionary biology and cladistics, which actively study and incorporate the mechanisms and processes of evolution into their methodologies. Therefore, the statement about species not evolving stands out as the one that does not align with the foundational assumptions of cladistics.