Final answer:
A distinctive characteristic of a group must have appeared at the evolutionary point of divergence, designating it as a shared derived character. This event marks the emergence and prevalence of a new trait, aiding in the phylogenetic species definition and clarifying relationships within a clade.
Step-by-step explanation:
A characteristic that is distinctive of a group must have appeared at the point of divergence in the evolutionary history of the group. This occurrence signifies a new trait becoming a shared derived character, which is distinct from a shared ancestral character that all members of a clade possess without change. Taking the amniotic egg as an example, only certain organisms possess this trait, classifying it as a shared derived character that arose at some stage in descent, helping to establish closer phylogenetic relationships within the group compared to those without the characteristic, such as fish or lancelets. Such a distinctive characteristic often becomes prevalent through a series of evolutionary processes, including genetic mutations leading to the emergence of a new trait, the spread of this trait across the population, and then the retention and frequency of this trait reaching a stable point. This pattern can often be linked back to the phylogenetic species definition, where a species can be defined by a unique characteristic that sets it apart from others.