Final answer:
A shared derived characteristic is a trait seen in descendants but not in all ancestors of those organisms, marking a point of evolutionary divergence. Examples include the amniotic egg in the Amniota clade and the presence of hair in mammals, both of which help map evolutionary relationships in phylogenetic trees.
Step-by-step explanation:
Shared Derived Characteristics
A shared derived characteristic is a trait that is found in an ancestor of a group and is present in its descendants, but not all ancestors of those organisms. These characteristics arise due to a change in the genetic makeup that leads to a new trait, becoming prevalent over time among modern descendants, thereby marking a point of divergence in the phylogenetic tree.
For example, the amniotic egg is a shared derived characteristic for the Amniota clade. This trait tells us that lizards, rabbits, and humans have an evolutionary relationship that is closer among themselves than with fish, lampreys, and lancelets, which do not have this characteristic.
Another example is the presence of hair, which is a shared derived characteristic for mammals. This particular trait shows us that mammals form a distinct group within the Amniota that differs from reptiles, which do not have hair.
In building phylogenetic trees, these characteristics help scientists determine the evolutionary relationships between different organisms, giving us an understanding of how species evolved and diversified from common ancestors.