What term describes a group of organisms that share a common ancestor?

The term that describes a group of organisms that share a common ancestor is "ancestor." In biological classification, the concept of a "common ancestor" is crucial in understanding the evolutionary relationships between different organisms. When we refer to a group of organisms sharing a common ancestor, we are discussing how these organisms have diverged over time from that original species through the process of evolution.

On the other hand, the other options, while related to classification, do not accurately encapsulate the idea of a shared common ancestor. For instance, "Phylum Chordata" designates a major taxonomic group that includes animals with a notochord, but it does not specifically indicate a group defined by shared ancestry at the level of individual organisms. "Species group" lacks the precision needed for defining an evolutionary relationship since it can refer to any grouping of species, without consideration of their phylogenetic links. Lastly, "Taxonomic Family" is a more specific ranking in biological classification that includes a group of related genera, but again, it doesn’t directly convey the idea of organisms unified by a singular lineage or common ancestor.

In summary, the term "ancestor" effectively captures the essence of a group of organisms linked through evolutionary history, highlighting their common origins.