∞ generated and posted on 2016.08.19 ∞
Single species from which two or more other species are derived.
| A Common Ancestor is a lineage that undergoes cladogenesis, that is, branching evolution, such that more than one species can trace its ancestry back to the common ancestor or, more accurately, to multiple common ancestors. |
An ancestral species is a common ancestor because two or more descendant species share it as an ancestor. When we speak of a trait being identical by descent, generally we are referring to a common ancestor sharing that trait with more than one descendant species.
The concept of common ancestor is complicated by horizontal gene transfer since, as a result of horizontal gene transfer, two organisms can share ancestry, but only in terms of small portions of individual common ancestors (e.g., individual genes or small clusters of genes). The result is what can be described as mosaic evolution, that is, where otherwise closely related species nevertheless can possess multiple ancestral species but, due to horizontal gene transfer, this occurs without their common ancestors necessarily completely overlapping and, indeed, with different parts of their genomes possessing different common ancestors.