Final answer:
A sister species or sibling species are two closely related species that evolved from a recent common ancestor but do not interbreed to produce fertile offspring in nature.
Step-by-step explanation:
What Is a Sister Species or Sibling Species?
A sister species or sibling species refers to two different species that have recently evolved from a common ancestor and are thus very closely related. The concept arises in the context of phylogenetic species, where the focus is on the evolutionary lineage and genetic ancestry of organisms. The biological species definition, which states that a species is a group of interbreeding organisms that produce fertile offspring, helps us to categorize and understand these relationships. Sister species are typically very similar and may even live in the same geographic area or have overlapping ranges, but they do not interbreed in nature or can only produce infertile hybrids, signifying that they have diverged enough to be considered separate species.
The difficulty in classifying sister species becomes apparent when we deal with fossils since it is challenging to ascertain if individuals belonged to different species (interspecific variation) or to the same species (intraspecific variation). For currently living species, genetic analysis alongside breeding behavior are key factors used to differentiate sister species. An example of sister species can be observed among birds of prey like the bald eagle (Haliaeetus leucocephalus) and the African fish eagle (Haliaeetus vocifer), which are closely related but distinct species.