Other Species Models

Because the morphological and biological concepts cannot be applied to every organism, biologists also use other species models.The phylogenetic species concept defines a species as the smallest group of organisms that share a common ancestor and can be distinguished from other groups by unique inherited characteristics. It focuses on evolutionary history (phylogeny). In practice, it is usually applied by comparing DNA base sequences or protein sequences and building a family tree. A species is a group that forms its own separate branch (lineage) on the tree.An advantage of the phylogenetic concept is that it does not depend on breeding, so it can be applied to organisms that reproduce asexually, such as bacteria, and to populations that never meet. However, because even small consistent genetic differences can mark a separate lineage, it tends to increase the number of recognised species. Populations once thought to be one species may be split into several.The ecological species concept defines a species as a group of organisms that share the same niche: they use the same resources and are adapted to the same environmental conditions. Two populations that look alike but occupy different niches would be different species.The evolutionary species concept sees a species as a single lineage of ancestor and descendant populations that keeps its own identity over time and has its own evolutionary future. This concept can link fossil forms to living species.Each model uses different evidence, so the same organisms can be classified differently depending on the model chosen.