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An evolutionary lineage is a temporal series of populations, organisms, cells, or genes connected by a continuous line of descent from ancestor to descendant.The University of California, Berkeley resource on understanding evolution defines a lineage as "A continuous line of descent; a series of , populations, cells, or genes connected by ancestor/descendant relationships." Understanding Evolution, Glossary of TermsThe Oxford English Dictionary defines biological lineage as "a sequence of species each of which is considered to have evolved from its predecessor." OED definition of lineage Lineages are subsets of the evolutionary tree of life. Lineages are often determined by the techniques of molecular systematics.
Members of a species are considered to evolve as a single unit (or lineage) when they repeatedly share the same genes. The nodes would represent a split in lineage due to a breaking of genetic connections: when a single lineage is divided into two subsets, with the individuals not exchanging genes, they will accumulate differences in genes. If they do not fuse back again, it will create a new distinct descendant clade.
Just as a map is a scaled approximation of true geography, a phylogenetic tree is an approximation of the true complete evolutionary relationships. For example, in a full tree of life, the entire clade of animals can be collapsed to a single branch of the tree. However, this is merely a limitation of rendering space. In theory, a true and complete tree for all living organisms or for any DNA sequence could be generated.
Nevertheless, phylogenies can sometimes appear in a non-treelike form. Branches on the tree of life may grow together, a phenomenon called reticulation, which occurs due to different biological processes. Another process, introgression, occurs when hybrids between distinct lineages transfer novel genetic material through subsequent crossing. In other cases, hybrid speciation takes place when lineages hybridize to form a new, distinct lineage. Horizontal gene transfer, involving the introgression of very few genes, usually appears as a treelike population history with some genes having a discordant history. Thus, the tree-like representation would be proper as long as introgression and hybrid speciation are rare or limited to closely related tips (of lineages). In some cases, evolutionary relantionships should be depicted better in the form of a network.
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