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Remi.b has suggested here:

The concept of species is poorly defined and is often misleading. The concept of lineage is much more helpful. IMO, the only usefulness of this poorly defined concept that is the "species" is to have a common vocabulary for naming lineages.

Definition of lineage from Wikipedia:

An evolutionary lineage is a temporal series of organisms, populations, cells, or genes connected by a continuous line of descent from ancestor to descendent. Lineages are subsets of the evolutionary tree of life

If we're talking about populations, how do we know if two individuals have the same lineage?

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Lineages have many levels, it is like using the word groups in some ways, they are stacked inside each other like the worlds biggest branching Russian nesting doll. All vertebrates is a lineage, all birds is another lineage, as is chickens, as is just rhode island reds. Two individuals of any earth life will always share some lineages. Lineage is not the same as clade however since they do not have to be monophyletic and usually refers to a stem line leading to a group, because of how lineage is named, it can be confusing. For instance the gorilla/human lineage could refer to basically all the great apes minus orangutans or it could refer to the sequence organisms leading to their common ancestor, or it could refer to the genetic lines to their common ancestor and back to themselves. And easy way to think of it is the outer boundaries of a lineage is not well defined. What is used to denote the lineage will help with what it is referring too.

You have to look at context. In paleontology because it is assumed you never have a direct ancestor it is used more vaguely almost like a "stem clade", in microbiology however it will be a far more precise term referring to one or more genetic lines.

The concept of species is misleading because people are trying to use our instinctual categorical thinking to describe a continuous series. It is like trying to define a color any category we invent to split the spectrum up will have many problems because it is not something with discrete differences.

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  • $\begingroup$ So, "lineage" is a synonym of "clade" or "monophyletic group"? I would agree with this definition, however wikipedia (A lineage is a single line of descent or linear chain within the tree, while a clade is a (usually branched) monophyletic group, containing a single ancestor and all its descendants.) and @mgkrebbs (in a comment under my now deleted answer; Lineage refers to a line from one individual to another, whereas a monophyletic group is a tree of descent from one individual to many, and so is not a synonym) disagree. $\endgroup$ – Remi.b Apr 18 '18 at 18:17
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    $\begingroup$ It can be synonymous with clade, it is not a well defined term. For instance the theropod lineage would basically just be the clade theropoda, but say the tyrannosaur lineage could mean the tyrannosaur clade or it could include the stem groups leading to tyrannosaurs as well. Context will usually tell you how it is being used. I'll be more clear. $\endgroup$ – John Apr 18 '18 at 22:13

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