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Sequence of ancestors and descendants.
Lineages can refer, equivalently, to sequences of ancestors and descendants within human families, sequences of ancestors and descendants within populations, and sequences of ancestors and descendants where each is ancestor and descendant is itself a population rather than individual organisms. Indeed, the only reason that these concepts might seem as though they are different is as a matter of convenience to those describing or instead listening to (or reading about) such a sequence.
It is perfectly legitimate, in other words, to speak of every single possible "begot" from the origin of life to the present. This by necessity would pass from one species to another, ancestral species to descendant species. It would, however, also be incredibly tedious to do so and, due to a lack of record keeping, difficult to keep track precisely of who parented who.
Nevertheless, just as you without question in principle should be able to trace your own lineage back for 2,000 or 10,000 or 100,000 years, so too in principle you can trace your ancestry back, one generation at a time, until all possible ancestors have been included. Such an exercise, in principle, is possible for every single organism that has ever existed, and in each case the result is a lineage.
Note that most lineages are not identical, except for full siblings. Nonetheless, many lineages are quite similar, and indeed the more closely related two individuals, the more similar their lineages. Thus, the lineage associated with members of your own family are more similar than the lineages of strangers than are the lineages of individual members of different species, etc. In all cases, nonetheless, a lineage exists which represents the passage of hereditary material from generation to generation, from parent to offspring, and from ancestors to descendants.
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