= Endosymbiosis
<Endosymbiosis> is an ecological association in which one <organism>, the <endosymbiont>, lives inside another. The definition specifies location, not that both partners benefit, nor that the internal partner has become an <organelle>. An intracellular association can remain an association between distinct <organisms>. <Endosymbiotic theory> addresses a different question: how particular ancestral <bacteria> became inherited, functionally integrated <organelles> during <evolution>.
The striking consequence is that a <eukaryotic> <biological cell> can combine histories: its nuclear lineage and its <mitochondria> need not have the same ancestry. <Mitochondria> have bacterial ancestry related to <Alphaproteobacteria>; the <plastids> of <plants> and many <algae>, including <chloroplasts>, have ancestry within <cyanobacteria>. These are ancestral lineages, not a claim that a named living bacterial <species> is the literal ancestor. Present-day <bacteria> and <organelles> have both evolved since their shared ancestors. <Origin and Evolution of Plastids and Photosynthesis in Eukaryotes by Geoffrey I. McFadden>[Opening paragraphs and “The endosymbiont: when and who?”: bacterial origins and uncertainty about closest relatives]{ref}
\Image[/endosymbiosis-organelle-integration.png]
{title=From an internal partner to an integrated <organelle>}
{description=Original conceptual schematic, not an observed historical sequence. Persistence, metabolic exchange, <endosymbiotic gene transfer>, <organelle protein import> and coordination of division illustrate integration; they are not dated stages in a compulsory order. The illustrated partner is generic, and the host outline does not specify the ancestral host of <mitochondria>. The protein-delivery arrow abbreviates nuclear <gene expression>, <biological translation> in the <cytosol> and <organelle protein import>; it does not place <protein> synthesis inside the <cell nucleus>.}
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