> When I was at school (many years ago!) we were taught that the difference between a species and a strain is that the former can't cross and produce fertile offspring

Surely you jest, Chris! Where does that leave species that do not reproduce sexually? 

> genetic material can be exchanged through viral transportation, through picking up stray DNA in the medium after a cell has broken apart, or by deliberate insertion of small rings of DNA, called plasmids. "Horizontal" or "lateral" genetic transfer is probably as old as life itself.

> some lizards are parthenogens; and have no males, where their nearest relatives are sexual, but in that case they are like their sexual cousins ecologically and morphologically 

> there are n+1 definitions of "species" in a room of n biologists.

-- John Wilkins

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