If the immediate consequences of the genetic change in question aren’t terribly deleterious then that first organism may very well have offspring, even without it conferring any particular advantage. And now those offspring do have siblings who share the gene.
[EDITED to add: oops, saw Viliam’s comment in Recent Comments and replied to it without noticing others had also done so making the same point.]
If the immediate consequences of the genetic change in question aren’t terribly deleterious then that first organism may very well have offspring, even without it conferring any particular advantage. And now those offspring do have siblings who share the gene.
[EDITED to add: oops, saw Viliam’s comment in Recent Comments and replied to it without noticing others had also done so making the same point.]