One /member/ forgoes reproduction, not one /species/. It's akin to a sibling deciding that they won't have kids so they can help their sibling raise their own. The sacrificing family member passes their DNA on because they share DNA with kin.

It's not that different from a multicellular organism. The vast majority of cells in your body are dead-end lineages. They've specialized into sterile forms so that a few cells in your gonads actually do have a chance to reproduce. Evolutionarily it works because all the cells have the same (or very similar, in the case of kin selection) genes.

Kin selection takes place within a single species:

> The DNA confirmed that both the sailors and tiny passengers inside the hemisphere belong to the same species.