Because that’s what makes it altruistic.
> I'm uncomfortable with the idea of individual organisms having
> fitness. I know this is not a new issue given "the selfish gene"
> thesis. Darwin's project was answering the question of how species
> originate. Fitness is a characteristic of species. Species are
> selected for or against.
We have covered this before, I am afraid. Darwin’s theory, as stated, and used upto the New Synthesis, applies to individual fitness among individuals within a species (or to be precise, phenotypes/traits manifested in individuals). New species emerge because of this fitness struggle, as differentiation between individuals leads to sexually incompatible groups (where applicable; similar and other mechanisms/considerations apply to asexual reproduction). With the New Synthesis, a genetic understanding of the underlying process was added, as well as an effort to explain such “anomalies” as altruism via ideas like kin selection or “inclusive fitness" (Hamilton, Williams, Trivers, et al). Whether such behaviour counts as “group level selection” is much debated, but at any rate, the conventional view of selection is that it occurs primarily at the individual - the “unit of reproduction” - level.
I mention Wilson/Sober because they go to the trouble of constructing particular population models and distributions (in terms of isolation and rate of interaction, so on) to demonstrate that group level selection is technically feasible. They also provide examples of how such group selective behaviour occurs in individuals. I am personally sympathetic, even partly convinced, that group level selection occurs (or at least that Occam’s Razor favours it’s occurrence), but I do not doubt - and I do not think any evolutionary biologist does either - the fact that traits manifest in individuals and are subjected to natural selection(*).
—ravi
(*) the strength and extent of natural selection’s part in evolutionary history is debatable and I think dismissal of Fodor, others, on this front has been premature.