Even microorganisms can choose genetically related individuals over unrelated competitors say Natasha J. Mehdiabadi et al., in this weeks Nature.

Dictyostelium normally live as asexually reproducing, unicellular amoebae in forest soils. But when starved of their bacterial food source, they aggregate in thousands to form a multicellular, motile 'slug'. This eventually becomes a fruiting body4,5, in which some amoebae in the group differentiate to form spores and other amoebae die to form a stalk structure that assists the dispersal of these spores. Stalk cells are therefore sacrificed to aid the others.

Because multicellularity in social amoebae is accomplished by aggregation of cells, fruiting bodies could consist of one or more clones. In the model organism Dictyostelium discoideum, genetically distinct clones can mix to form chimaeras, and one clone may sometimes exploit another by contributing less than its proportional share to the sterile stalk2. However, tests with several clones of other species suggest that mixing may not be the rule.
...
Randomization tests revealed that 12 of 14 pairwise experiments showed evidence of strong kin discrimination: although different isolates aggregated together, individual fruiting bodies consisted of predominantly one isolate or the other, with significantly higher variances in the proportion of fluorescent spores than were found in randomly mixed clonal controls

Further, the researchers tested whether the organisms would be selective when it is possible of a negative outcome, that is when there aren't enough individuals to readily form a slug, and discriminating might harm the chances of the group.

Strict exclusion of non-kin carries the risk of suboptimal group sizes when kin are rare9. To determine whether kin-discriminating clones of D. purpureum would mix with non-kin if kin were less abundant, we did 11 experiments at an amoeba density low enough to make fruiting bodies scarce (about 2times105 cells per cm2). There was less sorting in these low-density experiments than in the high-density ones (WSRT, Z=-2.667, n=11, P=0.0076; see supplementary information). This effect was lost when high- and low-density experiments were done simultaneously in six pairwise mixtures (WSRT, Z=-1.572, n=6, P=0.1159); however, the more important result is unambiguous. Dictyostelium purpureum preferentially associates with kin, and this remains true even at low density when partners are hard to find.

This kin discrimination means D. purpureum should avoid the disadvantages of forming chimaeras, and indeed only one clone was consistently cheated (see supplementary information). Our findings support the application of kin-selection theory to microorganisms and provide further evidence that social microbes can show sophisticated behaviour10 previously thought to occur only in higher organisms.

Isn't evolution grand?