Family Selection: Food-supply Certainty Gradients Carve Bimodal Social Architectures in Birds

The family constitutes a fundamental biosocial tier occupying an intermediate position between individual organisms and populations. As an independent entity to accomplish the reproduction and survival imperatives, the family evolved a series of life history strategies to achieve the success in generational gene transmission. However, the evolutionary mechanism ensuring the survival of family members themselves remains poorly elucidated. From the family's perspective, the greatest threat is the collective death of family members (CDFM). To mitigate this risk, the family entity employs a Family Selection strategy: Under low CDFM risk, close relatives hug together to form family-based societies (H-tactic); Under high CDFM risk, members disperse to form non-family social configurations (D-tactic). Through theoretical modeling and mathematical analysis, we identify diet composition, which may exert substantial ecological stress on foraging groups, as the paramount driver of family selection. Crucially, data on social structures and dietary distributions across all 9,993 extant bird species robustly support this: family-based societies predominantly evolved in birds with diets characterized by high food-supply certainty, a property derived from the multidimensional nature of food resources themselves. Our findings indicate that: Family selection embodies the family entity's active adaptation, prioritizing family genetic interests over individual fitness benefits. It reasonably explains the paradoxes in cooperative breeding research, facilitates to establishing a novel disciplinary domain centered on Family and will redefine the underlying logic in ecology, population genetics, evolutionary biology, and particularly in sociobiology.