Survival selection on group size, a genetically based trait in this To investigate temporal differences in the form and direction of Here, we use a 30-y field study of a colonially nesting songbird In group size distributions if the sign of directional and/or nonlinear selection regularly reversed such that small groups wereįavored in some years, medium-sized groups were best in others,Īnd large groups were advantageous in still other years. In the case of group size, fluctuating selection could hypothetically generate a long-term stasis (24, 25), and whether fluctuating selection is a common evolutionary process remains unclear.
Although perhapsĬommon in nature (23), only a relatively few studies have demonstrated statistically significant reversals in selection direction One mechanism that can maintain long-term stasis in traitĭistributions such as group size is temporally fluctuating selection, in which selection alternately favors traits in one directionĪnd then the opposite direction (20–22). Most perplexing, but unresolved, problems in behavioral biology Of apparent group size-related fitness costs remains one of the (and the associated variation in individual behavior) in the face
Group sizes (10–12), genetic differences among individuals inĭifferent-sized groups (13, 14), and behavioral specializations for Individuals in the less successful group sizes should be selectedĪgainst (9), especially given known heritable preferences for That are clearly disadvantageous relative to others (7, 8). Surprisingly, in almost all species, some group sizes occur Some of the costs, such as the spread of disease among g roups Has proven useful in studying both the benefits of sociality and Ost social animals exhibit wide variation in group size (1–ģ), with the smallest and largest groups often differing in | coloniality | group size | natural selection | social behavior Of different group sizes regularly oscillate over time in a natural The first case, to our knowledge, in which fitness advantages The wide range in cliff swallow group size is probably maintained by fluctuating survival selection and represents AveragedĪcross years, there was no net directional change in selection onĬolony size. Transfer, and/or the level of ectoparasitism, with the net benefit of sociality varying under these different conditions. Oscillating selection on colony size likely reflected annual differences inįood availability and the consequent importance of information In larger colonies tended to be favored in cooler and wetter years,Īnd birds in smaller colonies in hotter and drier years. Directional selection was predicted in part by drought conditions: birds Stabilizing and directional selection in different years, and reversals in the sign of directional selection regularly occurred. Pyrrhonota) to show that the survival advantages of differentĬolony sizes fluctuated among years. Recapture study of colonially breeding cliff swallows (Petrochelidon
Those in other groups, why wide group size variation persists in Because individuals inĬertain sizes of groups often have higher apparent fitness than Most animal groups vary extensively in size. Pageaĭepartment of Biological Sciences, University of Tulsa, Tulsa, OK 74104 Browna,1, Mary Bomberger Browna,2, Erin A. Fluctuating survival selection explains variation inĬharles R.
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