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The evolution of intraspecific variation in social organization
Many species show intraspecific variation in their social organization (IVSO), which means the composition of their social groups can change between solitary living, pair living, or living in groups. Understanding IVSO is important because it demonstrates species resilience to environmental change and can help us to study ultimate and proximate reasons for group living by comparing solitary and group-living individuals in a single species. It has long been realized that the environment plays a key role in explaining the occurrence of IVSO. IVSO is expected to have evolved in variable environments and can thus be a key adaptation to environmental change. It has previously been suggested that four different mechanisms relying on the environment exist that can lead to IVSO: environmental disrupters, genetic differentiation, developmental plasticity, and social flexibility. All four mechanisms depend on the environment such that focusing only on environmental factors alone cannot explain IVSO. Importantly, only three represent evolved mechanisms, while environmental disrupters leading to the death of important group members induce nonadaptive IVSO. Environmental disrupters can be expected to cause IVSO even in species where IVSO is also an adaptive response. Here, we focus on the questions of why IVSO occurs and why it evolved. To understand IVSO at the species level, it is important to conduct continuous long-term studies to differentiate between nonadaptive and adaptive IVSO. We predict that IVSO evolves in environments that vary in important ecological variables, such as rainfall, food availability, and population density. IVSO might also depend on life history factors, especially longevity. IVSO is predicted to be more common in species with a short life span and that breed only for one breeding season, being selected to respond optimally to the prevailing environmental situation. Finally, we emphasize the importance of accounting for IVSO when studying social evolution, especially in comparative studies, as not every species can be assigned to one single form of social organization. For such comparative studies, it is important to use data based on the primary literature.
alternative reproductive tactic, environmental disrupters, extrinsic factors, intraspecific variation in social organization, phenotypic flexibility, phenotypic plasticity
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