Local adaptation is assumed to occur under limited gene flow. However, habitat-matching theory predicts dispersal should favour rather than hinder local adaptation when individuals selectively disperse towards habitats maximizing their performance. We provide experimental evidence that local adaptation to the upper margin of a species' thermal niche is favoured by dispersal with habitat choice, but hindered under random dispersal. Our study challenges the idea that high gene flow precludes local adaptation, and provides unique experimental evidence of habitat choice as an overlooked mechanism responsible for adaptation under rapid environmental changes.

Aggregative groups entail costs that must be overcome for the evolution of complex social interactions. Understanding the mechanisms that allow aggregations to form and restrict costs of cheating can provide a resolution to the instability of social

Kin selection theory predicts that costly cooperative behaviors evolve most readily when directed toward kin. Dispersal plays a controversial role in the evolution of cooperation: dispersal decreases local population relatedness and thus opposes the evolution of cooperation, but limited dispersal increases kin competition and can negate the benefits of cooperation. Theoretical work has suggested that plasticity of dispersal, where individuals can adjust their dispersal decisions according to the social context, might help resolve this paradox and promote the evolution of cooperation. Here, we experimentally tested the hypothesis that conditional dispersal decisions are mediated by a cooperative strategy: we quantified the density-dependent dispersal decisions and subsequent colonization efficiency from single cells or groups of cells among six genetic strains of the unicellular Tetrahymena thermophila that differ in their aggregation level (high, medium, and low), a behavior associated with cooperation strategy. We found that the plastic reaction norms of dispersal rate relative to density differed according to aggregation level: highly aggregative genotypes showed negative density-dependent dispersal, whereas low-aggregation genotypes showed maximum dispersal rates at intermediate density, and medium-aggregation genotypes showed density-independent dispersal with intermediate dispersal rate. Dispersers from highly aggregative genotypes had specialized long-distance dispersal phenotypes, contrary to low-aggregation genotypes; medium-aggregation genotypes showing intermediate dispersal phenotype. Moreover, highly aggregation genotypes showed evidence for beneficial kin-cooperation during dispersal. Our experimental results should help to resolve the evolutionary conflict between cooperation and dispersal: cooperative individuals are expected to avoid kin-competition by dispersing long distances, but maintain the benefits of cooperation by dispersing in small groups.