Intraspecific competition drives increased resource use diversity within a natural population. Richard Svanbäck and Daniel I. Bolnick conducted an experiment using three-spine sticklebacks (Gasterosteus aculeatus) in enclosures in a natural lake to test whether competition increases resource use diversity. They manipulated population density and observed that increased density led to reduced prey availability, causing individuals to add alternative prey types to their diet. Phenotypically different individuals added different alternative prey, increasing diet variation among individuals. Competition also increased the diet-morphology correlations, indicating stronger frequency-dependent interactions in high competition. These results confirm that resource competition promotes niche variation within populations and can lead to increased diversity via behavioural plasticity, not evolutionary changes. The study shows that competition drives ecological diversification by altering foraging behaviour, leading to increased diet variation and stronger correlations between diet and morphology. This supports the idea that intraspecific competition can maintain phenotypic and genetic variation in natural populations. The findings suggest that behavioural diversification can occur rapidly and is easily reversible, and may facilitate evolutionary diversification by increasing frequency-dependent competition. The study highlights the importance of considering how changes in niche variation and frequency dependence affect evolutionary dynamics in competitive environments.Intraspecific competition drives increased resource use diversity within a natural population. Richard Svanbäck and Daniel I. Bolnick conducted an experiment using three-spine sticklebacks (Gasterosteus aculeatus) in enclosures in a natural lake to test whether competition increases resource use diversity. They manipulated population density and observed that increased density led to reduced prey availability, causing individuals to add alternative prey types to their diet. Phenotypically different individuals added different alternative prey, increasing diet variation among individuals. Competition also increased the diet-morphology correlations, indicating stronger frequency-dependent interactions in high competition. These results confirm that resource competition promotes niche variation within populations and can lead to increased diversity via behavioural plasticity, not evolutionary changes. The study shows that competition drives ecological diversification by altering foraging behaviour, leading to increased diet variation and stronger correlations between diet and morphology. This supports the idea that intraspecific competition can maintain phenotypic and genetic variation in natural populations. The findings suggest that behavioural diversification can occur rapidly and is easily reversible, and may facilitate evolutionary diversification by increasing frequency-dependent competition. The study highlights the importance of considering how changes in niche variation and frequency dependence affect evolutionary dynamics in competitive environments.