  Star clusters were historically considered simple stellar populations, with
all stars sharing the same age and initial chemical composition. However, the
presence of chemical anomalies in globular clusters (GCs), called multiple
stellar populations (MPs), has challenged star formation theories in dense
environments. Literature studies show that mass, metallicity, and age are
likely controlling parameters for the manifestation of MPs. Identifying the
limit between clusters with/without MPs in physical parameter space is crucial
to reveal the driving mechanism behind their presence. In this study, we look
for MP signals in Whiting 1, traditionally considered a young GC. Using the
Magellan telescope, we obtained low-resolution spectra within $\rm
\lambda\lambda = 3850-5500 \r{A}$ for eight giants of Whiting 1. We measured
the C and N abundances from the CN and CH spectral indices. C and N abundances
have variations comparable with their measurement errors ($\sim0.1$ dex),
suggesting that MPs are absent from Whiting 1. Combining these findings with
literature studies, we propose a limit in the metallicity vs. cluster
compactness index parameter space, which relatively clearly separates star
clusters with/without MPs (GCs/open clusters). This limit is physically
motivated. On a larger scale, the galactic environment determines cluster
compactness and metallicity, leading to metal-rich, diffuse, old clusters
formed ex situ. Our proposed limit also impacts our understanding of the
formation of the Sagittarius dwarf galaxy: star clusters formed after the first
starburst (age$\lesssim 8-10$ Gyr). These clusters are simple stellar
populations because the enriched galactic environment is no longer suitable for
MP formation.
