The study investigates the relationship between alternative splicing (AS) rates and effective population size (Ne) in metazoans, testing the "drift barrier" hypothesis. The authors analyzed transcriptome sequencing data from 53 metazoan species, covering a wide range of Ne values, to quantify AS rates. They found a negative correlation between AS rates and Ne proxies, such as body length, longevity, and dN/dS ratio, consistent with the drift barrier hypothesis. This pattern is dominated by low-abundance isoforms, which are likely errors, while abundant isoforms, enriched in functional AS events, show the opposite trend. The study also found that the proportion of frame-preserving variants among abundant splice variants is higher than among rare variants, suggesting that functional AS events are more prevalent in complex organisms. Additionally, the splicing rate of rare SVs is negatively correlated with gene expression levels, further supporting the hypothesis that the rate of splicing errors is shaped by the selection-mutation-drift balance. Overall, the findings support the idea that variation in AS rates across metazoans reflects the limits set by drift on the capacity of selection to prevent gene expression errors.The study investigates the relationship between alternative splicing (AS) rates and effective population size (Ne) in metazoans, testing the "drift barrier" hypothesis. The authors analyzed transcriptome sequencing data from 53 metazoan species, covering a wide range of Ne values, to quantify AS rates. They found a negative correlation between AS rates and Ne proxies, such as body length, longevity, and dN/dS ratio, consistent with the drift barrier hypothesis. This pattern is dominated by low-abundance isoforms, which are likely errors, while abundant isoforms, enriched in functional AS events, show the opposite trend. The study also found that the proportion of frame-preserving variants among abundant splice variants is higher than among rare variants, suggesting that functional AS events are more prevalent in complex organisms. Additionally, the splicing rate of rare SVs is negatively correlated with gene expression levels, further supporting the hypothesis that the rate of splicing errors is shaped by the selection-mutation-drift balance. Overall, the findings support the idea that variation in AS rates across metazoans reflects the limits set by drift on the capacity of selection to prevent gene expression errors.