Random genetic drift limits mRNA splicing accuracy in metazoans. Alternative splicing (AS) is widespread in eukaryotes, but its functional significance remains debated. While some studies suggest AS contributes to adaptive evolution by increasing functional diversity, others argue that higher AS rates in complex organisms may reflect increased splicing errors due to genetic drift. This study analyzed 53 metazoan species with varying effective population sizes (Ne) and found a negative correlation between Ne and genome-wide AS rates, supporting the 'drift barrier' hypothesis. Low-abundance splice variants, which make up most of the splicing repertoire, are enriched in errors, while abundant variants, which are more functionally relevant, show lower AS rates in complex species. These findings suggest that AS rates reflect the limits imposed by genetic drift on the ability of selection to prevent splicing errors. The study also shows that AS rates correlate with Ne proxies such as longevity and body size, and that functional splicing variants are more common in abundant splice variants. Analysis of splicing sites revealed stronger selective constraints on major splice sites compared to minor ones, supporting the idea that abundant splice variants are more likely to be functional. The study further shows that AS rates are negatively correlated with gene expression levels, consistent with the idea that highly expressed genes are under stronger selective pressure to minimize splicing errors. Overall, the results support the hypothesis that genetic drift sets an upper limit on the capacity of selection to prevent splicing errors, and that variation in AS rates across metazoans reflects this limit.Random genetic drift limits mRNA splicing accuracy in metazoans. Alternative splicing (AS) is widespread in eukaryotes, but its functional significance remains debated. While some studies suggest AS contributes to adaptive evolution by increasing functional diversity, others argue that higher AS rates in complex organisms may reflect increased splicing errors due to genetic drift. This study analyzed 53 metazoan species with varying effective population sizes (Ne) and found a negative correlation between Ne and genome-wide AS rates, supporting the 'drift barrier' hypothesis. Low-abundance splice variants, which make up most of the splicing repertoire, are enriched in errors, while abundant variants, which are more functionally relevant, show lower AS rates in complex species. These findings suggest that AS rates reflect the limits imposed by genetic drift on the ability of selection to prevent splicing errors. The study also shows that AS rates correlate with Ne proxies such as longevity and body size, and that functional splicing variants are more common in abundant splice variants. Analysis of splicing sites revealed stronger selective constraints on major splice sites compared to minor ones, supporting the idea that abundant splice variants are more likely to be functional. The study further shows that AS rates are negatively correlated with gene expression levels, consistent with the idea that highly expressed genes are under stronger selective pressure to minimize splicing errors. Overall, the results support the hypothesis that genetic drift sets an upper limit on the capacity of selection to prevent splicing errors, and that variation in AS rates across metazoans reflects this limit.