Standard deviation of GC content at different spatial scales. Genomes were partitioned into non-overlapping windows (5-, 20-, 80-, and 320-kb). As window size increases, variation in GC content naturally decreases. The western painted turtle exhibits a pattern consistent with high variation in nucleotide composition at smaller scales, rather than sustained isochoric variation at larger scales seen in mammals and birds. The expected pattern of decreasing standard variation assumes a compositionally homogeneous genome with a mean GC proportion of 0.41.