Gradual speciation is more likely to be associated with small changes building up in:

Eliminate incorrect options: Sudden large-scale mutations in a single generation are not gradual and are more associated with rapid speciation. Similarly, changes in the immediate environment without genetic change do not directly lead to speciation. Migration into a population may introduce new alleles but does not necessarily lead to gradual speciation unless it affects allele frequencies over time.

Connect gradual changes to evolutionary processes: Gradual speciation aligns with the concept of microevolution, where small genetic changes accumulate over generations. These changes can eventually lead to reproductive isolation, a key factor in speciation.