Final answer:
Crossing over is the process through which chromosomes physically exchange segments. It occurs during prophase I of meiosis and enhances the diversity of offspring.
Step-by-step explanation:
The first suggestion that chromosomes may physically exchange segments came from the microscopic identification of crossing over. Crossing over occurs during prophase I of meiosis, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes. This process creates unique combinations of genes and enhances the diversity of offspring.