Final answer:
Islam's spread before 1450 significantly unified disparate regions like West Africa and South Asia through a shared cultural and commercial network centered around the Islamic faith and the use of Arabic.
Step-by-step explanation:
One similarity in how Islam affected society in two regions where it spread to before 1450 is evident in the spheres of trade and commerce. Islam's spread contributed significantly to the increased trust among traders and merchants and established a cohesive network based on shared language, values, and traditions. For instance, the spread of Islam into West Africa and South Asia facilitated a vast trading network linked by a common Islamic culture and Arabic as the lingua franca. This unifying system allowed for enhanced commerce, as seen in the trans-Saharan trade routes and the expansion of market economies in India under Islamic rulers like those of the Mogul Empire.