Using experimental evolution, we investigated the contributions of ecological divergence, sexual selection, and genetic drift to the evolution of reproductive isolation in Caenorhabditis remanei. The nematodes were reared on two different environments for 100 generations. They were assayed for fitness on both environments after 30, 64, and 100 generations, and hybrid fitnesses were analyzed after 64 and 100 generations. Mating propensity within and between populations was also analyzed. The design allowed us to determine whether local adaptation was synchronous with pre- and post-zygotic reproductive isolation. Pre-zygotic isolation evolved quickly but was unconnected with adaptation to the divergent environments. Instead, pre-zygotic isolation was driven by mate preferences favoring individuals from the same replicate population. A bottleneck treatment, meant to enhance the opportunity for genetic drift, had no effect on pre-zygotic isolation. Post-zygotic isolation occurred in crosses where at least one population had a large fitness advantage in its 'home' environment. Taken together, our results suggest that pre-zygotic isolation did not depend on drift or adaptation to divergent environments, but instead resulted from differences in sexual interactions within individual replicates. Furthermore, our results suggest that post-zygotic isolation can occur between populations even when only one population has greater fitness in its home environment.