Ecological speciation can happen when different populations adapt to different environments, leading to reproductive isolation. A review found 16 examples where adaptation to different temperatures may cause populations to become reproductively isolated or coexist as separate species. These populations often live next to each other along a temperature gradient. Selection against hybrids or maladapted individuals plays a role in keeping populations separate. It's not clear if this process can lead to full speciation in the face of gene flow, but more research is needed to understand this better.