A new approach to sympatric speciation.

Sympatric speciation has always been a controversial issue. Recently, the debate has centred around the possibility that host race formation in phytophagous insects occurs as a first stage in this process. Now a new approach to host race formation - examining the population genetics of performance on alternative hosts - is challenging a major underlying assumption of this model. The evidence suggests that improved performance on one host is not generally accompanied by reduced performance on others.