[Truncated abstract] This study focussed on how isolation and environmental heterogeneity in salt lakes has influenced intraspecific variation in the calanoid copepod Calamoecia clitellata. Calamoecia clitellata relies on passive vectors for dispersal, and this, coupled with the insular nature of salt lakes, may promote genetic divergence at a molecular level. When contrasting environments are involved, genetic divergence may also occur at the life history level, possibly due to local adaptation. I examined the distribution of genetic variation among 14 populations in Western Australia using molecular genetic markers, and examined variation in life history traits among contrasting environments. To ascertain how isolation had influenced molecular genetic variation, I determined population genetic structure and used a phylogeographic approach to infer the impact of historical events. Environmentally induced variation was also evident in the field, with a switch from subitaneous egg production to resting egg production coinciding with changing environmental conditions. It is proposed that plasticity in life history traits has evolved in response to temporal environmental heterogeneity … It can be concluded that isolation in salt lakes in Western Australia has influenced molecular and phenotypic variation in C. clitellata in contrasting ways. At the molecular level, contemporary and historical isolation have promoted genetic divergence of populations, yet when coupled with environmental heterogeneity, marked phenotype plasticity has arisen. This study raises questions as to whether phenotype plasticity is a widespread phenomena in zooplankton found in temporary saline waters and an adaptive strategy to tolerate marked temporal environmental heterogeneity