Background
The authors write that "within-individual plasticity (acclimation) counteracts potentially negative physiological effects resulting from environmental changes and thereby maintains fitness across a broad range of environments," which fact suggests, in their words, that "the capacity for the acclimation of individuals may therefore determine the persistence of populations in variable environments [italics added]."

What was done
Seebacher et al. "determined phylogenetic relationships by Amplified Fragment Length Polymorphism (AFLP) analysis of six populations of mosquitofish (Gambusia holbrooki) from coastal and mountain environments and compared their capacity for thermal acclimation.

What was learned
The six scientists were able to demonstrate that mosquitofish populations "are divided into distinct genetic lineages and that populations within lineages have distinct genetic identities." In addition, they report "there were significant differences in the capacity for acclimation between traits (swimming performance, citrate synthase and lactate dehydrogenase activities), between lineages, and between populations within lineages," thereby demonstrating that "there can be substantial variation in thermal plasticity between populations within species," leading them to conclude that "similar responses are likely to be found in other species that comprise structured populations."

What it means
Noting that "many predictions of the impact of climate change on biodiversity assume a species-specific response to changing environments," Seebacher et al. proceed to argue, on the basis of their results, that "this resolution can be too coarse and that analysis of the impacts of climate change and other environmental variability should be resolved to a population level," since their findings suggest that some populations of a species may indeed be able to cope with a change of climate with which others cannot, thereby preventing the otherwise inevitable climate-induced extinction of the species.