Background
The authors write that "environmental change experienced by many contemporary populations of organisms poses a serious risk to their survival." However, they say that based on the theory of evolutionary rescue, "the combination of sex and genetic diversity [italics added] should increase the probability of survival by increasing variation and thereby the probability of generating a type that can tolerate the stressful environment."

What was done
Lachapelle and Bell tested this theory by comparing populations of Chlamydomonas reinhardtii - which originally differed in (1) their expression of sexuality and (2) their initial amount of genetic diversity - in a laboratory study where selected lines of the species were serially propagated in watery environments, and where the level of stress caused by periodic salt additions gradually increased over time, as the aquatic environments were transformed from a freshwater state to fully marine salinity conditions.

What was learned
"In the long term," in the words of the two Canadian researchers, "the combination of high diversity and obligate sexuality was most effective in supporting evolutionary rescue," such that "most of the adaptation to high-salt environments in the obligate sexual-high diversity lines [italics added] had occurred by midway through the experiment, indicating that positive genetic correlations of adaptation to lethal stress with adaptation to sub-lethal stress greatly increased the probability of evolutionary rescue."

What it means
The take-home message of Lachapelle and Bell's experimental work, as they describe it, is that "the evolutionary rescue events observed in this study provide evidence that major shifts in ways of life can arise within short time frames through the action of natural selection in sexual populations."