Climate change in recent years is rapid and with glaring consequences such as global warming and species extinctions. The situation may appear grim, but not dismal, especially when looked at from different angles. While climate scourge associated with depleting species is a fact, studies monitoring populations over a few decades indicate rapid adaptations in response to climate change too. Certainly, alternate hypotheses need testing to provide an unbiased estimate of climate-change associated species loss. A recent review published by the Experimental Ecology and Evolution (EEE) lab, Ahmedabad University, India, in Frontiers in Physiology offers fresh insights into how “clines” could be used to understand climate mediated species responses. 

A cline is a gradation exhibited for a character/ trait in a species across a geographic range. For instance, populations residing from lowlands to highlands could exhibit graded tolerance to desiccation response from low to high (or vice versa) and this could be interpreted as a cline for desiccation tolerance. Considering selection by rising temperatures (global warming proxy), populations with greater tolerance to desiccation could stand a higher chance of survival than populations with low desiccation tolerance levels. Population survival in pockets like these could maintain species continuity. 

Clines exist for several traits and across several taxa. Not all clines could confer survival advantage to climate induced challenges. The authors suggest exploring clines in the vinegar fly, Drosophila melanogaster, investigated extensively for its molecular repertoire and clines. The review points to the fact that D. melanogaster exhibits clines for diverse traits some of which could prove crucial to modulate temperature extremes, viz. thermal tolerance/ chill coma recovery/ pigmentation/ metabolism etc. Even though dispersal of D. melanogaster from its origins in Africa to the other continents is recent (< 20,000 years), there are considerable similarities and differences in clines and clinal trends across continents. A comparison of clinal trends is also provided among the Drosophila species which indicates both convergent and divergent clinal patterns in this group found across broadly differing spatial scales. 

How have these overlapping/ divergent clines evolved? Could they tell us about an organism’s (in this case the fly) response (adaptive or otherwise) to environmental fluctuations or are they merely local adaptations? Some cues could be gained from changes in the hereditary material (DNA) at the nucleotide (building blocks of DNA) level. Field studies demonstrate that nucleotides vary with respect to seasons which means that alleles’ (alternate forms of gene) frequencies correlate with seasons. If so, alleles for warmer seasons could be strongly selected and species/ populations possessing this allelic variation could survive the growing summer temperatures/ warming.

Clines could be potentially explored both at the phenotypic and genotypic levels to understand adaptive responses in changing environmental conditions. The review points out that clines are still under-explored across several geographic zones and in Drosophila species, clinal studies are largely biased to temperate regions. In the tropics, where the brunt of climate change could be stronger (rising temperatures), clinal studies are far less known. Subhash Rajpurohit, Ramanujan Fellow and Associate Professor at the School of Arts and Sciences, and the main author of the review, adds, “The Indian sub-continent offers a variety of terrains worth investigating clines which are hitherto unknown”. His laboratory website includes ‘Drosoclines’, an online repository of tropical Drosophilid clines. The review ends with suggestions for gaps in clinal studies, the need to revisit older clines and merging clinal investigations with molecular diagnostic tools which could make clines a valuable source to interpret and predict climate induced changes.