Evolution of a genetic polymorphism with climate change in a Mediterranean landscape

Abstract

Many species show changes in distribution and phenotypic trait variation in response to climatic warming. Evidence of genetically based trait responses to climate change is, however, less common. Here, we detected evolutionary variation in the landscape-scale distribution of a genetically based chemical polymorphism in Mediterranean wild thyme (Thymus vulgaris) in association with modified extreme winter freezing events. By comparing current data on morph distribution with that observed in the early 1970s, we detected a significant increase in the proportion of morphs that are sensitive to winter freezing. This increase in frequency was observed in 17 of the 24 populations in which, since the 1970s, annual extreme winter freezing temperatures have risen above the thresholds that cause mortality of freezing-sensitive morphs. Our results provide an original example of rapid ongoing evolutionary change associated with relaxed selection (less extreme freezing events) on a local landscape scale. In species whose distribution and genetic variability are shaped by strong selection gradients, there may be little time lag associated with their ecological and evolutionary response to longterm environmental change.

Evolution caused by extreme events

“…Nevo et al [64] used a common garden resurrection experiment to demonstrate rapid evolution of earlier flowering from 1980 to 2008 in wild cereal populations across Israel. Thompson et al [65] observed an increase in frequency of frost-sensitive but summer drought-tolerant phenolic chemotypes in populations of Mediterranean wild thyme (Thymus vulgaris) associated with a recent decline in severe freezing events.…”

An allelopathic plant facilitates species richness in the Mediterranean garrigue

“…Thymus vulgaris makes up a dominant component of this vegetation. Inside the valley, the variation in monoterpene production among several different thyme populations has been monitored for 40 years (Thompson 2005;Thompson et al 2013). This allowed us to choose sites where thyme populations are known to be homogenous in their monoterperne production over a long time period.…”

Elevational gradients in plant defences and insect herbivory: recent advances in the field and prospects for future research

“…By monitoring how insect herbivory and plant defence along elevational gradients change over time, the list of candidate abiotic drivers can be narrowed substantially as novel climates decouple these factors. For example, Thompson and colleagues used multi-decadal elevational surveys of chemical defences (terpenes) in wild-growing thyme Thymus vulgaris to show that less palatable, freeze-intolerant chemical types have colonized higher elevations as climate change has changed the elevations where freezing occurs (Amiot et al 2005, Thompson et al 2013). However, researchers should be aware that climate change may affect other abiotic drivers of plant-insect herbivore interactions that may not vary over elevational gradients.…”