Genomic Evidence for Adaptation to Global Warming??

The "burning embers" diagram above w...

The “burning embers” diagram above was produced by the IPCC in 2001. (Photo credit: Wikipedia)

Global Warming – The ultimate quibble of this century !! Or should i say the “haute” of this century. Why? Now, common ask yourselves, which single topic apart from the religion/atheism debate, you always hear in any gathering or book reading circles or conferences or on social platforms which is ready to divide people into two opposing camps. Books are being written, movies made, debates fought and what for – “The legitimacy of Global warming”. Despite numerous evidences detractors still love to question it. However, a new paper published this month in PNAS provides genomic evidence for phenotypic responses to climatic warming.

What’s it all about?

Ongoing changes in regional climates, especially the trend of warming winters and blazing summers are pushing many species (both plants and animals) to shift their distribution toward higher latitudes and altitudes. Such a change in the species distribution, with an expansion in previously hostile areas and contraction in their own habitats which are becoming less favorable, can occur rapidly both in plants and animals. However, not all species can migrate to lesser hostile areas, and there are many reasons proposed for it. Primarily among them is the increasing trend of Habitat Fragmentation. Habitat fragmentation can result from human expansion into wilder areas resulting in few phenomena:

  • Reduction in the total area of the habitat
  • Isolation of one habitat fragment from other areas of habitat
  • Breaking up of one patch of habitat into several smaller patches
  • Decrease in the average size of each patch of habitat

As, a result of such human activity many species especially plants can’t migrate into other areas resulting into their dwindling numbers. But some species do survive in such increasingly fragmented habitats and hence have adapted to the climatic warming. Though some previous studies in Drosophila melanogaster have shown adaptive trait variation in relation to climate change in both natural and experimental population, however in some cases, the evolutionary response to climate change may be slow due to genetic constraints causing a time lag between the environmental change and an observed evolutionary response. Hence,understanding how various species track climate warming by genetically based adaptive trait variation and which traits facilitate the evolution of such adaption is important.

What is the new evidence?

Thymus vulgaris

The authors decided to look at Mediterranean wild thyme (Thymus vulgaris), a low growing herbaceous plant which is native to Southern Europe and is often used as a culinary herb. The plant contains many oils and the chemical composition(phenolic or non-phenolic) of it varies in different regions based on the temperature. These oils make a plant adaptable to freezing and hence different climatic areas have plants with varying composition of oils (chemotypes). So it would be worthwhile to see if the recent trend of gradual warming of extreme winter freezing events, has brought about an evolutionary response in plants i.e, has their chemical composition changed over time? Interestingly, any such change in the respective oil compositions in different climactic areas with different temperatures would have a genetic basis. And this is what the authors looked about.

The study area had a Mediterranean climate with summer drought but also severe winter freezing temperatures within the basin as a result of a dramatic temperature inversion In this area, there are six different chemotypes that are the expression of a genetically controlled polymorphism in T. vulgaris. Two phenolic chemotypes (carvacrol and thymol) are largely dominant on the slopes above 250-m elevation and four nonphenolic chemotypes (linalool, thuyanol-4, α-terpineol, and geraniol) below 200m elevation, where they experience the winter temperature inversion. Hence, phenolic chemotypes are predominantly winter non-tolerant whereas non-phenolic types are winter tolerant.

There is thus a sharp gradient in the chemotype frequency over only 3–5 km that goes from 100%of either phenolic or nonphenolic chemotypes to 100% of the other form, with a narrow transitional zone. In short, nonphenolic chemotypes show adaptation to habitats, which in the past have experienced extreme freezing temperatures in early winter, whereas phenolic chemotypes are sensitive to intense early-winter freezing and occur in habitats where extreme summer drought can exclude nonphenolic chemotypes.

Coldest annual temperature from 1955 to 2010 at the weather station of SaintMartin-de-Londres (filled squares), which occurs in the zone dominated by freezing-tolerant nonphenolic chemotypes, and from1970 to 2010 at the Centre d’Ecologie Fonctionnelle et Evolutive–Centre National de la Recherche Scientifique experimental gardens on the northern periphery of Montpellier (open circles), where natural thyme populations are dominated by freezing-sensitive phenolic chemotypes.

Hypothesis: Phenolic chemotypes (thymol and carvacrol) now occur in sites where they were previously absent or have increased their frequency in the transitional sites due to a relaxation of selection pressure normally associated with extreme early winter freezing temperatures due to climatic warming.

To do so, they compared the chemotype composition of populations observed in the early 1970s  to that in 2009–2010 for 36 populations sampled along six transects. Each transect was <10 km long, each containing six populations, with two “phenolic,” “mixed,” and “nonphenolic” populations.

They found that the mean percentage of phenolic chemotypes in a population was significantly (df = 35, S = 68.5, P < 0.01) higher in the contemporary samples (overall value of 53.1%) than in those of the initial study (47.7%) of 1970’s. The changes in composition of the initial nonphenolic populations were associated with the appearance of the thymol chemotype in all eight of the populations whose composition changed and the carvacrol chemotype in three of them.

The changes reported involved a reduced intensity of freezing events and changes in frequency of freezing tolerant and nontolerant phenotypes in natural populations of the Mediterranean aromatic plant, Thymus vulgaris. A significant appearance of freezing-sensitive phenolic chemotypes in sites where they were historically absent and an increase in their frequency in previously mixed populations was observed. Such changes have occurred in 17 of the 24 populations where they could potentially occur.

Such studies, illustrate that a rapid evolutionary response to temperature modifications can occur where genetic variation is combined with a change in a previously strong selection pressure, even for a perennial woody plant. Hence, this provides quite a neat example of genetic changes brought about by climatic warming.  I guess, the detractors of global warming would be feeling quite uneasy now !!

More on this:

  1. Genetic consequences of climate change for northern plants, Alson, Proceedings of Royal Society B, 2012.
  2. Climate extremes: Observations, modeling, and impacts, Easterling DR, Science, 2000.
  3. Ecological and evolutionary responses to recent climate change, Parmesan C, Annu Rev Ecol Syst Evol, 2006.
  4. Ecological responses to recent climate change, Walther GR, Nature, 2002.
  5. Rapid shifts in plant distribution with recent climate change, Kelly AE, Goulden ML, Proceedings of National Academy of Sciences, 2008.

  6. The distributions of a wide range of taxonomic groups are expanding polewards, Hickling R,Global Change Biology, 2006.
  7. A globally coherent fingerprint of climate change impacts across natural systems, Parmesan C, Nature, 2003.

  8. Running to stand still: Adaptation and the response of plants to rapid climate change, Jump AS, Ecology Letters, 2005.
  9. Genetic response to rapid climate change: It’s seasonal timing that matters, Bradshaw WE, Molecular Ecology, 2008.

  10. Climate change and evolutionary adaptation, Hoffmann AA, Nature, 2011.

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