New research shows that species evolve faster to adapt to the climate getting colder than to it getting hotter, and that the rise in heat they can adapt to has limits.

Red honey ants endure temperatures above 50 °C in the Australian desert, while the springtail survives extreme cold of -30 °C in the Antarctic.

A new study, published today in Nature Communications, explores how evolution has prepared some species to withstand these temperatures and how fast different species evolve to changing temperatures.

Physiological tolerance to heat and cold determines where on the planet an organism can survive yet we have limited understanding of how this tolerance evolves over time. This is an area of increasing interest as the climate heats up more quickly than ever before.

The new research helps to fill this gap in our knowledge, showing that species evolve heat tolerance more slowly than cold tolerance. It also suggests that heat tolerance cannot keep evolving indefinitely, most likely because there are fundamental limits to how far membranes and proteins can withstand heat.

The study involved a group of renowned ecologists, physiologists and evolutionary biologists, including Dr Sally Keith, an ecologist from Lancaster University. They were gathered together by Professors M.Á. Olalla-Tárraga, from the University Rey Juan Carlos, Spain, and I. Morales-Castilla from the University of Alcalá, Spain, funded by the German Centre for Integrative Biodiversity Research (iDiv).

The first step was to commission Dr Joanne Bennett, who was working at iDiv, to create an unprecedented database on the ability of different species to tolerate heat extremes (GlobTherm database).

'It took me more than a year to collect data for more than 2000 diverse species, including multicellular algae, marine invertebrates, mammals, birds and plants. I had to synthesize scientific works published across multiple decades that used very diverse methodologies', said Dr Bennett, lead author on the study, who is now at the University of Canberra in Australia.

The authors then tested whether past climate 'legacies', current climatic extremes or physiological boundaries to evolution could best explain the enormous variation in thermal tolerance across species.

'Our work shows that the ability to adapt to cold has evolved up to twice as fast as the ability to adapt to heat', comments Olalla-Tárraga. 'The evolution of cold tolerance was fastest in endotherms [warm blooded species that can generate their own heat], perhaps reflecting their more recent evolution and expansion into cold climates,' adds Morales-Castilla.

Despite this varied evolutionary history, species appear best adapted to the extreme temperatures that they experience today, rather than to the temperatures that prevailed at the time they first evolved. However, the authors also detected evolutionary 'attractors', which suggest there is an upper limit to how far physiological processes that increase heat tolerance can evolve.

'These attractors suggest that, although species appear to have adapted so far to warming climates, the process cannot continue indefinitely. The implication for species survival under ongoing climate change is of big concern if Earth’s temperatures exceed what appears to be a fundamental physiological boundary,' said Dr Sally Keith from Lancaster University.

Read the research article (open access): Bennett, J.M., Sunday, J., Calosi, P. et al. 'The evolution of critical thermal limits of life on Earth' Nat Commun 12, 1198 (2021). https://www.nature.com/articles/s41467-021-21263-8