Rapid adaptation to temperature change… and its limits

ResearchBlogging.orgPeople often think of evolution as though natural selection were sitting around waiting for new mutations to promote or cull. But it’s not really like that. A great deal of variation exists in any population, much of which has little or no effect on the survival or reproductive success of individuals carrying that variation. However, a changing environment can alter all that.

Gasterosteus aculeatusBarrett et. al. (2010) were interested in how population of three-spine sticklebacks (Gasterosteus aculeatus) would respond to lower temperature extremes. They collected a sample of sticklebacks from both marine lagoons and freshwater lakes in British Columbia, Canada. First, they acclimated the fish to living in fresh water, as well at a consistent temperature and daylight length.

Lakes are far more variable in temperature than the oceans – they are warmer in summer and cooler in the winter – due to the smaller quantity of water which needs to gain or lose heat. Rather unsurprisingly, the researchers found that the lake-dwelling sticklebacks could tolerate significantly colder temperatures than their marine counterparts (both populations could tolerate much higher temperatures than they ever encountered in the environment). They also demonstrated that the degree of tolerance for cold extremes was heritable – even raised in the same environment, the offspring of lake-dwelling fish could tolerate lower temperatures, whereas marine sticklebacks could not (and hybrids were intermediate).

The interesting part, however, was when they got to raising populations of sticklebacks with marine ancestors in ponds, which could get even colder in the winter than the freshwater lakes. In just three generations (three years), the population evolved to tolerate temperatures 2.5°C colder than their marine forebears! This wouldn’t have been a new mutation – existing genes, already present (but perhaps rare) had become far more common in the population than they had previously been.

It wasn’t all good news for the sticklebacks, though. Genetic diversity is critical to maintaining populations, and a period of such strong natural selection will dramatically reduce a population’s diversity. Even if a population can adapt to one sudden shock, it may so deplete their genetic diversity that there won’t be any convenient alternative genes in the population when the next hit comes.

Canada temperature anomaly 2009 vs 2006-2008 from GISTEMPThe next year brought the coldest winter that part of Canada had seen for several decades, and despite all their adaptations, all three of the experimental populations were wiped out. It may be that it was just too cold, or perhaps the increased ice cover on the ponds reduced the oxygen levels in the water to below what the fish needed. Either way, it’s a grim prospect for conservation biologists if a population that seems, by all accounts, to be surviving and even adapting to the changes in its environment can suddenly hit an unpassable barrier and go extinct.

Sticklebacks have a history of being able to adapt to significantly changing temperatures over the last few millennia, and so they may have had an advantage in having genes for dealing with a changing climate already present in their populations. That may not be the case for all species, and this study has shown just how drastic effect a change in temperature extremes can have on populations.


Barrett, R., Paccard, A., Healy, T., Bergek, S., Schulte, P., Schluter, D., & Rogers, S. (2010). Rapid evolution of cold tolerance in stickleback Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.0923

Review: The Greatest Show on Earth

[cross-posted onto Young Australian Skeptics]

In previous books, Richard Dawkins has looked to promote new perspectives on natural selection, and at barriers to the understanding of evolution. In The Greatest Show on Earth, however, he looks at the evidence for evolution. You can read extracts from chapter one and chapter two online.

One of the book’s major strengths is the level of detail that Dawkins goes to in explaining difficult concepts and interesting experiments. For example, in the chapter on embryonic development, we learn how (well, one of the ways) in which mutations in the genome actually affect what a cell can do. This level of detail is a recurring feature. For example, we are treated 14 pages of glorious detail on Richard Lenski’s E. coli experiments, much of which I personally hadn’t picked up on when the story broke.

Dawkins considers molecular evidence to be the strongest line of evidence for evolution, and so the fossil record is just a bonus. He clearly outlines the problems with claiming that “gaps” in the fossil record make an argument against evolution. As far as fossils are concerned, the focus is on some of the more recent finds – and what a selection he has to choose from! He discusses tetrapods, whales, manatees, pinipeds (seals, sea lions & walruses) and turtles. It’s certainly a daunting prospect, to be claiming gaps in the fossil record, in the face of just these recent fossils. The discussion of homology was strong, not least for explicitly reverting to a pre-evolution definition of homology.

I particularly enjoyed the sections on development and molecular evidence – not least because of my unfamiliarity with these areas, and thus there was plenty for me to learn. I’m somewhat hopeful I’ll manage to work some self-organising systems into my doctorate somewhere.

Maybe I’m just a fan of cladograms, but I feel a couple of high-level cladograms – one of vertebrates (with a particular focus on the varieties of fish) and another of sauropsids – would have been worth a thousand words or so each. Another sour note was the mention of Andrew Schlafly in the discussion of the Lenski experiments, which mostly reeked of schadenfreude. I would have also avoided including any Haeckel drawings to illustrate any points – you just know that Haeckel’s crustaceans are going to give the history-deniers an irrelevant point to scream about whilst avoiding substantive discussion.

The depth of description – of the experiments, the discussion of human ancestry, and of the details of molecular & developmental biology is magnificent. As we’ve come to expect from Dawkins’ books, the writing is flowing and understandable, even on technical topics. And, as someone who had constantly had to maintain both my place in the main text and in the footnotes whilst reading The Selfish Gene, I was glad to see that the footnotes are at the bottom of each page, as opposed to at the back of the book.

Of course, Jerry Coyne published Why Evolution is True earlier this year. Is it worth reading both? The answer: yes, emphatically yes! Both books have very different lines of evidence on which they focus – Dawkins, for example, is highly focused on experiments, whereas Coyne focused more on observation in nature and the fossil record. Many lines of evidence, or topics for discussion, are only in one or other, or emphasised differently. There’s also a distinct difference in how they discuss creationism – Dawkins only mentions it occasionally and tends to give the evidence for evolution on its own merit (except in the chapter on biogeography, really).

So, yes, go and buy it. More importantly, go and read it!