Cool Fish - Rapid Evolution, Stickleback and Evolutionary Biology in Canada

Dr. Rowan Barrett, currently of Harvard but previously of UBC with Dolph Schluter, presented an interesting experiment at the CSEE conference last weekend that I wanted to share with you.  My goal in sharing these experiments are twofold:  first, I think the research being done in Canada needs to be celebrated, but we researchers do a poor job in communicating our findings to the public; and secondly, and more in the spirit of this blog, I think a lot of evangelical Christians hear ‘evolutionary biologist’ and immediately assume that they are biologists of fantasy, interpreting the world through a dishonest lens.  I hope that the Christians who read about these experiments will realize that these researchers are not trying to pull the wool over our eyes; instead they are following the evidence, trying their best to understand the complexity of this dynamic world.

Barrett’s talk was entitled ‘Rapid evolution of cold tolerance in stickleback.’

This is a stickleback.  These are the fish that I work with.  Immediately this experiment butts heads with a Young Earth interpretation of the world, as it begins by acknowledging the work of geologists and climatologists, whose overwhelming evidence suggest that 12 000 years ago not only was the world around, but much of Canada was covered in sheets of ice.  The freshwater of British Columbia and Alberta did not exist; the fish that currently inhabit these bodies of water had to migrate in from ‘refugia’ (places free of ice) or from the ocean itself once the glaciers had retreated.

Threespine stickleback are no exception.  Although they are currently found in numerous freshwater rivers and lakes along the coast of British Columbia, 12 000 years ago these habitats simply did not exist.  So where did the freshwater stickleback come from?  Unlike many fish, which can only survive in salt water or fresh water but not both, threespine stickleback come in several different forms: marine, freshwater, and anadromous (meaning they migrate between fresh and marine waters).  Within freshwater, there are lake and stream forms as well, and within some lakes there are bottom dwelling and open water forms, but those are beside the point.  The main thing you need to know is that marine and freshwater populations exist right next to one another in BC.  A lake on a small island between Vancouver Island and the mainland contains freshwater stickleback; a bay on the same island contains the marine stickleback.

It would seem that, when the glaciers retreated and opened up new freshwater lakes, threespine stickleback invaded these lakes from the ocean.  As the weight of the glaciers decreased, the land sprang up, isolating these lakes from the ocean and trapping the stickleback inside.

This might seem farfetched to some, but it is based on great geological work done in BC by Canadian researchers.  These researchers did not require any knowledge of sticklebacks or evolution to come up with their ideas.  Once they had put together a story, however, biologists were able to take that evidence and apply it to evolution.  And it turns out to have matched beautifully.  Genetic reconstructions of stickleback ancestry reveal that, indeed, the freshwater stickleback are closely related to marine stickleback, and that in many cases the freshwaters are genetically more similar to the nearby marines, than they are to other freshwaters.  This is exactly what you would expect if marine stickleback were invading new lakes all down the Pacific northwestern seaboard.

Freshwater (top) and marine (bottom) representatives of threespine stickleback, stained with Alizarin red to show differences in plate number.

What is also interesting is that, in each of these freshwater environments, threespine stickleback evolved in a similar manner.  One of the biggest changes involves body armour.  Stickleback are scaleless fish.  Marine populations are instead covered in bony plates which protect them from fish predators.  Freshwater populations, however, on average have very few plates.  I have seen this for myself, and the differences really are staggering.  Low-plated individuals are very rare in the ocean, and completely-plated individuals are very rare in lakes.  Many other differences have also been detected, from smaller defensive spines in the freshwater populations to extreme differences in body shape.

All of this evolution must have occurred in less than 12 000 generations, since the age to maturity of stickleback is one to two years.  This is incredibly rapid evolution.

Barrett took all of this information and he wondered how stickleback were able to survive in the freshwater environment at all.  After all, freshwater temperatures are much higher during the summer and lower during the winter than are ever experienced in the ocean.  Marine stickleback invading freshwater would have had to adapt fairly rapidly to these new temperatures.

So Barrett collected wild marine and freshwater stickleback from a few locations in BC and brought them back to his lab, where he subjected them to either increasingly colder temperatures or increasingly warmer temperatures.  The point was to determine at what temperature the fish were‘ecologically dead’ – that is, at what temperature were the fish no longer able to stay upright, making them vulnerable to predation (fish were able to recover after being brought back down to normal temperatures).  This experiment showed, first of all, that both marine and freshwater stickleback can tolerate very high temperatures (on average over thirty degrees, which is at least eight degrees warmer than the freshwater environment ever experiences!), and that both freshwater and marine stickleback tolerated the same warm temperatures.  This is pretty interesting, because it suggests, first of all, that invading marine stickleback did not need to adapt to survive the high summer temperatures of freshwater (caveat: this only tested their ability to survive for a brief amount of time, and did not assess long-term effects of exposure to high temperatures), and second it suggested that freshwater populations can tolerate a good amount of environmental warming (with the same caveat as above).

The cold temperature trials were more interesting.  The marine fish could only tolerate temperatures that were as cold as their environment typically got – between 5-6 degrees C.  The freshwater populations, on the other hand, could tolerate an average just above 1.5 degrees.  There was some variation, however, with some individual marines tolerating lower temperatures, and some freshwater marines being unable to tolerate their population's average.

Keep in mind that these freshwater populations were once marine populations.  This is amazingly rapid evolution.

But the evolution turned out to be faster than anyone had anticipated.

Barrett took his marine fish with their known cold temperature tolerance of just above 5 degrees C, and he transferred them into three experimental freshwater ponds on the UBC campus, which experienced normal freshwater temperatures during the course of the year.  These marine stickleback survived in the freshwater and bred and ate and lived and after three years he sampled their third-generation offspring.  He then subjected these fish to the same cold tolerance trials that he had subjected their ancestors to.  And guess what?  Within three generations, they had evolved to tolerate temperatures 2.5 degrees lower than their ancestors.  Their cold tolerance ability was now similar to that of the freshwater fish.

This is being touted as the most rapid rate of evolution to be detected in a natural population.

That is cool.*

*Get it?  Cool, like the temperature and like the experiment!

Pictures from http://wanderinweeta.blogspot.com/2010/07/tracking-invisible-fish.html
and from myself.