For salmon trying to make it upriver to spawn before a hot summer hits, slow and steady loses the evolutionary race.
Salmon DNA records stretching back over 30 years show that nature has increasingly selected for fish that migrate from the ocean earlier in the year. It is among the first pieces of genetic evidence that climate change is driving the evolution of a species.
Many species have changed their migration patterns over the past few decades in response to warmer temperatures. What is difficult to tell is whether the species are changing their behaviour or evolving genetically – or both.
Thanks to an old experiment, researchers at the University of Alaska at Fairbanks were able to confirm that genes play a role in at least one animal's response to warmer temperatures – the pink salmon (Oncorhynchus gorbuscha). Its migration from the ocean to the river is controlled largely by its genes.
In the 1970s, Anthony Gharrett was studying why some salmon migrate up Alaska's Auke Creek a month later than the rest of the population. Gharrett selectively bred some late-migrating fish so that they would share a rare genetic mutation that had no impact on the salmons' survival prospects. This allowed him to identify the late migrators even before the migration began.
By the time Gharrett ended these salmon-breeding investigations in 1985, 26 per cent of the late-migrating salmon population had the genetic marker, compared to 3 per cent of the rest of the population. He continued to collect DNA from the migrating salmon population every few years.
Fast forward to 2011, when Ryan Kovach and David Tallmon analysed Gharrett's 32-year-long fish DNA record. They found that the number of salmon with the genetic marker – more likely to be late-migrating fish – was relatively stable throughout the 1980s.