Climate change drives salmon evolution
July 2012 by Sara Reardon
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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 latemigrating 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.
Disappearing marker
But between 1989 and 1993 there was a steep decline in the marker's frequency. It is now found in
about 3 per cent of all fish, and it is no longer possible to distinguish early and late-migrating
salmon by studying the frequency of the genetic marker in the population.
The speed with which the marker has disappeared from the population suggests that something in
the salmon's environment changed between 1989 and 1993, making late-migrating fish less fit and
increasing the number of fish in the general population with an early-migrating heritage. Stream
temperatures in 1989, during the peak migration time, were the second highest on record, the
researchers note.
It is an exciting result, says Lisa Crozier of the US National Oceanic and Atmospheric
Administration in Seattle. The findings fit with many observations of salmon changing their
migration patterns, although each population is different: some are migrating later in the autumn
rather than earlier in the spring, for example. If these shifts have a genetic component, she says,
the selection of certain salmon could have long-term effects on their genetic diversity.
While it's good to know that the salmon are adapting, Kovach says, an altered salmon migration
pattern is "worrisome" for other animals, such as those that depend on eating the fatty fish in order
to hibernate or produce milk for their young on schedule.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2012.1158