Data from: Contemporary evolution of an at-risk stickleback population during a severe drought

Populations can be granted conservation status because they harbour a set
of unique traits, evolutionary histories, or ecological roles. Such
populations are often isolated and specialized and, as such, can be
particularly vulnerable to environmental disturbances. Even if distinct
populations survive and adapt to severe disturbances, they could show
changes in the very traits that made them distinct in the first place.
Here, we leverage a natural “experiment” involving an unarmoured
population of threespine stickleback (Gasterosteus aculeatus) in Rouge
Lake (Haida Gwaii, BC) – a population listed as Special Concern under the
Canadian Species at Risk Act. In 2015, Rouge Lake nearly dried up during a
severe drought event; yet the stickleback population appeared to have
fully recovered its abundance in subsequent years. Using phenotypic
measurements, we assessed the extent to which evolution in this population
was impacted by the drought. We document important shifts in several
phenotypic traits, with the largest occurring in precisely the trait that
made the population distinct and prompted its original conservation
designation. Specifically, fish with no lateral plates (i.e.,
“unarmoured”) made up 51 % of the population before the drought, but only
13 % after the drought. This shift held (13-16 % unarmoured) over the four
years of our post-drought monitoring. Field observations support a strong
demographic bottleneck, which we suggest might have been coupled with a
shift in the selective regime. These findings underscore how populations
of conservation concern are not only at risk of extinction; they are also
at risk of losing the characteristics that make them unique. These
dynamics highlight the need for policies to consider a population’s
evolutionary potential and develop more flexible approaches than simply
considering single-timepoint assessments of diversity.