Alternative reproductive tactics and evolutionary rescue

Almost all life on earth is facing environmental change, and understanding
how populations will respond to these changes is of urgent importance. One
factor that is known to affect the speed by which a population can evolve
when faced with changes in the environment is strong sexual selection.
This increases the adaptive capacity of a population by increasing
reproductive skew towards well-adapted (usually) males who will, on
average, be best able to compete for matings. This effect could
potentially be disrupted when males pursue alternative reproductive
tactics (ARTs), whereby males within a species exhibit qualitatively
different behaviours in their pursuit of matings. ARTs are diverse but one
common class is those expressed through condition-dependent polyphenism
such that high-quality, well-adapted males compete aggressively for mates
and low-quality, poorly adapted males attempt to acquire matings via
other, non-aggressive behaviours. Here, using an individual-based
modelling approach, we consider the possible impacts of ARTs on adaptation
and evolutionary rescue. When the ART is simultaneous, meaning that
low-quality males do engage in contests but also pursue other tactics,
adaptive capacity is reduced and evolutionary rescue, where a population
avoids extinction by adapting to a changing environment, becomes less
likely. This is because the use of the ART allows low-quality males to
contribute more maladaptive genes to the population than would happen
otherwise. When the ART is fixed, however, such that low-quality males
will only use the alternative tactic and do not engage in contests, we
find the opposite: adaptation happens more quickly and evolutionary rescue
when the environment changes is more likely. This surprising effect is
caused by an increase in the mating success of the highest quality males
who face many fewer competitors in this scenario—counterintuitively, the
presence of males pursuing the ART increases reproductive skew towards
those males in the best condition.