No change in genetic variance
The effect of selection on genetic variation is a key topic in
evolutionary biology. Strong stabilizing or directional selection is
expected to erode genetic variance (Barton and Turelli, 1989; Estes and
Arnold, 2007). However, when many loci are contributing to variation in
the trait under selection, selection might leave no detectable signature
on the genetic variance associated with that trait (Bulmer, 1971;
Johnson and Barton, 2005). Although our truncated selection increased
and decreased the phenotypic mean by three-quarters of a standard
deviation in the high and low line, respectively, we observed no
significant decrease in the coefficient of additive genetic variance.
There are several explanations for this observation. First, several
genetic factors may be involved. Previously, genetic mapping studies
revealed several QTL underlying difference in sex pheromone composition
in H. subflexa (Groot et al., 2013). Since variation in pheromone
composition in H. subflexa is also explained by fitness variation
(Blankers et al., 2021), many different genetic factors likely
contribute small additional fractions to the total variance. Another
explanation comes from population genetics: re-orientation of genetic
covariances in response to bottlenecks may free up additive genetic
variance, thereby paradoxically increasing levels of genetic variance
(Carson, 1990; Templeton, 2008). Since truncation selection is
effectively a non-random bottleneck of the population, this may also
explain the maintenance of genetic variance that we observed. Lastly, to
avoid inbreeding depression, we maintained large populations and avoided
mating first and second-degree relatives. Such a mating scheme likely
counteracts some loss in genetic variability due to selection (Du et
al., 2021).