Species evolutionary potential is linked to the interplay between gene flow, mutation, genetic drift and selection, impacting both the amount and distribution of genetic variation important for adaptation. In rapidly changing environments the ability to evolve depends largely on the distribution of this variation, both within and across populations. Maintenance of evolutionary potential in rare species represents a challenge as these species are often isolated and exhibit reduced population size, exacerbating the genetic consequences of rarity. Thus, one of the major outstanding questions for rare species asks whether rare species have the necessary variation to evolve and if genetic variation is lacking, what may be the consequences of genetic rescue, defined as the intentional immigration of new alleles between populations with the aim to increase population fitness and associated demographic vital rates.
To evaluate evolutionary potential of rare species, we assessed variation in traits important to plant fitness using common garden experiments in Torrey pine (Pinus torreyana) one of the rarest pines in the world, restricted to one mainland and one island population. We compared phenotypic differences between mainland, island and F1 hybrid individuals over ten years, comparing early germination traits and annual fitness metrics height and fecundity following establishment.
Results/Conclusions
Phenotypic variance within the common garden suggests population-specific growth trajectories have evolved in situ, underlying genetic differences between populations. Indeed, the island population exhibited a substantially reduced growth rate and little genetic variance relative to the mainland or hybrid population, indicating increased inbreeding which may reduce the island individuals’ ability to respond to environmental stochasticity. F1 hybrids, however, exhibited increased fitness relative to mainland and island individuals at all development stages, suggesting a potential role for genetic rescue via intraspecific hybridization in this genetically depauperate species. However, unidirectional hybridization within the F1s suggests barriers to reproduction may have evolved between these populations, indicating between-population crosses may not necessarily represent a viable option to conserve evolutionary potential. This long-term dataset provides an invaluable resource to test predictions regarding the use of genetic rescue in a rare, long-lived species and may be used to direct conservation strategies to preserve the evolutionary potential of Torrey pine and design ex situ conservation strategies long term.