Adaptation and gene flow in central and peripheral populations
Peripheral populations are usually smaller and inhabit less optimal environments than core populations. Gene flow can accelerate local adaptation by providing genetic variation into peripheral populations, or it can inhibit local adaptation at range peripheries by introducing maladapted alleles. Disjunct peripheral populations may be 1) locally adapted due to selection and isolation or 2) severely maladapted due to small population size, low genetic diversity and inbreeding. Understanding how position within a species’ range affects local adaptation will provide us with a guideline for better ex situ conservation strategies. We examined gamete pools of pollen, a major determinant of gene flow in wind-pollinated conifers, and evaluated some fitness-related quantitative growth traits in Sitka spruce to understand the evolutionary dynamics and fitness of peripheral populations. Paternity analysis using maximum likelihood methods estimated that outcrossing rate significantly decreased in peripheral populations compared to central populations. The estimated number of effective pollen donors per family ranged from 2.1 to 18.9, being highest in the central continuous population and lowest in the disjunct peripheral population. Multiple common garden experiments indicated that the disjunct peripheral populations did not have low average fitness despite high biparental inbreeding rates. Thus, these results suggest that wind-pollinated conifer species may, at least in the short term, show resilience to isolation and inbreeding because of their high fecundity and longevity, and isolation may even accelerate local selection. This implies that peripheral isolated populations of conifer species are potential sources of adaptive genes for extreme environments and for recovery of populations.