Development of genetic diversity, differentiation and structure over 500 years in four ponderosa pine populations

Population history plays an important role in shaping contemporary levels of genetic variation and geographic structure. This is especially true in small, isolated range-margin populations, where effects of inbreeding, genetic drift and gene flow may be more pronounced than in large continuous populations. Effects of landscape fragmentation and isolation distance may have implications for persistence of range-margin populations if they are demographic sinks. We studied four small, disjunct populations of ponderosa pine over a 500-year period. We coupled demographic data obtained through dendroecological methods with microsatellite data to discern how and when contemporary levels of allelic diversity, among and within-population levels of differentiation, and geographic structure, arose. Alleles accumulated rapidly following initial colonization, demonstrating proportionally high levels of gene flow into the populations. At population sizes of approximately 100 individuals, allele accumulation saturated. Levels of genetic differentiation among populations (FST and Jost's Dest) and diversity within populations (FIS) remained stable through time. There was no evidence of geographic genetic structure at any time in the populations' history. Proportionally, high gene flow in the early stages of population growth resulted in rapid accumulation of alleles and quickly created relatively homogenous genetic patterns among populations. Our study demonstrates that contemporary levels of genetic diversity were formed quickly and early in population development. How contemporary genetic diversity accumulates over time is a key facet of understanding population growth and development. This is especially relevant given the extent and speed at which species ranges are predicted to shift in the coming century.

种群历史对塑造当代遗传变异水平与地理格局具有重要作用。这一点在小型、孤立的分布边缘种群中尤为突出，近交、遗传漂变与基因流的效应相较于大型连续分布种群更为显著。若分布边缘种群为种群汇，则景观破碎化与隔离距离的效应可能对其存续产生影响。本研究针对4个小型、离散分布的美国黄松种群，开展了基于500年时间尺度的分析。我们将通过树轮生态学方法获取的种群统计数据与微卫星数据相结合，以解析当代等位基因多样性水平、种群间与种群内的遗传分化水平以及地理格局的形成过程与时间节点。初始种群定植后，等位基因快速积累，表明迁入种群的基因流水平相对较高。当种群规模达到约100个个体时，等位基因积累达到饱和状态。种群间的遗传分化水平（FST与Jost's Dest）以及种群内的遗传多样性（FIS）随时间推移保持稳定。在该种群的整个历史进程中，未发现任何时期存在地理遗传结构。种群增长早期相对较高的基因流，推动了等位基因的快速积累，并迅速使种群间形成了相对同质化的遗传模式。本研究表明，当代遗传多样性水平在种群发展的早期阶段便快速形成。解析当代遗传多样性随时间的积累过程，是理解种群生长与发展的核心维度之一。考虑到未来百年内物种分布范围预计发生的变化幅度与速度，这一点尤为重要。