Data from: Long-distance dispersal suppresses introgression of local alleles during range expansions

During range expansions, even low levels of interbreeding can lead to massive introgression of local alleles into an invader's genome. Nonetheless, this pattern is not always observed in human populations. For instance, European Americans in North America are barely introgressed by Amerindian genes in spite of known contact and admixture. With coalescent spatially explicit simulations, we examined the impact of long-distance dispersal (LDD) events on introgression of local alleles into the invading population using a set of different demographic scenarios applicable to a diverse range of natural populations and species. More specifically, we consider two distinct LDD models: one where LDD events originate in the range core and targets only the expansion front and a second one where LDD events can occur from any area to any other. We find that LDD generally prevents introgression, but that LDD events specifically targeting the expansion front are most efficient in suppressing introgression. This is likely due to the fact that LDD allows for the presence of a larger number of invader alleles at the wave front, where effective population size is thus increased and local introgressed alleles are rapidly outnumbered. We postulate that the documented settlement of pioneers directly on the wave front in North America has contributed to low levels of Amerindian admixture observed in European Americans and that this phenomenon may well explain the lack of introgression after a range expansion in natural populations without the need to evoke other mechanisms such as natural selection.

在物种分布范围扩张（range expansion）过程中，即使是低水平的种群间杂交，也会导致本地等位基因（allele）大量渐渗（introgression）进入入侵种群的基因组。尽管如此，这一规律在人类种群中却并非总能观测到。例如，尽管已知北美欧洲裔人群与美洲原住民存在接触和基因混合（admixture），但前者几乎未受到后者基因的渐渗影响。我们借助空间显式溯祖模拟（coalescent spatially explicit simulations），针对长距离扩散（long-distance dispersal, LDD）事件对本地等位基因渐渗进入入侵种群的影响展开了分析，所采用的一系列种群统计情景可覆盖多样的自然种群与物种类群。具体而言，我们设计了两种不同的LDD模型：其一，LDD事件起源于分布范围的核心区域，且仅针对扩张前沿（expansion front）；其二，LDD事件可发生于任意区域之间。研究结果表明，LDD通常会抑制渐渗过程，而专门针对扩张前沿的LDD事件则是抑制渐渗效果最为显著的情形。这一现象的潜在原因在于，LDD使得扩张波前沿的入侵者等位基因数量得以增加，进而提升了该区域的有效种群大小（effective population size），使本地渐渗等位基因的占比被快速赶超。我们据此提出假说：北美地区已被文献记录的、先锋拓殖种群直接定居于扩张波前沿的现象，正是导致北美欧洲裔人群中美洲原住民基因混合水平较低的原因之一；且该现象可充分解释自然种群在范围扩张后未出现明显渐渗的现象，无需借助自然选择等其他机制。