Computational Genetics Laboratory, Dartmouth College, 707 Rubin Building, HB 7937, One Medical Center Drive, Dartmouth Hitchcock Medical Center, Lebanon, Hanover, NH 03755, USA.
J Theor Biol. 2011 Mar 21;273(1):147-55. doi: 10.1016/j.jtbi.2010.12.036. Epub 2010 Dec 29.
Dispersal modulates gene flow throughout a population's spatial range. Gene flow affects adaptation at local spatial scales, and consequently impacts the evolution of reproductive isolation. A recent theoretical investigation has demonstrated that local adaptation along an environmental gradient, facilitated by the evolution of limited dispersal, can lead to parapatric speciation even in the absence of assortative mating. This and other studies assumed unconditional dispersal, so individuals start dispersing without regard to local environmental conditions. However, many species disperse conditionally; their propensity to disperse is contingent upon environmental cues, such as the degree of local crowding or the availability of suitable mates. Here, we use an individual-based model in continuous space to investigate by numerical simulation the relationship between the evolution of threshold-based conditional dispersal and parapatric speciation driven by frequency-dependent competition along environmental gradients. We find that, as with unconditional dispersal, parapatric speciation occurs under a broad range of conditions when reproduction is asexual, and under a more restricted range of conditions when reproduction is sexual. In both the asexual and sexual cases, the evolution of conditional dispersal is strongly influenced by the slope of the environmental gradient: shallow environmental gradients result in low dispersal thresholds and high dispersal distances, while steep environmental gradients result in high dispersal thresholds and low dispersal distances. The latter, however, remain higher than under unconditional dispersal, thus undermining isolation by distance, and hindering speciation in sexual populations. Consequently, the speciation of sexual populations under conditional dispersal is triggered by a steeper gradient than under unconditional dispersal. Enhancing the disruptiveness of frequency-dependent selection, more box-shaped competition kernels dramatically lower the speciation-enabling slope of the environmental gradient.
扩散调节了种群空间范围内的基因流动。基因流动影响局部空间尺度的适应性,从而影响生殖隔离的进化。最近的一项理论研究表明,在环境梯度上的局部适应,通过有限扩散的进化来促进,即使在没有交配偏好的情况下,也能导致邻域物种形成。这项研究和其他研究都假设无条件扩散,即个体在不考虑当地环境条件的情况下开始扩散。然而,许多物种是有条件扩散的;它们的扩散倾向取决于环境线索,如局部拥挤程度或合适伴侣的可用性。在这里,我们使用连续空间中的个体基础模型,通过数值模拟来研究基于阈值的条件扩散的进化与频率依赖竞争驱动的沿环境梯度的邻域物种形成之间的关系。我们发现,与无条件扩散一样,当繁殖是无性的时,在广泛的条件下会发生邻域物种形成,而当繁殖是有性的时,在更受限制的条件下会发生邻域物种形成。在无性和有性的情况下,条件扩散的进化都受到环境梯度斜率的强烈影响:环境梯度较浅会导致较低的扩散阈值和较高的扩散距离,而环境梯度较陡则会导致较高的扩散阈值和较低的扩散距离。然而,后者仍然高于无条件扩散,从而破坏了距离隔离,并阻碍了有性种群的物种形成。因此,有条件扩散下的有性种群的物种形成是由比无条件扩散更陡峭的梯度触发的。增强频率依赖选择的破坏性,更箱形的竞争核会显著降低环境梯度的物种形成有效斜率。