Abstract
The hypothesis of ecological speciation states that as populations diverge in different niches, reproductive isolation evolves as a by-product of adaptation to these different environments. In this context, we used Nolina parviflora as a model to test if this species evolved via ecological speciation and to explore current and historical gene flow among its populations. Nolina parviflora is a montane species endemic to Mexico with its geographical distribution restricted largely to the Trans-Mexican Volcanic Belt. This mountain range is one of the most complex geological regions in Mexico, having undergone volcanism from the mid-Miocene to the present. Ecologically, the Trans-Mexican Volcanic Belt possesses different types of vegetation, including tropical dry forest; oak, pine, pine-oak, and pine-juniper forests; and xerophytic scrub - all of which maintain populations of N. parviflora. Using species distribution models, climatic analyses, spatial connectivity and morphological comparisons, we found significant differences in climatic and morphological variables between populations of N. parviflora in two distinct Trans-Mexican Volcanic Belt regions (east vs. west). This could mean that the geographically isolated populations diverged from one another via niche divergence, indicating ecological speciation. Spatial connectivity analysis revealed no connectivity between these regions under the present or last glacial maximum climate models, indicating a lack of gene flow between the populations of the two regions. The results imply that these populations may encompass more than a single species.
Highlights
The ecological speciation hypothesis postulates that incipient species adapt to different niches, which are influenced by the ecological environment [1], resulting in niche differentiation and leading to speciation
Svensson [1] defines three critical conditions necessary for ecological speciation: 1) niche differences must exist with sufficient magnitude between species such that divergent selection can drive speciation, 2) niche differences must persist for a sufficiently long period of time to allow reproductive isolation to evolve, and 3) ecological differentiation must evolve prior to reproductive isolation such that the ecological differentiation is driving the process
Even given these criteria, ecological speciation is only one factor in the speciation process, with other geographical processes often contributing to reproductive isolation, and with confounding factors associated with ecological differentiation having an effect
Summary
The ecological speciation hypothesis postulates that incipient species adapt to different niches, which are influenced by the ecological environment (e.g. resources, predators or abiotic factors) [1], resulting in niche differentiation and leading to speciation. Svensson [1] defines three critical conditions necessary for ecological speciation: 1) niche differences must exist with sufficient magnitude between species such that divergent selection can drive speciation, 2) niche differences must persist for a sufficiently long period of time to allow reproductive isolation to evolve, and 3) ecological differentiation must evolve prior to reproductive isolation such that the ecological differentiation is driving the process. Even given these criteria, ecological speciation is only one factor in the speciation process, with other geographical processes often contributing to reproductive isolation, and with confounding factors associated with ecological differentiation having an effect (e.g. allopatric, parapatric, and sympatric modes of speciation; [2,3]). The niche conservatism hypothesis proposes that species distribution patterns are governed by ancestral climatic affinities [10]
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