Plant speciation in the Namib Desert: origin of a widespread derivative species from a narrow endemic

Background: Parapatric (or budding) speciation is increasingly recognized as an important phenomenon in plant evolution but its role in extreme (e.g. desert) environments is poorly documented. Aims: To test this speciation model in a hypothesized sister pair, the Southwest and North African disjunct Senecio flavus and its putative progenitor, the Namibian Desert endemic S. englerianus. Methods: Phylogenetic inferences were combined with niche divergence tests, morphometrics, and experimental genetic approaches. We also evaluated the potential role of an African Dry Corridor (ADC) in promoting the hypothesized northward expansion of S. flavus (from Namibia), using palaeodistribution models. Results: Belonging to an isolated (potentially relict) clade, the two morphologically distinct species show pronounced niche divergence in Namibia and signs of digenic epistatic hybrid incompatibility (based on F2 pollen fertility). The presence of connate fluked pappus hairs in S. flavus, likely increasing dispersal ability, is controlled by a single gene locus. Conclusions: Our results provide support for a rare example of budding speciation in which a wider ranged derivative (S. flavus) originated at the periphery of a smaller ranged progenitor (S. englerianus) in the Namib Desert region. The Southwest and North African disjunction of S. flavus could have been established by dispersal across intermediate ADC areas during periods of (Late) Pleistocene aridification.

Ecological niche differentiation in Chiroxiphia and Antilophia manakins (Aves: Pipridae)

Species distribution models are useful for identifying the ecological characteristics that may limit a species’ geographic range and for inferring patterns of speciation. Here, we test a hypothesis of niche conservatism across evolutionary time in a group of manakins (Aves: Pipridae), with a focus on Chiroxiphia boliviana, and examine the degree of ecological differentiation with other Chiroxiphia and Antilophia manakins. We tested whether allopatric sister species were more or less similar in environmental space than expected given their phylogenetic distances, which would suggest, respectively, ecological niche conservatism over time or ecologically mediated selection (i.e. niche divergence). We modeled the distribution of nine manakin taxa (C. boliviana, C. caudata, C. lanceolata, C. linearis, C. p. pareola, C. p. regina, C. p. napensis, Antilophia galeata and A. bokermanni) using Maxent. We first performed models for each taxon and compared them. To test our hypothesis we followed three approaches: (1) we tested whether C. boliviana could predict the distribution of the other manakin taxa and vice versa; (2) we compared the ecological niches by using metrics of niche overlap, niche equivalency and niche similarity; and (3) lastly, we tested whether niche differentiation corresponded to phylogenetic distances calculated from two recent phylogenies. All models had high training and test AUC values. Mean AUC ratios were high (>0.8) for most taxa, indicating performance better than random. Results suggested niche conservatism, and high niche overlap and equivalency between C. boliviana and C. caudata, but we found very low values between C. boliviana and the rest of the taxa. We found a negative, but not significant, relationship between niche overlap and phylogenetic distance, suggesting an increase in ecological differentiation and niche divergence over evolutionary time. Overall, we give some insights into the evolution of C. boliviana, proposing that ecological selection may have influenced its speciation.