High dispersal ability inhibits speciation in a continental radiation of passerine birds

Abstract

Dispersal can stimulate speciation by facilitating geographical expansion across barriers or inhibit speciation by maintaining gene flow among populations. Therefore, the relationship between dispersal ability and speciation rates can be positive or negative. Furthermore, an 'intermediate dispersal' model that combines positive and negative effects predicts a unimodal relationship between dispersal and diversification. Because both dispersal ability and speciation rates are difficult to quantify, empirical evidence for the relationship between dispersal and diversification remains scarce. Using a surrogate for flight performance and a species-level DNA-based phylogeny of a large South American bird radiation (the Furnariidae), we found that lineages with higher dispersal ability experienced lower speciation rates. We propose that the degree of fragmentation or permeability of the geographical setting together with the intermediate dispersal model are crucial in reconciling previous, often contradictory findings regarding the relationship between dispersal and diversification.

Figure 1.

Basic elements of the intermediate dispersal model, which predicts a unimodal relationship between dispersal ability and speciation rates. Lineages with low dispersal ability are restricted to small areas where opportunities for geographical subdivision are few, resulting in low chances of speciation. Lineages with intermediate dispersal ability expand their geographical range and differentiate across barriers. Lineages with high dispersal ability maintain high levels of gene flow across extensive geographical ranges.

Figure 2.

Linear measurements used to calculate the hand-wing index: (WL) ‘wing length’ from the carpal joint to the tip of the longest primary feather; (SL) ‘secondary length’ from the carpal joint to the tip of the first secondary feather. Both measurements were taken on closed wings without flattening their natural curvature. Here, WL and SL are traced on a wing outline of Xenops rutilans (University of Washington Burke Museum 77 384) to show their relationship with the extent and width of the wing.

Figure 3.

Relationship between aspect ratio and the hand-wing index in 35 species of Furnariidae. Regression line from a phylogenetic generalized least-squares model (R² = 0.66, F = 68.4, d.f. = 1, p < 0.001): aspect ratio = 0.045 (±0.005) × hand-wing index + 3.7 (±0.01).

Figure 4.

Species-level phylogeny of the Furnariidae (Bayesian maximum clade credibility tree [33]) showing hand-wing index values for extant species as well as maximum likelihood ancestral reconstructions.

Figure 5.

Relationship between the hand-wing index and speciation rates in the Furnariidae as inferred from quantitative state speciation and extinction models (QuaSSE). The nine models with varying speciation rates are shown in shades of grey proportional to their probability w (table 2) (some model predictions overlap so only six lines are shown). The black line represents the model-averaged prediction.