Vicariance across major marine biogeographic barriers: temporal concordance and the relative intensity of hard versus soft barriers

Abstract

The marine tropics contain five major biogeographic regions (East Pacific, Atlantic, Indian Ocean, Indo-Australian Archipelago (IAA) and Central Pacific). These regions are separated by both hard and soft barriers. Reconstructing ancestral vicariance, we evaluate the extent of temporal concordance in vicariance events across three major barriers (Terminal Tethyan Event (TTE), Isthmus of Panama (IOP), East Pacific Barrier, EPB) and two incomplete barriers (either side of the IAA) for the Labridae, Pomacentridae and Chaetodontidae. We found a marked lack of temporal congruence within and among the three fish families in vicariance events associated with the EPB, TTE and IOP. Vicariance across hard barriers separating the Atlantic and Indo-Pacific (TTE, IOP) is temporally diffuse, with many vicariance events preceding barrier formation. In marked contrast, soft barriers either side of the IAA hotspot support tightly concordant vicariance events (2.5 Myr on Indian Ocean side; 6 Myr on Central Pacific side). Temporal concordance in vicariance points to large-scale temporally restricted gene flow during the Late Miocene and Pliocene. Despite different and often complex histories, both hard and soft barriers have comparably strong effects on the evolution of coral reef taxa.

Keywords: DEC model; Lagrange; ancestral biogeography; barriers; coral reef fishes; vicariance.

Figure 1.

(a) Frequency histogram of vicariance events across all barriers in all three families. (b) Observed (solid line) versus expected (dashed line) frequency of vicariance events in four time periods (P, Plio/Pleistocene; M, Miocene; O, Oligocene; E, Eocene) for each of the five barriers. Expected frequencies are based on the product of the total vicariance associated with each barrier and the relative frequency of vicariance in each time period (based on the distribution among times in the entire dataset).

Figure 2.

Distribution of reconstructed vicariance events associated with barriers between biogeographic regions. (a) Schematic diagram of world map identifying boundaries between regions (dashed lines) with known historical barriers: IOP and TTE (see text). Lines for IOP and TTE are extended down indicating the timing of known final barrier formation; (b) mean age (±s.e.) of vicariance events associated barriers (L, Labridae; P, Pomacentridae; C, Chaetodontidae) and (c) distribution of vicariance events across each barrier. Each circle represents a vicariance event across the associated barrier as implied from the Lagrange reconstruction. Family is indicated by circle colour as in (b) (Labridae, grey; Pomacentridae, white; Chaetodontidae, black). (Online version in colour.)