Evolutionary Disequilibrium Among Indo-Pacific Corals

About half (53%) of living Indo-Pacific coral reefs lie on continental shelves between Australia and Asia. These shelf reefs are often exposed to considerable terrigenous influences and may be wholly or partially isolated from oceanic conditions. Their upper surfaces (<20 m deep) have probably been important habitats for the evolution of shallow water Indo-Pacific scleractinians since the mid-Pliocene.

In contrast to the general assumption that Indo-Pacific corals were little affected by Pliocene-Quaternary glaciations, I suggest they were subjected to chronic evolutionary disturbance by high frequency sea level fluctuations. The average time that any given bathymetric level remained in the zone of active coral growth (<20 m) was only 3,200 years. Such periods probably were too short for populations of long-lived corals to complete enough generations to approach evolutionary equilibrium before their descendants colonized new habitats.

I suggest these evolutionary disturbances did not cause faunal changes (speciation or extinction), but instead, that they maximized the expression of intraspecific variation in shallow water corals, while largely inhibiting processes of population differentiation (including speciation). The faunal homogeneity of Indo-Pacific corals, the apparent paucity of consistent patterns of differentiation within species, and the general absence of endemic species in isolated peripheral localities are all interpreted as results of prolonged evolutionary disequilibrium.