Tracing the long-term evolution of a species complex: Examples from the Montastraea “annularis” complex

Recent molecular work has revealed numerous species complexes of scleractinian reef corals. Although species within complexes are distinct through much of their distribution, hybridization has been discovered at species margins, and has been hypothesized as playing an important role in mediating responses to changing environments. In the present study, I examine the long-term evolution of the Montastraea “annularis” complex over the past 5 million years to determine when speciation, extinction, and hybridization took place over the past 6.5 million years, with the eventual aim of understanding how these events corresponded with environmental changes in the Caribbean region. The material consists of colonies collected in the Mio-Pliocene of the Dominican Republic and the Plio-Pleistocene of Costa Rica and Panama. Genetically characterized colonies from the Recent of Panama are included in the analyses for comparison. Species are distinguished in the fossil material using a landmark-based morphometric approach that focuses on the size and shape of architectural features within corallites in transverse thin sections. Evolutionary relationships among species are examined using phylogenetic analyses based on parsimony. Phylogenetic characters are derived from the results of multiple comparisons tests, which statistically evaluated differences among species using morphometric data. sults show that the Montastraea “annularis” complex originated during late Miocene time, and consisted of >12 species during the Pliocene, with a maximum of 4–5 species co-occurring at any one time. The three modern species do not form a monophyletic group but belong to separate clades within the complex. The ranges of two of the three modern species may extend back to 2.9–3.5 Ma, indicating that they are survivors of the Plio-Pleistocene extinction event in which ∼80% of Caribbean reef coral species became extinct. Morphologic differences among species (disparity) were higher during the Pliocene than they are today.