Systematics and Morphology
DOI link for Systematics and Morphology
Systematics and Morphology book
H.A. Lessios Smithsonian Tropical Research Institute, Balboa, Panama
ABSTRACT: Data relevant to processes that give rise to new species of echinoids are reviewed. Phylogeographic information from mitochondrial DNA is used to ask whether speciation in sea urchins fits the allopatric model, which predicts that if reproductive isolation accumulates steadily with time, then young, closely related species would tend to occur on two sides of a geographic barrier. The conclusion of this analysis is that most genera show a strong signature of allopatric speciation, but that Echinometra, Lytechinus and Strongylocentrotus also contain recently diverged sister species that are sympatric. The reason for these exceptions is probably not that sympatric speciation has occurred, but rater that reproductive isolation is not a function of divergence time alone. Although postzygotic isolation (lower hybrid fitness) in echinoids is correlated with divergence time, prezygotic isolation seems to arise due to additional factors, which at are not necessarily related to the time that species have remained separate. There is no correlation between gametic incompatibility and time; bindin, one of the molecules responsible for gametic isolation, evolves under strong selection in some genera, but neutrally in others. Even though there is some evidence for reinforcement as a selective force on bindin, the differences in intensity of selection between the genera are more likely caused by intraspecific processes, such as variation in local sperm density. If so, age of species would not be a good predictor of geographic overlap, because young species may be reproductively isolated, while old species may be compatible, regardless of whether they arose sympatrically or allopatrically. The paucity of extant species in echinoids indicates that even though gametic reproductive isolation can arise rapidly, such events, leading to speciation, do not happen often.