Fish Speciation in a Western North American Pliocene Rift Lake

Abstract

The extinct sculpins of Pliocene Lake Idaho exhibit radiation of diverse species into deep lacustrine habitats from ancestors in a shallow, offshore habitat. The eight known lineages of sculpins in the system apparently evolved within the lake from two or three immigrant species. Intralacustrine speciation is possibly an important aspect of evolution in ancient lakes. Although a single factor cannot explain all intralacustrine speciation, the process must involve ecological and ethological mechanisms for partitioning resources and for inhibiting gene flow between populations. In sculpins the process seems to involve colonization of deep zones, relatively uninhabited by competitors, by founder populations which diverge as their genetic contact with the ancestral form is cut off by differentiation of spawning time and place. The mechanism for selection against intermediates is not yet demonstrated, but may involve reducedfitness of individuals with intermediate spawning times and places. Intermediate juvenile forms probably fall between peaks of abundance of their zooplankton food. The study of speciation in ancient lakes requires fossil samples that provide information about spatial and temporal distribution of populations through the lake's histoty. It is also necessary to have information about colonization from adjacent lakes and streams. Stratigraphy of the Lake Idaho example is based on a simplified transgression-regression model of sedimentation. In the model, inshore sediments (and habitats) are recognized as coarser sands and associated paleobiotic indicators that form the transgressive base and regressive top of the lacustrine wedge. Time lines dip basinward into deep lacustrine silts. The lake stage examined in this study spanned from about 3.5 to 2.4 million years ago.