Evolution of species flocks of cichlid fishes in African lakes

Speciation in rapidly diverging systems: lessons from Lake Malawi

Rapid evolutionary radiations provide insight into the fundamental processes involved in species formation. Here we examine the diversification of one such group, the cichlid fishes of Lake Malawi, which have radiated from a single ancestor into more than 400 species over the past 700 000 years. The phylogenetic history of this group suggests: (i) that their divergence has proceeded in three major bursts of cladogenesis; and (ii) that different selective forces have dominated each cladogenic event. The first episode resulted in the divergence of two major lineages, the sand- and rock-dwellers, each adapted to a major benthic macrohabitat. Among the rock-dwellers, competition for trophic resources then drove a second burst of cladogenesis, which resulted in the differentiation of trophic morphology. The third episode of cladogenesis is associated with differentiation of male nuptial colouration, most likely in response to divergent sexual selection. We discuss models of speciation in relation to this observed pattern. We advocate a model, divergence with gene flow, which reconciles the disparate selective forces responsible for the diversification of this group and suggest that the nonadaptive nature of the tertiary episode has significantly contributed to the extraordinary species richness of this group.

The oldest fossil cichlids (Teleostei: Perciformes): indication of a 45 million-year-old species flock

Five closely related species of fossil cichlids collected from an Eocene site in Tanzania, East Africa, represent the oldest known cichlids. The specimens are whole-body, articulated fishes that are extremely well preserved and, therefore, have the potential to add to our knowledge of the history of this family. Modern cichlids are particularly well known for the numerous species flocks of the East African Great Lakes. A great deal of research is ongoing regarding all aspects of the fishes in these flocks, including their evolutionary history The new collection of fossils reported here is interpreted as representing a species flock that arose in a small crater lake. These fossils indicate that cichlids' ability to form species flocks evolved early in the history of this family.

Divergence with gene flow in the rock-dwelling cichlids of Lake Malawi

Within the past two million years, more than 450 species of haplochromine cichlids have diverged from a single common ancestor in Lake Malawi. Several factors have been implicated in the diversification of this monophyletic clade, including changes in lake level and low levels of gene flow across limited geographic scales. The objectives of this study were to determine the effect of recent lake-level fluctuations on patterns of allelic diversity in the genus Metriaclima, to describe the patterns of population structure within this genus, and to identify barriers to migration. This was accomplished through an analysis of allele frequencies at four microsatellite loci. Twelve populations spanning four species within Metriaclima were surveyed. The effect of lake-level fluctuations can be seen in the reduced genetic diversity of the most recently colonized sites; however, genetic diversity is not depressed at the species level. Low levels of population structure exist among populations, yet some gene flow persists across long stretches of inhospitable habitat. No general barrier to migration was identified. The results of this study are interpreted with respect to several speciation models. Divergence via population bottlenecks is unlikely due to the large allelic diversity observed within each species. Genetic drift and microallopatric divergence are also rejected because some gene flow does occur between adjacent populations. However, the reduced levels of gene flow between populations does suggest that minor changes in the selective environment could cause the divergence of populations.

Major low levels of Lake Malawi and their implications for speciation rates in cichlid fishes

Lake Malawi, the third largest lake in Africa, is several million years old. Lake levels have fluctuated to a considerable extent in the late Pleistocene. Although tectonism may have influenced earlier level changes, the more recent changes have been climatically controlled. Major recessions occurred in the period before 25000 years ago and 10740 ± 130 years ago, with further large falls between 1150 and 1250 A. D. and within the period 1500-1850. The 1500-1850 lake recession-refilling cycle is documented by using a variety of techniques. Sediment cores show an erosional hiatus stretching across the southern area of Lake Malawi down to water depths of at least 121 m. Diatoms sharply decline in abundance and diversity across this break, withMelosira nyassensisdominating in the post-erosion period. During the low stage, exposed littoral sands were reworked into aeolian dune-fields along windward shorelines. Oral histories reflect a group memory of this low period, which is supported by14C dated archaeological finds in beach ridges surrounding the lake. Dating by210Pb methods show that lacustrine sedimentation had resumed by about 1860. At this time, early explorers, such as Livingstone, were reporting evidence of rising lake levels. Hydrological modelling shows that the lake-level changes indicated are possible in the timespan available. Various permutations of rainfall and timescale are discussed, e. g. a drop of 110 m over 250 years would require rainfall at 50% of modern values. The changes in lake level imply longterm changes in climate; these are highly relevant in the field of drought-risk assessment. The species flock of rocky-shore dwelling Lake Malawi cichlids known as ‘Mbuna’ contains about 200 species in Malawi’s waters. Mitochondrial DNA differentiation shows that the flock as a whole is of extremely recent origin. Almost every rocky outcrop and island has a unique Mbuna fauna, with endemic colour forms and species. As many of these islands and outcrops were dry land within the last 200-300 years, the establishment of the faunas has taken place within that time. The evolution of distinct forms in such a brief timespan is discussed in relation to current ideas on allopatric speciation. The present diversity of the Malawi cichlid-species flock, and particularly the Mbuna, may be readily explained by the rapidity with which small founder populations can diverge from the parent population, as demonstrated by the present chronological evidence on changes in lake levels and by the Mbuna distribution data. The repeated recessions and refillings of the lake have provided numerous opportunities for the establishment of different founder populations and consequently different selection pressures, leading to further bouts of speciation.