How does the taxonomic status of allopatric populations influence species richness within African cichlid fish assemblages?

Abstract

Aim Current estimates of species richness within rapidly evolving species flocks are often highly dependent on the species status of allopatric populations that differ in phenotypic traits. These traits may be unreliable indicators of biological species status and systematists may have inconsistently assigned species among lineages or locations on the basis of these traits, thus hampering comparative studies of regional species richness and speciation rates. Our aim was to develop a method of generating standardized estimates of regional species richness suitable for comparative analysis, and to use these estimates to examine the extent and consistency of species assignment of allopatric populations within rapidly evolving cichlid fish flocks present in three east African lakes. Location Lakes Malawi, Victoria and Tanganyika. Methods Using published taxon co‐occurrence data, a novel approach was employed to calculate standardized ‘minimum’ estimates of regional species richness for hard substrate associated complexes of cichlids within each of the lakes. Minimum estimates were based on an explicit assumption that if taxa present on equivalent habitats have disjunct distributions, then they are allopatric forms of the same species. These estimates were compared with current observed ‘high‐end’ regional species richness estimates for those complexes to determine the consistency of species assignment of allopatric populations between lineages within a lake. A ‘sympatry’ index was developed to enable comparisons of levels of species assignment of allopatric populations between‐lakes to be made. Results Within each lake, the minimum and high‐end estimates for species richness were significantly correlated across complexes, indicating that the complexes that contain more recognized species contain the most genuine biological species. However, comparisons of complexes among lakes revealed considerable differences. For equivalent geographical areas, substantially higher proportions of recognized species were totally allopatric within the studied Lake Malawi and Lake Victoria complexes, than those of Lake Tanganyika. Main Conclusions Among African lakes, levels of assignment to species status of allopatric populations were found to be distinctly different. It is unclear whether the discrepancies are a consequence of differences between the lake faunas in degrees of phenotypic divergence among allopatric populations, or are simply the result of inconsistent taxonomic practices. In either case, these results have considerable wider relevance for they emphasize that quantitative measures of regional and beta diversity are critically dependent on the species status of allopatric populations, an issue usually neglected in comparative studies of species richness. The technique introduced here can be used to standardize measures of regional diversity of lineages for comparative analyses, potentially enabling more accurate identification of processes influencing rates of speciation.

The species flocks of East African cichlid fishes: recent advances in molecular phylogenetics and population genetics

“…While the timing of the evolutionary processes in East African cichlids is fairly well documented (see above), there are ongoing debates on the taxonomic status of many cichlid species, lineages, and even species flocks (see e.g., Snoeks et al 1994;Turner et al 2001;Genner et al 2004). The taxonomy of the Tanganyikan cichlids is relatively well established (Poll 1986), which is mostly due to the existence of morphological characteristics that distinguish the different tribes and species.…”

“…Within these tribes, the more recent radiation and speciation processes seem to be as well related to trophic morphologies (see, e.g., Sturmbauer et al 2003), whereas colorational differences are found within arrays of closely related species, for instance of the Cyprichromini and the Tropheini. Unlike in Lake Malawi, however, in Lake Tanganyika the different colored morphs are usually not considered as separate species (Poll 1986;Snoeks et al 1994;Genner et al 2004).…”

Parallel life history evolution in mouthbrooding cichlids from the African Great Lakes

“…An estimated 660-1,319 species of cichlid fishes have been recorded from Lakes Malawi, Tanganyika, and Victoria, most endemic to a single lake catchment (12). These species flocks are so rich that, collectively, they provide the best example of rapid adaptive radiation in vertebrates (13)(14)(15)(16).…”

Geographical ancestry of Lake Malawi's cichlid fish diversity

“…A greater understanding of geographical and phylogenetic ancestry of cichlids is required to test whether functional genetic variation under divergent selection within lake radiations is present within riverine ancestors, and whether this variation has been shared among riverine cichlids through intraspecific gene flow and interspecific hybridization [9]. Lake Malawi contains a radiating flock of at least 450 haplochromine species [11]. Early phylogenetic reconstructions suggested that the lake radiation was monophyletic [12,13].…”