Colour diversification in Malawi cichlids: evidence for adaptation, reinforcement or sexual selection?

Abstract

The mechanisms responsible for the explosive radiations of haplochromine cichlid fishes in Africa's great lakes remain controversial. Since species thought to be closely related often differ most apparendy in male breeding colours, I examined patterns of male colour variation in rock‐dwelling cichlids from Lake Malawi to test whether initial divergence between species is likely to have been caused by adaptation to differing habitats, by selection against hybridization (reinforcement or character displacement), or by sexual selection. I found that all significant variation in overall male colour occurs within, not between, species complexes, in contrast to variation in habitat and behavioural traits, which differed significandy between complexes. Male colour does not vary significandy with habitat characteristics such as water depth, and sympatric species differ no more in colour than allopatric ones. These results fail to provide support for adaptation or reinforcement as initial causes of colour divergence, and are consistent with the possibility that divergence may instead have been caused by sexual selection, but a direct test of sexual selection is not yet possible.

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

“…Trophic traits related to ecological differences in the food uptake would be diverging secondarily, in particular, once sympatry was re-established as a form of character displacement after secondary contact (Mayr 1984). Empirical observations show that sibling cichlid species tend to differ in coloration rather than in trophic differences, lending support to this hypothesis (Deutsch 1997;Reinthal and Meyer 1997;. Recent molecular studies on sympatric speciation in a trophically and colorationally polymorphic Nicaraguan crater lake cichlid species (Wilson et al 2000;Barluenga and Meyer 2004) also revealed more pronounced differences between color but not pharyngeal jaw morphologies (a trait related to trophic differences; see Meyer 1989Meyer , 1990aMeyer , 1990b If, however, cichlid assemblages of entire lake habitats are considered, it seems more likely that, initially, ecological factors lead to a spatial separation into more benthic and more limnetic lineages (Danley and Kocher 2001).…”

“…A second factor has to do with the cichlid's sophisticated breeding behavior and in particular with the various modes of broodcare behavior that are found in the East African lake cichlids (Fryer and Iles 1972;Goodwin et al 1998;Stiassny and Meyer 1999;Barlow 2000;Kornfield and Smith 2000;Klett and Meyer 2002). More recent studies suggested that sexual selection through female choice of male (nuptial) coloration might also play an important role in the evolution of East African cichlids (Dominey 1984;Meyer 1993;Turner and Burrows 1995;Deutsch 1997;Barlow 2000;Danley and Kocher 2001). Speciation on the basis of sexual selection in East African cichlids has been proposed on the basis of field observations ) as well as mate choice experiments in the laboratory (Knight et al 1998;.…”

“…Mayr's and Dobzhansky's views have both been used to explain the evolution of isolating mechanisms in East African lake cichlids (see, e.g., Fryer and Iles 1972;Deutsch 1997;Kornfield and Smith 2000). However, the extreme pace of species formation in the East African cichlids poses difficulties in determining which mode-isolating mechanisms were acquired and possibly perfected secondarily.…”

“…However, it is likely that the adaptive radiation in Lake Malawi is at a different stage than that of Lake Tanganyika, and that the Lake Tanganyika species assemblage had been more species-rich at some point in the past Ecological factors vs. sexual selection Ernst Mayr (1984) stated that "the probable causal factors in the origin of isolating mechanisms has recently been discovered in sexual selection" and refers to Dominey (1984) in the same compendium, who was the first to discuss the Fisherian runaway sexual selection model with respect to cichlid fishes. Sexual selection via female choice of male (nuptial) coloration is now regarded as important factor in cichlid evolution (see, e.g., Meyer 1993;Turner and Burrows 1995;Deutsch 1997;Kornfield and Smith 2000;Allender et al 2003). Assortative mating was also seen as a possible mechanism by which sympatric speciation might occur (Turner and Burrows 1995;Wilson et al 2000;Schliewen et al 2001;Allender et al 2003;Barluenga and Meyer 2004), particularly after simulations predicted that sympatric speciation is the theoretical outcome of such set-ups (Dieckmann and Doebeli 1999;Higashi et al 1999;Kondrashov and Kondrashov 1999;Gavrilets and Waxman 2002).…”

The underwater photic environment of Cape Maclear, Lake Malawi: comparison between rock- and sand-bottom habitats and implications for cichlid fish vision

“…Bar patterns are typically found in cichlids associated with structurally complex habitats (including rocky habitats) whereas horizontal stripes are more often associated with shoaling behavior, which is typical for open-water and sand-dwelling cichlids (Seehausen et al, 1999). The color pattern of Malawi cichlids was previously characterized by Deutsch (Deutsch, 1997) and McElroy (McElroy, 1991). However, both studies relied on human visual inspection of photographs and thus have limited significance for understanding the spectral reflectance of cichlids.…”