Cichlid fish species of Lake Victoria can interbreed without loss of fertility but are sexually isolated by mate choice. Mate choice is determined on the basis of coloration, and strong assortative mating can quickly lead to sexual isolation of color morphs. Dull fish coloration, few color morphs, and low species diversity are found in areas that have become turbid as a result of recent eutrophication. By constraining color vision, turbidity interferes with mate choice, relaxes sexual selection, and blocks the mechanism of reproductive isolation. In this way, human activities that increase turbidity destroy both the mechanism of diversification and that which maintains diversity.In the Great Lakes of Africa, large and diverse species flocks of cichlid fish have evolved rapidly (1, 2). Lake Victoria, the largest of these lakes, had until recently at least 500 species of haplochromine cichlids (3). They were ecologically so diverse that thev utilized almost all resources available to freshwater fishes in general (2), despite having evolved in perhaps as little as 12,400 years (I ) and from a single ancestral species (4). This s~ecies flock is the most notable . . example of vertebrate explosive evolution known today. Many of its species have vanished within two decades (5, 6 ) , which can only partly be explained by predation by the introduced Nile perch (Lates spp.). Stenotopic rock-dwelling cichlids, of which there are more than 200 species (7), are rarely eaten by Nile perch (8). Yet, many such species have disappeared in the past 10 years (6, 7). Because gene flow between island ~o~ulations of these cichlids is effectivelv iiGited by stretches of sand and mud botto4 (9), populations underlie local selection regimes. Here we demonstrate that increasing turbidity, by curbing the impact of sexual selection on sexual isolation, is responsible for the decline in cichlid diversity.The seven Great Lake basins of tro~ical Africa in which haplochromine cichlids formed endemic s~ecies flocks have distinctly clearer waters than the five in which they did not (300 to 2200 versus 20 to 130 cm maximum Secchi disc readings; n = 12, t = 2.99, P = 0.015). Significance increases when the three very large lakes Victoria, 'Malawi, and Tanganyika are excluded (n = 9, t = 3.83, P = 0.009), ruling out the alternative hypothesis that lake size explains the difference. Lake Victoria has rapidly eutrophied (10) and become turbid. Water transparency decreased in deep open waters from 5.5 to 8 m in the 1920s to 1.3 to 3 m in the 1990s and decreased in the littoral zone from 3 to 1.5 m within the past decade (I I). We have investigated the effect of these changes on the cichlids. Postmating reproductive barriers have not been found among Lake Victoria cichlids (12), presumably because of their phylogenetic youth. Reproductive isolation among sympatric species is maintained only by mate choice. Haplochromines have vibrantly colored males and usually cryptically colored females (2,7). Their eyes are equipped with three retinal cone pigments ...

Natural selection is considered to play an important role in driving speciation (Funk, Nosil, & Etges, 2006; Sobel, Chen, Watt, & Schemske, 2010). Divergent selection can contribute to speciation when allopatric populations encounter different habitats with different selective pressures (Schluter, 2001, 2009). Under such circumstances, it is expected that loci that underlie local adaptation will show allele frequency differences in populations under different environments (Schluter, 2009). Similarly, if allopatric populations encounter environments with similar selection pressures (i.e., uniform selection), different adaptive mutations may be selected for (Schluter, 2001, 2009) because different mutations may result in similar optimal phenotypes. If populations that are experiencing either divergent or uniform selection maintain geographic isolation, thus restricting gene flow between them, divergence will

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International audienceEvolutionary loss of traits can result from negative selection on a specific phenotype, or if the trait is selectively neutral, because the phenotype associated with the trait has become redundant. Even essential traits may be lost, however, if the resulting phenotypic deficiencies can be compensated for by the environment or a symbiotic partner. Here we demonstrate that loss of an essential metabolic trait in parasitic wasps has evolved through environmental compensation. We tested 24 species for the ability to synthesize lipids de novo and collected additional data from the literature. We found the majority of adult parasitoid species to be incapable of synthesizing lipids, and phylogenetic analyses showed that the evolution of lack of lipogenesis is concurrent with that of parasitism in insects. Exploitive host manipulation, in which the host is forced to synthesize lipids to the benefit of the parasitoid, presumably facilitates loss of lipogenesis through environmental compensation. Lipogenesis re-evolved in a small number of parasitoid species, particularly host generalists. The wide range of host species inwhich generalists are able to develop may impede effective host manipulation and could have resulted in regaining of lipogenic ability in generalist parasitoids. As trait loss through environmental compensation is unnoticed at the phenotypic level, it may be more common than currently anticipated, especially in species involved in intricate symbiotic relationships with other species

The haplochromine cichlids of Lake Victoria constitute a classical example of explosive speciation. Extensive intra- and interspecific variation in male nuptial coloration and female mating preferences, in the absence of postzygotic isolation between species, has inspired the hypothesis that sexual selection has been a driving force in the origin of this species flock. This hypothesis rests on the premise that the phenotypic traits that underlie behavioural reproductive isolation between sister species diverged under sexual selection within a species. We test this premise in a Lake Victoria cichlid, by using laboratory experiments and field observations. We report that a male colour trait, which has previously been shown to be important for behavioural reproductive isolation between this species and a close relative, is under directional sexual selection by female mate choice within this species. This is consistent with the hypothesis that female choice has driven the divergence in male coloration between the two species. We also find that male territoriality is vital for male reproductive success and that multiple mating by females is common.

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African cichlid fishes have undergone outbursts of explosive speciation in several lakes, accompanied by rapid radiations in coloration and ecology. Little is known about the evolutionary forces that triggered these events but a hypothesis, published by Wallace Dominey in 1984, has figured prominently. It states that the evolution of colour patterns is driven by sexual selection and that these colour patterns are important in interspecific mate choice, a combination which holds the potential for rapid speciation. Here we present phylogenetic analyses that describe major events in colour evolution and test predictions yielded by Dominey's hypothesis. We assembled information on stripe patterns and the presence or absence of nuptial coloration from more than 700 cichlid species representing more than 90 taxa for which molecular phylogenetic hypotheses were available. We show that sexual selection is most likely the selection force that made male nuptial coloration arise and evolve quickly. In contrast, stripe patterns, though phylogenetically not conserved either, are constrained ecologically. The evolution of vertical bar patterns is associated with structurally complex habitats, such as rocky substrates or vegetation. The evolution of a horizontal stripe is associated with a piscivorous feeding mode. Horizontal stripes are also associated with shoaling behaviour. Strength of sexual selection, measured in terms of the mating system (weak in monogamous, strong in promiscuous species), has no detectable effects on stripe pattern evolution. In promiscuous species the frequency of difference between sister species in nuptial hue is higher than in pair bonding and harem forming species, but the frequency of difference in stripe pattern is lower. We argue that differences between the two components of coloration in their exposure to natural selection explain their very different evolutionary behaviour. Finally, we suggest that habitat‐mediated selection upon chromomotor flexibility, a special form of phenotypic plasticity found in the river‐dwelling outgroups of the lake‐dwelling cichlids, explains the rapid and recurrent ecology‐associated radiation of stripe patterns in lake environments, a new hypothesis that yields experimentally testable predictions.

The role of selection in speciation is a central yet poorly understood problem in evolutionary biology. The rapid radiations of extremely colorful cichlid fish in African lakes have fueled the hypothesis that sexual selection can drive species divergence without geographical isolation. Here we present experimental evidence for a mechanism by which sexual selection becomes divergent: in two sibling species from Lake Victoria, female mating preferences for red and blue male nuptial coloration coincide with their context-independent sensitivities to red and blue light, which in turn correspond to a difference in ambient light in the natural habitat of the species. These results suggest that natural selection on visual performance, favoring different visual properties in different spectral environments, may lead to divergent sexual selection on male nuptial coloration. This interplay of ecological and sexual selection along a light gradient may provide a mechanism of rapid speciation through divergent sensory drive.