Peripheral Isolate Speciation of a Lake Malawi Cichlid Fish from Shallow Muddy Habitats (original) (raw)
Related papers
Molecular Ecology, 2001
Although there is mounting evidence that speciation can occur under sympatric conditions, unambiguous examples from nature are rare and it is almost always possible to propose alternative allopatric or parapatric scenarios. To identify an unequivocal case of sympatric speciation it is, therefore, necessary to analyse natural settings where recent monophyletic species flocks have evolved within a small and confined spatial range. We have studied such a case with a cichlid species flock that comprises five Tilapia forms endemic to a tiny lake (Lake Ejagham with a surface area of approximately 0.49 km2) in Western Cameroon. Analysis of mitochondrial D-Loop sequences shows that the flock is very young (approximately 104 years) and has originated from an adjacent riverine founder population. We have focused our study on a particular pair of forms within the lake that currently appears to be in the process of speciation. This pair is characterized by an unique breeding colouration and specific morphological aspects, which can serve as synapomorphic characters to prove monophyly. It has differentiated into a large inshore and a small pelagic form, apparently as a response to differential utilization of food resources. Still, breeding and brood care occurs in overlapping areas, both in time and space. Analysis of nuclear gene flow on the basis of microsatellite polymorphisms shows a highly restricted gene flow between the forms, suggesting reproductive isolation between them. This reproductive isolation is apparently achieved by size assortative mating, although occasional mixed pairs can be observed. Our findings are congruent with recent theoretical models for sympatric speciation, which show that differential ecological adaptations in combination with assortative mating could easily lead to speciation in sympatry.
Constraints to speciation despite divergence in an old haplochromine cichlid lineage
Evolution
Most of the 500+ cichlid species of Lake Victoria evolved very rapidly in the wake of an adaptive radiation within the last 15,000 years. All 500 species have evolved from just one out of five old cichlid lineages that colonized the lake. Endemic to the Lake Victoria region, Astatoreochromis alluaudi is a member of an old haplochromine lineage that never speciated in the region. Even though the species occurs in a wide range of habitats, there were no indications of evolutionary diversification. Here, we tested predictions of several hypothetical mechanisms that might constrain speciation, including high dispersal rates, a generalist life style and the lack of behavioral assortative mating. Genomic analyses of individuals from 13 populations revealed several genomically distinct groups, associated with major habitat classes, indicating the existence of two distinct ecotypes. We found significant phenotypic differences between these ecotypes in the wild, which were retained under com...
2021
Understanding the drivers and dynamics of diversification is a central topic in evolutionary biology. Here, we investigated the dynamics of diversification in the cichlid fish Astatotilapia burtoni that diverged along a lake-stream environmental gradient. Whole-genome and morphometric analyses revealed that divergent selection was essential at the early stages of diversification, but that periods in allopatry were likely involved towards the completion of speciation. While morphological differentiation was continuous, genomic differentiation was not, as shown by two clearly separated categories of genomic differentiation. Reproductive isolation increased along a continuum of genomic divergence, with a “grey zone” of speciation at ∼0.1% net nucleotide divergence. The quantification of the extent of (non-)parallelism in nine lake-stream population pairs from four cichlid species by means of multivariate analyses revealed one parallel axis of genomic and morphological differentiation a...
Genomic islands of speciation separate cichlid ecomorphs in an East African crater lake
Science (New York, N.Y.), 2015
The genomic causes and effects of divergent ecological selection during speciation are still poorly understood. Here we report the discovery and detailed characterization of early-stage adaptive divergence of two cichlid fish ecomorphs in a small (700 meters in diameter) isolated crater lake in Tanzania. The ecomorphs differ in depth preference, male breeding color, body shape, diet, and trophic morphology. With whole-genome sequences of 146 fish, we identified 98 clearly demarcated genomic "islands" of high differentiation and demonstrated the association of genotypes across these islands with divergent mate preferences. The islands contain candidate adaptive genes enriched for functions in sensory perception (including rhodopsin and other twilight-vision-associated genes), hormone signaling, and morphogenesis. Our study suggests mechanisms and genomic regions that may play a role in the closely related mega-radiation of Lake Malawi.