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Original article:
Jackson, G.D.; Standish, T.; Çetintaş, O.; Zinenko, O.; Shechonge, A.H.; Yanchukov, A. Population Genetics and Gene Flow in Cyphotilapia frontosa and Cyphotilapia gibberosa Along the East Coast of Lake Tanganyika. Fishes 2024, 9, 481. https://doi.org/10.3390/fishes9120481
Text and image from the original paper.
Cichlids are common aquarium fish. Hundreds of different cichlid species have been found in the African rift lakes, including Lake Tanganyika. Among biologists, these fish are famous for their rapid speciation, with new species appearing within very few generations. Our ultimate interest is in the mechanism underlying this rapid speciation. It is already suspected that there are certain reproductive behaviors, such as mouth brooding, that may play a role, but this does not address the question of where the observed variability originates.
To study cichlid speciation, we collected a deepwater cichlid genus suspected to be made up of two species from Lake Tanganyika and studied individual’s genomes using restriction site-associated DNA sequencing (RADSeq). This technique revealed thousands of genetic markers in our samples allowing us to confirm the existence of two genetically separate populations, consistent with distinct species. The species in the northern part of the lake, Cyphotilapia frontosa, was shown to fall into multiple subpopulations, while the species to the south, Cyphotilapia gibberosa, formed a single population. This population structure is consistent with the history of Lake Tanganyika, which is believed to have split into multiple smaller lakes when the water level was lower in the past. As there is a shallow region transecting near the center of the long and narrow lake, this separated populations allowing development of the two species in the northern and southern parts of the modern lake. Since the most recent rise in water level, the two species have come back into contact over this relatively shallow area.
Possibly of greatest interest was the discovery that genetic material is moving disproportionally from C. frontosa into the C. gibberosa population. This points toward one possible element of the mechanism by which new species are produced with such startling speed. Various cichlid species may not be completely genetically isolated. This means that novel genetic combinations via introgression (or hybridization) produce new phenotypic variability rapidly. Among these new forms some may be more fit than others to thrive in the conditions they encounter. This accounts for the rapid appearance of variability upon which selection can act without having to wait long periods of time for fortuitous unguided DNA mutations to produce variants among which some might be adaptive. This mechanism of speciation via genetic introgression is different from the neo-Darwinian mechanism via mutation and harks back to the mechanism of speciation via hybridization advocated by some at the time of Gregor Mendel and Charles Darwin.