Biogeography: A Very Short Introduction by Mark V. Lomolino

Author:Mark V. Lomolino [Lomolino, Mark V.]
Language: eng
Format: epub
ISBN: 9780192590251
Google: qWjxDwAAQBAJ
Publisher: OUP Oxford
Published: 2020-07-22T23:00:00+00:00

The cichlids of Africa’s Rift Valley Lakes

The lakes of east Africa’s Rift Valley harbor some of the most diverse flocks of hyper-endemic species in the world. More than 1,400 species of cichlids (Family Cichlidae) inhabit the twelve largest lakes, together comprising some 60% of the global diversity of cichlids. The diversity of endemic cichlids is, consistent with patterns described for terrestrial islands, highest in the largest lakes—Victoria, Malawi, and Tanganyika, with over 300, 200, and 170 endemic species, respectively.

Comparisons between the radiations of these freshwater fish and the terrestrial lineages discussed earlier may at first glance seem confounded because we are comparing starkly different types of species and ecosystems. Yet research on adaptive radiations of cichlid fish across these lakes confirms many of the lessons garnered from the above descriptions of adaptive radiations in terrestrial systems. Analogous to the positive correlation between area and maximum elevation on land, lake area co-varies with maximum depth. Recalling our earlier discussion on plate tectonics, the Rift Valley is a tectonically active area where the continental plate of Africa is splitting apart, creating expansive depressions that over the past 30 million years filled in with rainfall and runoff to create the Great Lakes of this region.

In addition to these more long-term, geological dynamics, Africa’s Great Lakes also were dynamic on a much shorter time scale. Each of the repeated climatic cycles of the Pleistocene repeatedly transformed the lakes—first isolating their deepest pools into separate lakes during glacial periods that brought drought to this region, and then refilling and reconnecting the glacial-period lakes during the much wetter interglacials. Cichlids thus experienced periods of diversification in isolation during glacial periods, followed by further diversification accelerated by ecological interactions when diverging species co-occurred during interglacial periods—all this repeated some twenty to twenty-five times during the past 2.6 million years.

In addition to these geological and geographic factors, the diversification rates of cichlids were strongly influenced by species traits as well. The cichlids of these lakes share a combination of characteristics that inhibit gene flow and promote evolutionary divergence and, at some early stage in their evolutionary history, they developed a particular morphological trait—an evolutionary innovation intricately involved in their incredible bouts of ecological and evolutionary divergence. In addition to exhibiting high habitat specificity—being limited to particular depths and restricted to various microhabitats by features such as sandy or rocky sediments—these cichlids have limited home ranges, are highly territorial, and exhibit strong philopatry: regardless of how far they disperse as fry, they return to their natal site to breed. So despite the open, three-dimensional nature of these aquatic ecosystems and the abilities of these fish to disperse to other parts of their lake, gene flow is spatially restricted often to just a few meters of their natal sites. As a result, these fish are ecologically as well as evolutionarily coarse-grained—undergoing divergence at relatively fine (local) scales within each lake.

The evolutionary innovation alluded to above is a second set of jaws (pharyngeal jaws), which join the typical, and more anterior, oral jaws.

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