Monday, 22 July 2019

New Light on Cichlid Evolution in Africa

Just a few of the Lake Victoria Cichlids
New light on cichlid evolution in Africa - LMU Munich.

Cichlids are a ubiquitous group of freshwater fish in the tropics and are notable for the wide range of morphologies and patterns of behaviour that have adapted them to exploit a wide range of freshwater habitats. There are some 1100 different African species alone, most of them belonging to the Haplotilapiini group.

Now a collaborative research project carried out under the auspices of the GeoBio-Center at Ludwig-Maximilians-Universität München, Germany (LMU), has helped shed light on the origins and causes of this central paradigm in evolutionary biology.

The most notable example of this diversification is in the Great Lakes, Lakes Tanganyika, Malawi and Victoria, and others, in the East African Rift Valley where we can date the beginnings of this radiation with a fair degree of accuracy because several of these lakes are know to have dried up at particular geological times. This effectively zeros the local evolutionary process at the point where the lakes fill again.

East African Lakes
As I said in 2012:

Between 17,000 and 16,000 years ago, towards the end of the last Ice Age, there was a surge of icebergs and glacial meltwater into the North Atlantic which altered ocean currents and changed the weather pattern over the African and Asian monsoon areas, which experienced a resulting mega-drought. Analysis of sediments show that this caused Lake Victoria, Lake Albert and Lake Tana to dry up and disappear.

A similar event 14,000 - 15,000 years ago caused Lake Victoria to dry up again and a subsequent lowering of water levels 5,000 years ago left a small satellite lake, Lake Nabugabo, isolated. So, from this we know how long ago each lake received its founding population of cichlids from their feeder rivers. In the case of Lake Victoria, this was between 14,000 and 15,000 years ago. We also know that the micro-lake, Lake Nabugabo, has been isolated for just 5,000 years. By contrast, nearby Lake Tanganyika is tens of millions of years old and has remained filled for all that time. See AfricaPaleo - FOCUS 1: Lake levels and evolution.

Cichlid diversification in East Africa has become a central paradigm in evolutionary biology. As a consequence, dating the onset of the process and understanding the mechanisms that drive it are issues of great interest to evolutionary biologists and paleobiologists.

Professor Bettina Reichenbacher, lead author
LMU paleontologist
What was lacking was a clear evolutionary tree for these very clearly related species due mostly to the lack of a detailed fossil record. However, the research team led by Bettina Reichenbacher has now described a new fossil cichlid discovered in a fossil fish rich sedimentary deposit known as a Lagerstätte, in Central Kenya's Tugen Hills. They believe this, on the basis of detailed study and comparison with other specimens, to be the oldest member of the Oreochromini tribe of cichlids and have assigned it to a new genus, Oreochromimos because of its resemblance to members of the Oreochromini tribe (mimos in the name means mimic). Oreochromimos is about 12.5 million years old.

(a1–a6) Paratype OCO-2a-10a, b of †Oreochromimos kabchorensis gen. et sp. nov. (a1, a4) Articulated skeleton of part and counterpart. (a2) Close-up of the neurocranium showing the putative nuchal hump, scales, and the supraorbital sensory canal (all indicated with arrows). (a3) Reconstruction of the caudal skeleton, dotted lines indicate tentative outline due to preservation. (a5) Reconstruction of the posterior lateral line segment (complemented based on holotype). (a6) Flank scales visible between the neural spines beneath the soft rayed part of the dorsal fin. Scale bars: 5 mm (a1–a5), 1 mm (a6). Photos of a1 and a4 by M. Schellenberger at the SNSB - Bavarian State Collection of Palaeontology and Geology (BSPG). Abbreviations: ep, epural; hp, hypurapophysis; hs, haemal spine; hy, hypural plate; ns, neural spine; ph, parhypural; pu, preural vertebra; un, uroneural; us, urostyle.

Their finding were published open access a few days ago in Scientific Reports.

A new genus and species of fossil cichlid fishes of middle Miocene age (12.5 Ma) is described from the Ngorora fish Lagerstätte (Tugen Hills, Kenya) in the East African Rift Valley. Parsimony analysis of morphological characters using published phylogenetic frameworks for extant cichlids combined with the application of a comprehensive best-fit approach based on morphology was employed to place the new fossil taxon in the phylogenetic context of the African cichlids. The data reveal that the fossil specimens can be assigned to the tribe Oreochromini within the haplotilapiines. †Oreochromimos kabchorensis gen. et sp. nov. shows a mosaic set of characters bearing many similarities to the almost pan-African Oreochromis and the East African lake-endemic Alcolapia. As the striking diversity of present-day African cichlids, with 1100 recognised species, has remained largely invisible in the fossil record, the material described here adds significantly to our knowledge of the Miocene diversity of the group. It effectively doubles the age of a fossil calibration point, which has hitherto been used to calibrate divergence times of the East African cichlids in molecular phylogenetic investigations. Furthermore, the comparative dataset derived from extant cichlids presented here will greatly facilitate the classification of fossil cichlids in future studies.

This species is thus a member of the stem group of cichlids that gave rise to the Haplotilapiini group, which is the progenitor group of most African cichlids but especially of the Great Lakes radiation.

Notable, from a creationist point of view is the complete absence of any hint that the ToE is a theory in crisis. It looks for all the world like the grand unifying theory of biology, which of course it is.

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