Welcome!  This page contains some introductory information pertinent to the study of evolution in Lake Malawi Cichlid fishes.  Below you will find descriptions of a hierarchy of biological diversity in Lake Malawi and a description of the Lake itself.  With this background information on the Malawi system it is possible to begin to compose questions about the ultimate causes of this great diversity of life.  The work of science is to pose questions that we can approach objectively.  Formulating and testing coherent hypotheses is the way that we can begin to answer some of our questions.  I am approaching the question of the origin of new species by studying the population genetics, systematics, and ecology of the Metriaclima zebra species complex.  Members of this complex are very recently derived, and some may currently be undergoing the process of speciation.  An intensive focus on this group of species allows me to rigorously test important hypotheses while still keeping my focus on the larger question -- what is responsible for the great diversity of cichlid species in the African great lakes??
 

Cichlid diversity
The Cichlid fish species flocks of East Africa compose the most species rich assemblage of vertebrates on earth.  Good estimates of the number of species that exist are not easy to come by.  In the early years of Cichlid research, scientists were overburdened with the task of cataloging and formally describing new species.  Tijs Goldshmidt, a Dutch Cichlid biologist, describes the great diversity of species in Lake Victoria in his book, Darwin's Dream Pond - Drama in Lake Victoria.

                " I myself once caught  and unnamed, exceedingly lively, fiery-tempered male with
                purple flanks and a pitch-black mask but then let him go because at the time I couldn't
                face discovering another new species.  I don't think it was ever caught again!"

The number of described species in Lake Malawi is now around 450 .  Many Malawian species are still known only by informal names, such as M. zebra 'aggressive bars' -- an unnamed aggressive, barred member of the Metriaclima zebra species complex.  Given the relatively short period of time that these species may have arisen, the rate of speciation is possibly of a magnitude greater than what previously would have been considered possible.

Cichlidae
African Cichlids are widely known for their stunning diversity of trophic morphologies and behaviors.  All of these species and their elaborate adaptations have evolved from a single ancestor within Lake Malawi (this is also true of Lake Victoria).  The scale of this adaptive radiation is truly amazing.  Some biologists attribute this diversity, in part, to a structure in the pharynx of all cichlids -- the pharyngeal jaw (diagrammed in the middle of the image on the right).  It is thought that this structure acts as a "key innovation" - a trait that facilitates the adaptive change of other structures by lessening the selective pressures on them.  That is to say, that because cichlids have this set of bones in their pharyngeal region, their outer jaws are under less pressure to process all of the food that the fish eats.  Therefore the outer jaws are 'free' to evolve new methods of gathering food.  Whether this is the case or not, the result of the adaptive radiation of cichlids within Lake Malawi (and similarly in Victoria and Tanganyika) has been the evolution of species that fill essentially every imaginable niche within the lake -- a truly amazing feat for a single family of animals.  How have the pharyngeal jaw and ecological specialization contributed to the cichlid diversity in the African great lakes?
 

mbuna

The rock fish of Lake Malawi are known by their local name, mbuna.  There are thirteen genera of mbuna containing several hundred described species and countless undescribed taxa and geographical variants.  The mbuna are generally small, colorful, obligate rock dwellers.  Their ties to rocky habitat generally limits migration between even adjacent populations of mbuna species.  With male territoriality, female mouth brooding, and multiple mating in both sexes (polygynandry), the mbuna mating system is intriguing.  How do limited migration and this extraordinary mating system contribute to the species diversity among the mbuna?
 
 

Metriaclima zebra ~ species complex
A species complex is a monophyletic group of very closely related species.  The Metriaclima zebra species complex is composed of dozens of species from all around Lake Malawi.  An in-depth look at the M. zebra complex provides a great challenge for systematists and taxonomists to sort out relationships in this diverse group.  This species complex, like Lake Malawi in general, contains many extremely narrow endemics.  These groups tend to differ from each other principally in one or more elements of male reproductive coloration. While nearly all Malawian species are endemic to the Lake, these narrow endemic taxa often exist on a single rocky island, reef, or outcropping!  Groups within this species complex are often referred to as species, but are also frequently called populations of a single (or a few) species.  In fact, many of these groups may be incipient species -- populations undergoing differentiation which, in the fullness of time, will become separate species.  Endemism in the M. zebra species complex is so extreme that splitting endemic geographical variants into separate species leads to a 'species on every rock' situation.  What can we learn about the speciation process by looking at sibling species or populations of the same species?

Left: M. zebra "BB" (Blue-Black) from Chiofu Bay. Right: M. zebra "red dorsal" from Nakantenga Island (M. cyneusmarginatus - Stauffer) Two members of the zebra complex, which differ only in dorsal fin coloration.

This work on Malawi Cichlid fishes is supported by the National Science Foundation
NSF DEB 9707532