Lake Malawi, the third largest lake in Africa, is several million years old. Lake levels have fluctuated to a considerable extent in the late Pleistocene. Although tectonism may have influenced earlier level changes, the more recent changes have been climatically controlled. Major recessions occurred in the period before 25 000 years ago and 10 740 ± 130 years ago, with further large falls between 1150 and 1250 A.D. and within the period 1500-1850.
The 1500-1850 lake recession-refilling cycle is documented by using a variety of techniques. Sediment cores show an erosional hiatus stretching across the southern area of Lake Malawi down to water depths of at least 121 m. Diatoms sharply decline in abundance and diversity across this break, with Melosira nyassensis dominating in the post-erosion period. During the low stage, exposed littoral sands were reworked into aeolian dune-fields along windward shorelines. Oral histories reflect a group memory of this low period, which is supported by 14C dated archaeological finds in beach ridges surrounding the lake. Dating by 210Pb methods show that lacustrine sedimentation had resumed by about 1860. At this time, early explorers, such as Livingstone, were reporting evidence of rising lake levels.
Hydrological modelling shows that the lake-level changes indicated are possible in the timespan available. Various permutations of rainfall and timescale are discussed, e.g. a drop of 110 m over 250 years would require rainfall at 50% of modern values. The changes in lake level imply long-term changes in climate; these are highly relevant in the field of drought risk assessment.
The species flock of rocky-shore dwelling Lake Malawi cichlids known as 'Mbuna' contains about
200 species in Malawi's waters. Mitochondrial DNA differentiation shows that the flock as a
whole is of extremely recent origin. Almost every rocky outcrop and island has a unique Mbuna
fauna, with endemic colour forms and species. As many of these islands and outcrops were dry
land within the last 200-300 years, the establishment of the faunas has taken place within that
time. The evolution of distinct forms in such a brief timespan is discussed in relation to
current ideas on allopatric speciation. The present diversity of the Malawi cichlid-species
flock, and particularly the Mbuna, may be readily explained by the rapidity with which small
founder populations can diverge from the parent population, as demonstrated by the present
chronological evidence on changes in lake levels and by the Mbuna distribution data. The
repeated recessions and refillings of the lake have provided numerous opportunities for the
establishment of different founder populations and consequently different selection pressures,
leading to further bouts of speciation.