Summary of Part II: The Ecology and Evolution of a Group of Rock-Frequenting Nyasan Cichlid Fishes Known as the "Mbuna"
An account is given of the biology, ecology and evolution of a group of closely related fishes which inhabit rocky shores of Lake Nyasa, to which lake they are confined. The group, which is believed to be monophyletic in origin, and which is spoken of as the "Mbuna", is defined.
Some species are structurally stable; others are very variable. Although there is a basic colour pattern the actual coloration is very variable both between, and sometimes within specific units. This colour variation, sexual dimorphism and colour polymorphism are discussed.
Notes are given on the distribution, general ecology, habits and food preferences of the group as a whole and of individual species. All species are to a greater or lesser degree bound to rocky shores; sandy beaches constituting very effective barriers to inter-habitat migrations.
Males become territorial during the breeding periods. There is a definite courtship behaviour before laying and fertilisation of the eggs which are subsequently carried in the mouth of the female, which continues to brood the resulting young at least until the yolk sac is absorbed.
There is no definite evidence to indicate the existence of intraspecific homogamy.
Competition for breeding grounds (territories) seems to be obviated by "continuous" breeding throughout the year, only a small percentage of the population requiring territories at any one time.
The adaptive radiation of the group is summarised and an attempt is made to trace the origin of the Mbuna, the phylogenetic relationships of the various species, and the evolutionary history of the group as a whole.
From the known geological history of Lake Nyasa the history And distribution of environments populated by Mbuna is reconstructed. Against this background the probable sequence of events during the evolution of the various species of Mbuna is reconstructed from the knowledge of the structure, ecology and habits of the present-day species. The evidence is wholly compatible with the theory of allopatric speciation, and there is no occasion to to have recourse to the theory of sympatric speciation to explain the evolution of the group.
The effects of predation on the maintenance of dynamic balance is pointed out. Predation,
by keeping a check on the numerical density of populations, obviates interspecific competition
and favours the survival of individual species.