The evolution of the house sparrow (Passer domesticus) in southern Africa.
The house sparrow, Passer domesticus, is one of the most successful invading bird species in the world. It was introduced to southern Africa around 1900 and has since spread through the region. Its dispersal was characterised by an initial slow phase followed by a rapid increase in the rate of spread. Following 50 years of slow spread, the rate of dispersal accelerated to over 80 km/year. The initial slow rate can be attributed to an Allee effect, defined as "a disproportionate reduction in reproduction below a threshold population density due to reduced probability of finding a mate". The rapid phase involved a combination of long-range jumps (leap-frogging dispersal) and diffusive movement over short distances. Dispersal was significantly faster along railway lines. Introduction of the house sparrow, Passer domesticus, to southern Africa involved unknown numbers of both the domesticus race of Europe and indicus of Asia, resulting in the establishment of a genetically diverse founder population along coastal South Africa. The birds have undergone significant differentiation since introduction about 100 years ago. Significant sexual size dimorphism was detected among southern African house sparrows, especially in flight structures. Males were larger than females in all characters except tarsus and claw length. Overall body size variation was clinaly ordered with a general increase in size with latitude in conformity with Bergmann's rule. Tarsus length also increased southwards, with the longest tarsi in birds of coastal sites in South Africa and the shortest in Zimbabwe. Patterns of variation in morphological characters paralleled climatic trends, especially minimum temperature and humidity. Beak size and shape of Zimbabwean birds appeared to be under the greatest influence of climatic factors. Shorter and more conicaly shaped beaks were selected for in females in Zimbabwe. Natural selection was modifying the morphological characters resulting in adaptive radiation in morphology of southern African populations. Few studies of microevolution (change in morphology over a short period) have been conducted in birds and none in invading bird species in the tropics. A founding population comprising both the Asian and the European races of the house sparrow, P. d. domesticus and P. d. indicus first arrived in Zimbabwe 30 years ago. Because of its recent introduction to Zimbabwe and because of its known potential for rapid adaptation and differentiation elsewhere in its new range, the house sparrow provided the ideal case study in microevolution in tropical Africa. Morphological differentiation in Zimbabwean populations of the house sparrow was analysed to determine temporal variation in local samples and the extent of variation from parent populations of Asia and Europe. Samples collected since arrival in Zimbabwe up to 1980 were compared with those collected from current populations in 1998/1999 to determine local changes over time. The Zimbabwean samples were then contrasted with samples from Asian and European populations to determine the extent of differentiation in the introduced birds of Zimbabwe. Zimbabwean populations had differentiated from their Asian parents in six of the seven morphological characters examined. The greatest differentiation was in beak size and shape for both males and females. Males developed larger beaks and shorter wings than the Asian birds and female beaks became more conical. A large proportion of the potential phenetic diversity of the founding population of both domesticus and indicus genes had been realised in Zimbabwe.