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TABLE 3. Estimated times to the cenancestor of the world populations of P. falciparum

Estimated mutation rate × 10-9

µa

µ b

t95

t50

7.12

2.22

24,511

5,670

3.03

0.95

57,481

13,296

Adapted from Rich et al. (1998) and Ayala et al. (1998). t95 and t50 are the upper boundaries of the confidence intervals. Thus, in thefirst row the cenancestor lived less than 24,511 years ago with a95% probability, and less than 5,670 years ago with a 50% probability.µa and µb are the estimated neutral mutation rates of 4-fold and 2-fold degeneratecodons, respectively.

de Zulueta, 1973, 1994; Coluzzi, 1997, 1999; Sherman, 1998). Genetic changes that have increased the affinity within the parasite-vector-host system also seem to be a viable explanation for a recent expansion. Coluzzi (1997, 1999) has cogently argued that the worldwide distribution of P. falciparum is recent and has come about, in part, as a consequence of a recent dramatic rise in vectorial capacity caused by repeated speciation events in Africa of the most anthropophilic members of the species complexes of the Anopheles gambiae and Anopheles funestus mosquito vectors. The biological processes implied by this account may have, in turn, been associated with, and even depended on the onset of agricultural societies in Africa and climatic changes, specifically the gradual increase in ambient temperatures after the Würm glaciation, so that about 6,000 years ago climatic conditions in the Mediterranean region and the Middle East made possible the spread of P. falciparum and its vectors beyond tropical Africa (de Zulueta, 1973, 1994; Coluzzi, 1997, 1999).

Sherman (1998) has noticed the late introduction and low incidence of falciparum malaria in the Mediterranean region, which postdates historical times. Hippocrates (460–370 B.C.) describes quartan and tertian fevers, but there is no mention of severe malignant tertian fevers, which suggests that P. falciparum infections did not yet occur in classical Greece, as recently as 2,400 years ago. The late introduction of falciparum malaria into the Mediterranean region and the Middle East has been attributed to the low vectorial efficiency of the indigenous anopheline mosquitoes (Coluzzi, 1997, 1999). Once the demographic and climate conditions became suitable for the propagation of P. falciparum, natural selection would have facilitated the evolution of Anopheles species that were highly anthropophilic and effective falciparum vectors (de Zulueta, 1973; Coluzzi, 1997, 1999).

The selective sweep hypothesis (iv) is, in a way, a special case of the demographic sweep hypothesis (v); i.e., a particular strain may have spread throughout the world and replaced all other strains impelled by



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