expected to be high based on results from protein electrophoresis studies (Nason et al., 1998), Table 7.2 shows that the observed level of DNA polymorphism in these species is not uncommonly high for the three surveyed genes. In fact, DNA polymorphism in these three species is much lower than that observed in other outbred plant species. Average values of θ, Watterson’s estimator of the population mutation rate (4Nμ) (Watterson, 1975), for F. popenoei (0.00292), F. bullenei (0.00403), F. neartrigonata (0.00961), are approximately two to nine times lower than averages values at silent sites from the two outcrossing plants (Zea mays ssp. parviglumis and Arabidopsis lyrata ssp. petraea) that have the highest levels of nucleotide diversity that have been measured (Wright and Gaut, 2005). However, our survey of variation is based on a small sample of genes and species, and thus, genetic studies that include more loci and species would help to resolve this last point.
In the past, it has been difficult to explain the origin of the remarkable global and local diversity of fig species given the assumption of strict-sense cospeciation and one-to-one pollinator species-specificity. As a result, there is no generally accepted view on the mechanisms of speciation in figs and their pollinators (Michaloud et al., 1996; Weiblen, 2002). A few authors have proposed that speciation in these organisms happens by allopatric isolation (Janzen, 1979; Michaloud et al., 1996; Ramirez, 1970a). However, genetic studies using paternity analyses have shown that neotropical figs have the highest documented distances of gene flow of any tropical plant (Nason et al., 1996, 1998). Despite growing at very low densities (1–10 individuals per km2) and having asynchronous flowering within populations, individual fig trees receive pollinating wasps (and pollen) from a large number of individual trees. Conservative estimates suggest that fig wasps routinely disperse pollen over distances of >10 km and that breeding populations of figs constitute hundreds of individuals spread over areas >100 km2 (Nason et al., 1998). Therefore, allopatric isolation in monoecious neotropical species is highly unlikely. Other authors have proposed that temporal isolation (allochrony) generated by flower asynchrony could lead to isolation in sympatry (Kiester et al., 1984). However, population genetic studies of the pollinators reveal no evidence of population subdivision (Molbo et al., 2004), an essential condition for the allochronic speciation model (Kiester et al., 1984).
In a short essay published in 1961, Baker proposed pollinator generalization (lack of species specificity) and hybridization among fig species as a potential explanation for the large diversity of recognized species of