The relationship of genetic distance with pre- and postmating isolation, geographic overlap and fitness in the estrildid finches (Aves: Estrildidae)

Speciation studies guided by the biological species concept focus on the evolution of mechanisms preventing hybridization between species and the relative fitness of their hybrid offspring. As incipient species diverge, interspecific crosses will yield offspring having progressively reduced fitness. Previous research suggests that in birds postzygotic isolation develops late in the divergence process and that hybrid sterility precedes inviability suggesting that prezygotic barriers are primarily responsible at the onset of speciation. This study corroborates most of the patterns found previously. Genetic distance increases monotonically in interspecific crosses producing hybrids of both sexes, crosses producing only fertile males, and crosses that produce hybrids with no information on their fertility. The oldest hybridization recorded occurred at a genetic distance of 0.248 ca. 12.4 my if a clock of 1 my per 2% sequence divergence is used. There were not hybridization events recorded at genetic distances of 0.250 to 0.306; ca. 12.5 to 15.3 my. Also, interspecific crosses that breeders use in mixed-flock aviaries and for which no hybrids are known suggest that postzygotic isolation can be achieved as early as a genetic distance of 0.070 or ca. 3.5 my. Mating trials show that females lay eggs regardless they were paired inter- or interspecifically or genetic distance. Information gathered in this study indicates that prezygotic isolation might be relatively easy to surmount allowing hybridization event suggesting; thus suggesting that prezygotic isolation must operate in conjuction with other forces to be effective.;Morphological differentiation could be the byproduct of adaptation to different geographical settings. However, when species attain secondary sympatry further morphological differentiation could result from several processes of species interaction. Observed patterns of geographical overlap for African species agree with previous findings such as a positive relationship between the degree of co-occurrence and the time of divergence from a common ancestor. In South East Asia and Australia, however, the pattern differs suggesting possibly taxon cycles in this geographic scenario or at least that most species stabilize then geographic ranges after reaching their island's limits. The similitude of morphological distances of species living in sympatry and allopatry possibly suggests phylogenetic niche conservatism where phylogenetic constraints limit the possibility of morphological changes than bodies can attain or the absence of competitive exclusion as a shaping force in secondary sympatry. On the other hand, it is noteworthy to observe that plumage dissimilarity is consistent with some sort of reproductive character displacement, where signals based on plumage pattern emerge as an important component for ethological isolation that could lead inevitably to speciation once the process has started. On average, sympatric species differ in three body regions more than their allopatric counterparts.;The phytohemagglutinin (PHA) test assesses some components of T-cell mediated response related to fitness. PHA response has been linked to life history attributes such a body size, development, and longevity and PHA responses are good predictors of survival and parasite resistance. In this study, the PHA test was administered to several species of estrildid finches and their hybrids to determine whether the performance of one component of the immune system decreases as a result of species divergence. When comparing PHA responses between hybrids and their parental species, analyses did not yield significant differences in most cases. Only when PHA responses of hybrids were compared with the mid-parent values did the response decrease with increasing genetic divergence of the parental species. Contingency analyses of crosses binned within intervals of genetic distance corroborated a trend suggesting that as genetic distance increases hybrids show a reduced inflammatory response and possibly a reduction in fitness.