Summary of problems:The hybridization observed in the finches is not enough to merge two species, and observations in the field have actually shown substantial evidence of incipient speciation.
Full discussion:Explore Evolution argues that
after the rains returned [to the Gal pagos], the Grants notices that several separate species of finches were interbreeding. No only were no new species springing forth, but existing varieties actually seemed to be merging. Critics therefore conclude that the finch beak example of microevolution actually suggests that biological change has limits.Explore Evolution , p. 93
Given that this passage follows right after their complaints about the dangers of extrapolation, it is rich to find them claiming that anything that didn't happen in a 30 year study could never happen. The sole basis for the claim that limits exists seems to be that something did not occur in the course of a single study. By that standard, I could predict that I will never die, since I am thirty and have not been observed to die.
Furthermore, the citation the authors use to explain that hybridization occurred points out that genetic novelty accompanies such unusual matings, and explains why those matings do not mean that the species are merging. The ongoing changes in beak shape (shown in part F of the figure above) cannot be explained by natural selection alone. They can only be explained by invoking the combination of two of the 4 major evolutionary mechanisms: natural selection and gene flow. Natural selection explains the initial changes, but the flow of genes between species provided the ongoing evolutionary pressure towards blunter beaks. Peter and Rosemary Grant explain:
The proportionally greater gene flow from G. fortis to G. scandens than vice versa has an ecological explanation. Adult sex ratios of G. scandens became male biased after [the extremely hot and wet] 1983 as a result of heavy mortality of the socially subordinate females. High mortality was caused by the decline of their principal dry-season food, Opuntia cactus seeds and flowers; rampantly growing vines smothered the bushes. G. fortis, more dependent on small seeds of several other plant species, retained a sex ratio close to 1:1. Thus, when breeding resumed in 1987 after 2 years of drought, competition among females for mates was greater in G. fortis than in G. scandens. All 23 G. scandens females paired with G. scandens males, but two of 115 G. fortis females paired interspecifically. All their F1 offspring later bred with G. scandens because choice of mates is largely determined by a sexual imprinting-like process on paternal song.Grant, P. R. and B. R. Grant (2002), "Unpredictable evolution in a 30-year study of Darwin's finches." Science 296:707-711
There are two important things to understand about that. First, that the hybridization was a result of unusual environmental conditions and an excess in the number of males of one species. Those males competed for access to any female at all, and were prepared to overcome pre-mating barriers to hybridization. Second, the main force limiting hybridization is that different species of finches select mates with songs similar to those of their fathers. Elsewhere, the Grants explain:
Hybridization occurs sometimes as a result of miscopying of song by a male; a female pairs with a heterospecific male that sings the same song as that sung by her misimprinted father. On Daphne Major island, hybrid females bred with males that sang the same species song as their fathers. All G. fortis G. scandens F1 hybrid females whose fathers sang a G. fortis song paired with G. fortis males, whereas all those whose fathers sang a G. scandens song paired with G. scandens males. Offspring of the two hybrid groups (the backcrosses) paired within their own song groups as well. The same consistency was shown by the G. fortis G. fuliginosa F1 hybrid females and all their daughters, which backcrossed to G. fortis. Thus mating of females was strictly along the lines of paternal song.Peter R. Grant and B. Rosemary Grant (1997). "Genetics and the origin of bird species," Proceedings of the National Academy of Sciences, 94:7768 7775
The Grants go on to discuss how this helps explain the speciation of Gal pagos finches.
Experiments show that birds are less responsive to the songs of conspecifics from different islands than to songs from their own island. Even though the changes are small, the process of cultural drift is enough to begin isolating these populations. In addition, the finches show a preference for the morphology of birds from their own island to members of the same species from other islands, even independent of song differences. The forces driving natural selection on different islands will differ, and that will produce morphological differences, which will combine with differences in songs to make hybridization less likely.
Thus, the observation of hybridization does not provide evidence that two species will merge into one. Instead, it helps test the process by which species separate. Furthermore, the hybridization observed had an effect which directly contradicts the claim that novelty cannot originate through evolutionary processes. Because of genes flowing in from another species, G. scandens experienced substantial evolutionary change, and acquired novel traits.
The Grants point out that this sort of gene exchange is important to understanding speciation and evolution in general:
Introgressive hybridization [as seen in Darwin's finches] has the potential of leading to further evolutionary change as a result of enhancing genetic variances, in some cases lowering genetic covariances), introducing new alleles, and creating new combinations of alleles, some of which might be favored by natural selection or sexual selection. Sv rdson believed that introgression in coregonid fishes has replaced mutation as the major source of evolutionary novelty. Introgression and mutation are not independent; introgressive hybridization may elevate mutation rates.
The relevance to speciation lies in the fact that regions of introgression are peripheral areas, which could become isolated from the main range of the species through a change in climate and habitat: they are potential sites of speciation.Grant, P. R. and B. R. Grant (1997)
Far from demonstrating limits to the power of evolution, the rare hybridizations between species demonstrate how strong pre-mating isolation is, illustrate an important source of variation, and provide evidence about the process of speciation and the origins of genetic novelty. The fact that Explore Evolution never mentions two of the four major mechanisms of evolution (gene flow and genetic drift), speaks poorly of the authors' commitment to a serious examination of evolutionary biology.