The Galapagos Islands have long been recognised as the home of numerous endemic species, stimulating questions about how such species came into being. Those responding with answers have supported their views more by theory than observation. But Peter and Rosemary Grant are different, because they have pioneered longitudinal studies of the Galapagos finches, particularly on the small (and relatively isolated) island of Daphne Major. A newly reported study of an immigrant male ground finch (Geospiza fortis) covers the period 1981 to the present.
"We have followed the survival and reproduction of this individual and all of its known descendants, here termed the immigrant lineage, for seven generations (F0 to F6) spanning 28 years."
This finch's odd beak and song make it unpopular with the locals. (Credit: Peter Grant/PNAS, Source here)
The Grants have observed a clustering of male territories of the immigrant lineage and have established that these birds formed a new, albeit small, breeding population. The story is one of interbreeding followed by inbreeding. The authors consider they have gained an "important insight into the process of speciation" - although they hasten to add that they do not regard these birds as a new species.
"How many generations of exclusively within-group mating are needed before the group is recognized as a separate species that deserves taxonomic status? There is no nonarbitrary answer. We treat the endogamous group as an incipient species because it has been reproductively isolated from sympatric G. fortis for three generations and possibly longer."
This is not a story involving new mutations, or any significant genetic change outside the normal range of groundfinches. It is relevant to discussion of sympatric speciation, because the new population coexists with other medium groundfinches on the same island and interbreeding did take place in the earlier generations of the lineage. So what factors are associated with a barrier to interbreeding? The Grants identify two factors: one is morphological (immigrants have larger beak dimensions) and the other is song (immigrants differ from residents lower maximum frequency and higher note repetition rate).
"Our observations provide insight into speciation and hence, into the origin of a new species. They show how a barrier to interbreeding can arise behaviorally and without genetic change in sympatry. A necessary condition was prior ecological divergence, and introgressive hybridization was possibly another. Evidently it takes only a single diploid immigrant to start the process by breeding with a resident, and tolerance of the effects of inbreeding is needed to complete it."
These observations, and the emerging ecological perspective on speciation is interesting - but how does it relate to the broader issues faced by evolutionary biologists? In particular, does it help to understand how innovation occurs? The answer must be - no. There are no genetic changes that can be associated with novel characteristics and although this population is currently reproductively isolated, a few environmental traumas could easily lead to unification. Darwinism is exactly where it has always been - explaining the origin of complexity by appealing to theory and the imagination.
On the other hand, these data can be understood readily in terms of speciation by gene pool reduction. Although normally presented in terms of allopatry, this model considers the possible loss of alleles when a breeding population is split. The resultant populations may not have the genetic diversity of the parent population, and whilst this may be workable in the short-term, environmental change may reveal that the daughter population(s) are unfit and the end result may be extinction. Clearly, this perspective on the data has nothing to give Darwinism but it is compatible with several non-Darwinian perspectives on speciation.
Science reports of stories relevant to evolutionary theory can degenerate to the level of cheer-leading for a favoured cause. One account of the Grants' research refers to "a real-time record of evolution in action. In the PNAS paper, they describe something Darwin could only have dreamed of watching: the birth of a new species." For more, please refer to Jonathan Wells comments here.
The secondary contact phase of allopatric speciation in Darwin's finches
Peter R. Grant and B. Rosemary Grant
Proceedings of the National Academy of Sciences, Early Edition, Nov. 16, 2009 | doi 10.1073/pnas.0911761106
Abstract: Speciation, the process by which two species form from one, involves the development of reproductive isolation of two divergent lineages. Here, we report the establishment and persistence of a reproductively isolated population of Darwin's finches on the small Galapagos Island of Daphne Major in the secondary contact phase of speciation. In 1981, an immigrant medium ground finch (Geospiza fortis) arrived on the island. It was unusually large, especially in beak width, sang an unusual song, and carried some Geospiza scandens alleles. We followed the fate of this individual and its descendants for seven generations over a period of 28 years. In the fourth generation, after a severe drought, the lineage was reduced to a single brother and sister, who bred with each other. From then on this lineage, inheriting unusual song, morphology, and a uniquely homozygous marker allele, was reproductively isolated, because their own descendants bred with each other and with no other member of the resident G. fortis population. These observations agree with some expectations of an ecological theory of speciation in that a barrier to interbreeding arises as a correlated effect of adaptive divergence in morphology. However, the important, culturally transmitted, song component of the barrier appears to have arisen by chance through an initial imperfect copying of local song by the immigrant. The study reveals additional stochastic elements of speciation, in which divergence is initiated in allopatry; immigration to a new area of a single male hybrid and initial breeding with a rare hybrid female
Cressey, D. Darwin's finches tracked to reveal evolution in action, Nature News (16 November 2009) | doi:10.1038/news.2009.1089
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