under single-gene or polygenic control (plumage and morphology), others are culturally inherited (song). The slow evolution of postmating isolation implies that the scope for reinforcement of premating isolating mechanisms is minimal. Involvement of culturally inherited traits may be partly responsible for the relatively rapid rate of speciation in bird.

Speciation in Birds

Speciation has been most thoroughly investigated, and for many years, in Darwin’s finches (Geospizinae; refs. 47). We therefore begin by describing a model that was devised specifically for these birds on the Galápagos Islands (8). We examine the evidence for various aspects of the model focusing on genetic factors where possible, and consider alternatives. Then we ask what needs to be added to the model to make it a comprehensive statement of speciation in birds in general.

A Model of Allopatric Speciation

Fig. 1 portrays three stages in the cycle of events leading to the division of one species into two. The choice of islands to illustrate these stages is arbitrary. In step 1, the archipelago is colonized from continental South or Central America. A breeding population becomes established, and its size increases. In step 2 some individuals disperse to another island and establish a new breeding population. Some evolutionary change takes place in the new environment through selection and drift. Step 2 may be repeated several times, giving rise to several differentiated populations of the same species. Step 3 is the contact, through dispersal of members of two populations possessing different mate signaling and recognition systems. This is the secondary sympatric phase of the cycle, and there are two types of outcomes. In one, members of the two populations do not interbreed, or if they do their offspring are inviable or infertile; the process of speciation in this case has been completed in allopatry. Alternatively the populations are only partly reproductively isolated, interbreeding occurs, and some of the hybrids survive to breed. Reinforcement of the differences between the species then may occur if the hybrids have relatively low fitness.

FIG. 1. Allopatric speciation of Darwin’s finches in the Galápagos archipelago (8). After an initial colonization of the archipelago (step 1), dispersal and the colonization of new islands (step 2) gives rise to allopatric populations, which diverge through selection and drift. The process is completed with the establishment of sympatry (step 3). The choice of islands to illustrate the process is arbitrary. [Reproduced with permission from ref. 7 (Copyright 1996, The Royal Society)].

Step 1 probably occurred once, or at most a few times, given the large distance separating the islands from the continent, which was greater at the time of initial colonization than at present (9). An argument from major histocompatibility complex variation suggests that there must have been a minimum of 30 individuals in the colonizing population (10). Stages 2 and 3 were repeated several times, giving rise to several species over a period of time estimated to be less than 3 million years (11). The ecological conditions would have varied from one cycle to another, but the essential features were repeated. The varying conditions include the length of the period of the allopatric phase (stage 2) before secondary contact, population sizes and hence the scope for drift, and the difference in the island environments and hence the scope for directional selection. Another important factor was the creation of new islands by volcanic activity (7, 9) and the recent periodic lowering of sea level (12). Over the last 3 million years there has been a net increase in the number of islands despite some disappearing through submergence, paralleling the increase in number of species (7). Thirteen species are recognized on the basis of morphological and biological criteria (4, 6), with as many as 10 occurring on a single island. A 14th species inhabits Cocos Island.

Evidence from Field Studies of Darwin’s Finches

We observe closely related species in sympatry and infer how they evolved from a common ancestor. Therefore we first consider how species are reproductively isolated, and then work back to their allopatric origin.

Reproductive Isolation

Species can be recognized by their morphological characteristics and songs (13, 14). With rare exceptions sympatric species pair and breed conspecifically, and as a result are reproductively isolated from each other. They choose mates on the basis of song, sung by males only, and morphological appearance, in which beak size and shape and body size play a part but plumage does not. Imprinting on adult features early in life appears to guide the choice of mates (7, 15, 16). The role of morphology in mate choice has been demonstrated experimentally with tests that show that several pairs of sympatric species of ground finches (Geospiza) discriminate between conspecific and heterospecific visual cues (17). Separately, experiments have shown that males can discriminate between conspecific and heterospecific auditory cues (18). Females were not tested in these acoustic experiments, but it would be surprising if they were not capable of making the same discriminations. The evolution of reproductive isolation in Darwin’s finches is therefore the evolution of differences in song and in morphology.

Reproductive isolation is not complete; species hybridize, rarely, and are capable of producing fertile hybrids that backcross to the parental species (12, 19, 20). The rare interbreeding of species and the mating pattern of the hybrids provide further evidence of the importance of song in mate choice. Hybridization occurs some times 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 (16). On Daphne Major island hybrid females bred with males that sang the same species song as their fathers (20). 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. The independent role of morphology in mate

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