is a tract of repetitive DNA in which certain DNA motifs, ranging in length from 2 to 5 base pairs, are repeated, with the number of repeats varying in alleles of individuals.) A measure of the genetic difference between species is shown by the length of the horizontal lines. The feeding habits of the various species are also shown. Drawings of the birds’ heads are proportional to actual body size. The maximum amount of black colouring in male plumage and the average body mass are shown for each species. C, Camarhynchus; Ce, Certhidea; G, Geospiza; P, Platyspiza; Pi, Pinaroloxias. (c) Gene flow for the four species on Daphne Major, through interbreeding with other species on the island and with immigrants of the same and other species from the nearby islands. Flow is measured as the effective number of individuals per generation. For genes to flow, the first‐generation hybrid offspring must themselves mate with one of the parental species. Genes flow from G. fortis to G. scandens when the hybrid sings the G. scandens song (because its father did) and vice versa for genes flowing from G. scandens to G. fortis. The population of G. fulginosa on Daphne Major is very small, and hence the flow of genes into G. fortis comes from immigrants from other islands.
Source: (b) After Petren et al. (1999). (c) After Grant & Grant (2010).
Isolation – both of the archipelago itself and of individual islands within it – has led to an original evolutionary line radiating into a series of species, each matching its own environment. Populations of ancestor species became reproductively isolated, most likely after chance colonisation of different islands within the archipelago, and evolved separately for a time. Secondary contact phases subsequently occurred as a result of movements between islands that brought non‐hybridising biospecies together that then evolved to fill different niches that elsewhere in the world are filled by quite unrelated species. Members of one group, including Geospiza fuliginosa and G. fortis, have strong bills and hop and scratch for seeds on the ground. G. scandens has a narrower and slightly longer bill and feeds on the flowers and pulp of the prickly pears as well as on seeds. Finches of a third group have parrot‐like bills and feed on leaves, buds, flowers and fruits, and a fourth group with a parrot‐like bill (Camarhynchus psittacula) has become insectivorous, feeding on beetles and other insects in the canopy of trees. A so‐called woodpecker finch, Camarhynchus (Cactospiza) pallida, extracts insects from crevices by holding a spine or a twig in its bill, while yet a further group includes the warbler finch, which flits around actively and collects small insects in the forest canopy and in the air.
However, the biospecies compartments are not water‐tight. A study of the four species on the small island of Daphne Major, and of their possible interbreeding with birds from larger nearby islands, again using molecular techniques, is summarised in Figure 1.9c. The two most abundant species, Geospiza fortis and G. scandens, were subject to a greater flow of genes between one another than they were to genes from immigrants of their own species from other islands. Indeed, in the case of G. fortis, there was also a substantial flow of genes from G. fuliginosa immigrants from other islands. Thus, the ‘ideal’ of gene flow within a species but not between them is not borne out by the data. But the fact that there are ‘grey areas’ partway through the process does not diminish the importance of either the process of speciation or the concept of biospecies.
ring species – perfect examples of speciation in action, but why so rare?
That speciation is a process rather than an event is beautifully illustrated by the existence of ring species. In these, races or subspecies of a species that fall short of being full species themselves (i.e. distinct forms that are nonetheless capable of producing fertile hybrids) are arranged along a geographic gradient in such a way that the two ends of the gradient themselves meet, hence forming a ring, and where they do, they behave as good species despite being linked, back around the ring, by the series of interbreeding races. Thus, what would normally be a temporal sequence of events, that we can only presume to have happened, becomes frozen in space. That the phenomenon is theoretically feasible has been demonstrated using mathematical models (e.g. de Brito Martins & de Aguiar, 2016). But actual examples are rare, and several that have been proposed in the past have been called into question by modern molecular studies, leading Pereira and Wake (2015) to wonder whether ring species are an unfulfilled promise or, worse still, wish‐fulfilment fantasy.
The classic example is the extraordinary case of two species of sea gull. The lesser black‐backed gull (Larus fuscus) originated in Siberia and colonised progressively to the west, forming a chain or cline of different forms, spreading from Siberia to Britain and Iceland. The neighbouring forms along the cline are distinctive, but were assumed to hybridise readily in nature. Neighbouring populations are regarded as part of the same species and taxonomists give them only ‘subspecific’ status (e.g. L. fuscus graellsii, L. fuscus fuscus). Populations of the gull have, however, also spread east from Siberia, again forming a cline of freely hybridising forms. Together, the populations spreading east and west encircle the northern hemisphere. They meet and overlap in northern Europe. There, the eastward and westward clines have diverged so far that it is easy to tell them apart, and they are recognised as two different species, the lesser black‐backed gull (L. fuscus) and the herring gull (L. argentatus). Moreover, the two species do not hybridise: they have become true biospecies. In this remarkable example, then, we can see how two distinct species seem to have evolved from one primal stock, and that the stages of their divergence remain frozen in the cline that connects them.
However, modern molecular techniques to determine genetic relationships have revealed a more complex picture. Thus, while ancestral populations expanded in a roughly circular fashion, there have been intermittent periods of allopatric fragmentation and subsequent range expansion, leading to areas of secondary contact where hybridisation currently occurs. Population divergence, therefore, proceeded at least partly in allopatry, not exclusively through isolation by distance throughout a contiguous range, as the ring species concept requires. Moreover, adjacent subspecies have been found not necessarily to be each other’s closest relatives and evidence is lacking of closure of the circumpolar ring by colonisation of Europe by North American herring gulls, a cornerstone of the ring species concept (Martens & Packert, 2007).
A more convincing example involves bulbuls in the genus Alophoixus in montane habitats of the Indo‐Malayan bioregion. Fuchs et al. (2015) have shown that diversification is consistent with most criteria expected for ring species (Figure 1.10a). First, molecular analysis shows that the seven taxa (Figure 1.10b) are all descendants of a single ancestral species, and probably derive from a single colonisation from Sundaland. Second, neighbouring taxa are most closely related, suggesting that taxa have diverged from a stepping stone colonisation of the high‐elevation forest around Thailand’s lowlands (lowland ‘barriers’ A and B in Figure 1.10a). The current distribution suggests that divergence can be explained by isolation by distance, as assumed by the ring species concept (but also, partly, by periods of geographic isolation that probably occurred during climatic cycles following initial diversification of the complex). Third, gene flow between neighbouring taxa suggests that divergence and secondary contact between taxa around the ring have resulted in genetic intergradation. And fourth, demographic analyses indicate a recent expansion and geographic overlap of the oldest taxon (1) and its most distant relative (7), leading to closure of the ring. However, hybrids sampled at the terminus of the ring (where taxon 1 meets taxon 7) indicate that divergence has not been sufficient for complete reproductive isolation to evolve.
