Ecology. Michael Begon

Ecology - Michael  Begon


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of ‘secondary contact’, the two subpopulations re‐meet. The hybrids between individuals from the different subpopulations are now of low fitness, because they are literally neither one thing nor the other. Natural selection will then favour any feature in either subpopulation that reinforces reproductive isolation, especially pre‐zygotic characteristics, preventing the production of low‐fitness hybrid offspring. These breeding barriers then cement the distinction between what have now become separate species.

Graph depicts the orthodox picture of ecological speciation. A uniform species with a large range (1) differentiates (2) into subpopulations (for example, separated by geographic barriers or dispersed onto different islands), which become genetically isolated from each other. (3) After evolution in isolation they may meet again, when they are either already unable to hybridise (4a) and have become true biospecies, or they produce hybrids of lower fitness (4b), in which case evolution may favour features that prevent interbreeding between the emerging species until they are true biospecies.

      Darwin’s finches

Schematic illustration of different species of Darwins finches that have evolved on the Galápagos Islands. (a) Map of the Galápagos Islands showing their position relative to Central America; on the equator 5° equals approximately 560 km. (b) A reconstruction of the evolutionary history of the Galápagos finches based on variation in the length of microsatellite DNA. 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. (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.

      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.

      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


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