Subdivided populations

The distributions of species are extremely varied. No species that we know of has a truly worldwide distribution, although humans and some of their associates (dogs, rats, lice) come very close. Possibly the widest-distributed flowering plant is the common reed Phragmites australis, which is found on every continent except Antarctica. At the other end of the scale are many endemic species that have extremely restricted ranges, such as the giant Galapagos tortoises Geochelone nigra. Most of the 11 surviving subspecies are restricted to single islands within the archipelago, and in the case of G. n. abingdonii the entire subspecies is reduced to a single male known as Lonesome George who now lives at the Charles Darwin Research Station on the Island of Santa Cruz. All other species on Earth can be placed somewhere along the geographical continuum from humans to Lonesome George. Equally variable are species' patterns of distribution, with some forming essentially continuous populations throughout their range and others having extremely disjunct distributions. Examples of the former once again include humans, and examples of the latter include the strawberry tree Arbutus unedo, which is native to much of Mediterranean Europe and also Ireland, and the springtail Tetracanthella arctica, which is common in Iceland, Spitzbergen and Greenland and is found also in the Pyrenees Mountains between France and Spain and in the Tatra Mountains between Poland and the Czech Republic.

Dispersal and vicariance

Disjunct populations, whether separated by thousands of kilometres or only a few kilometres, are isolated from one another either because they were founded following colonization events (dispersal), or because something has severed the connections between formerly continuous populations (vicariance). We have spent some time discussing dispersal in the previous chapter, so will touch only briefly on it here. Dispersal influences phylogeographic patterns through ongoing gene flow, which can have profound effects on population subdivision, Ne and genetic diversity. Another way in which dispersal is important to phylogeography is through rare long-distance movements. These often entail the colonization of new habitats such as oceanic islands. Gigantic land tortoises in the past have colonized not just the Galapagos archipelago but also a number of other oceanic islands, including the Seychelles, Mauritius and Albemarle Island. They may have dispersed to these islands by riding on rafts of floating vegetation across hundreds or even thousands of kilometres of open ocean.

Vicariance is the term given to the splitting of formerly continuous populations by barriers such as rivers or mountains. The uplifting of the Isthmus of Panama, for example, was a vicariant event that caused the Atlantic and Pacific populations of numerous plant and animal species to become isolated from one another (Figure 5.9). Vicariance may also result if two populations become separated by an exaggerated intervening distance following the extinction of intermediate populations.

Examples of dispersal and vicariance as promoters of population differentiation are given in Table 5.3. There are two ways in which sequence data can help us to decide whether populations were separated by dispersal or vicariance. The first is to use an appropriate molecular clock to estimate the time since lineages diverged from one another and see if this coincides with the timing of a known vicariant event, such as the separation of continents following continental drift. When a molecular clock was applied to chloroplast sequences from species of the southern beech subgenus Fuscospora in Australasia and South America, it became apparent that some lineages diverged from each other at around the time that the two regions became separated, and therefore a vicariant event that occurred approximately 35 million years ago may explain the current distributions of these species (Knapp et al., 2005).

Figure 5.9 A red mangrove tree (Rhizophora mangle). This is an unusually salt-tolerant tree that grows along coastlines. Uplifting of the Isthmus of Panama approximately 3 million years ago was a vicariant event that caused red mangrove populations along the Atlantic and Pacific coasts to become isolated from one another (Nunez-Farfan et a/., 2002). The bird on this mangrove tree is a brown pelican (Pe/ecanus occidenta/is). Author's photograph

Figure 5.9 A red mangrove tree (Rhizophora mangle). This is an unusually salt-tolerant tree that grows along coastlines. Uplifting of the Isthmus of Panama approximately 3 million years ago was a vicariant event that caused red mangrove populations along the Atlantic and Pacific coasts to become isolated from one another (Nunez-Farfan et a/., 2002). The bird on this mangrove tree is a brown pelican (Pe/ecanus occidenta/is). Author's photograph

A second approach for differentiating between dispersal and vicariance is to look at the branching order of gene genealogies; by comparing the evolutionary relationships of populations to their geographical distribution, we can gain some insight into the relative importance of past dispersal versus vicariant events (Figure 5.10). This method was used to investigate which force promoted the speciation of Queensland spiny mountain crayfish (genus Euastacus) in the upland rainforests of Eastern Australia (Ponniah and Hughes, 2004). Each of these rainforests, which are separated by lowlands, is home to a unique species of Euastacus, and two competing hypotheses could explain their current distribution.

Table 5.3 Some examples in which either vicariance or dispersal has been identified as the most likely explanation for population differentiation and, in most cases, speciation

Species

Rationale

Reference

Vicariance

Sonoran Desert cactophilic flies (Drosophila pachea)

Marine gastropods (Tegula viridula and T. verrucosa)

Sand gobies (genera Pomatoschistus, Gobiusculus, Knipowitschia, and Economidichthys)

Genetic differentiation between, but not within, the continental and peninsular populations (barrier is Sea of Cortez) Sister species located either side of the Isthmus of Panama

Rapid evolution dating to the salinity crisis (end of the Miocene) in the Mediterranean Sea

Hurtado et al. (2004)

Vermeij (1978)

Huyse, Van Houdt and Volckaert (2004)

Dispersal

Mouse-sized opposums (Marmosops spp.) in Guiana Region

Two frogs in the genera Mantidactylus and Boophis (species not yet described) Freshwater invertebrates (Daphnia laevis, Cristatella mucedo)

Genetic data suggest recent origin of populations, rapid population growth, and dispersal from small ancestral population Recently discovered in Mayotte, an island in the Comoro archipelago (Indian Ocean) Genetic lineages roughly follow waterfowl migratory routes

Steiner and Catzeflis (2004)

Vences et al. (2003)

Taylor, Finston and

Hebert (1998); Freeland, Noble and Okamura (2000)

The first hypothesis states that a widespread ancestor was subdivided into populations by 'simultaneous vicariance' such as habitat fragmentation, after which time each population would have followed its own evolutionary path. Alternatively, a dispersal hypothesis states that colonization of each rainforest occurred in a northwards stepping-stone manner.

Because spiny mountain crayfish are known to have originated in the south, Ponniah and Hughes (2004) assumed that populations originally followed a north-south pattern of isolation by distance. From this they reasoned that if a single vicariant event had occurred, and all populations were split simultaneously, a pair of neighbouring populations in the south should now show a similar level of genetic differentiation to a pair of neighbouring populations in the north. Alternatively, if a stepping-stone dispersal pattern had occurred then southern populations should show greater genetic differentiation than northern populations

Site

Site

Site 1

Site 2

Site 2

Site 2

Site 1

Z Site 3

Site 2

Figure 5.10 The phylogenetic relationships of populations or species are expected to vary, depending on whether they arose following dispersal (a) or vicariance (b). Under a dispersal scenario, sites 2 and 3 are colonized by species (or populations) X and Z. If populations in sites 2 and 3 remain reproductively isolated from the populations in site 1, the descendants of the original populations eventually will evolve into pairs of related species (X-1 and X-2, Z-1 and Z-2), a pattern that is reflected in the phylogenetic tree. Under a vicariance scenario, site 1 first is split into sites 1 and 2, which leads to the evolution of species X-1 and Y from the ancestral species X. After site 2 is split into sites 2 and 3, the descendants of species Y in site 3 evolve into species Z. Meanwhile, speciation is also occurring within sites 1 and 2, leading to closely related species pairs (X-1 and X-2, Y-1 and Y-2). Note that in the vicariance phylogenetic tree those species from the same site are most closely related to one another, whereas the nearest neighbours in the dispersal phylogenetic tree are from different sites. Adapted from Futuyma (1998)

because they would have had a longer time to evolve population-specific haplotypes. The two hypotheses were tested using mitochondrial sequence data, which provided a genealogy consistent with the former scenario. The authors therefore concluded that vicariance was a more plausible explanation than dispersal for the current distribution of Euastacus. However, it is important to note that past events in this and other studies may be obscured by factors that cannot be controlled for easily, including unknown historical population sizes, the amount of time that has passed since populations diverged, and the fact that vicariance and dispersal may not be mutually exclusive. We will pursue this further later in the chapter, but first will look at how the genealogical relationships of two reproductively isolated populations are likely to change over time.

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