niches is the main driving force.
sympatric speciation – divergence with gene flow or microallopatric speciation?
A final critical question is whether a case thought to have arisen by sympatric speciation is truly a result of species diverging while gene flow was occurring (sympatric) or merely ‘microallopatric’ speciation. A small‐scale geographic barrier (analogous to ocean habitat between islands) may occur, for example, as an underwater ridge in a lake. Moreover, host‐specific parasites and phytophagous insects might also have broadly overlapping geographic ranges and yet never encounter one another because of their distinct ecological niches. In other words, populations might overlap at a coarse grain if they occupy the same geographic region, but not co‐occur at a finer grain if they occupy different habitats within that region. Thus, whether populations are described as sympatric is somewhat at the discretion of the observer (Fitzpatrick et al., 2008).
a mechanism for sympatric speciation: AITs?
It is easy to see how geographically isolated populations have diverged, because they are also reproductively isolated, but not so straightforward to conceive how assortative mating can evolve sympatrically in populations that are not geographically isolated but experience divergent selective pressures. This may occur via ‘automatic isolating traits’ (AITs). An example would be where a particular locus or set of loci interacts with the environment to express different mating behaviours under different environmental conditions, regardless of genotype, such as the timing of flowering in plants. For example, the most recent common ancestor of two sympatric sister Howea palms on the tiny Lord Howe Island, 600 km off the coast of Australia, may have exhibited different flowering times when growing in different soil types so that a difference in physiology elicited by environmental differences, rather than a difference in genotype, could have enforced mating fidelity by soil type rather than genotype and increased the likelihood that divergence was possibly despite broad‐scale sympatry (Figure 1.12). Papadopulos et al. (2011) describe other examples of sympatric speciation of plants in the genera Metrosideros and Coprosma on Lord Howe Island. Further possible cases where AITs may operate include fish with colour polymorphisms, genes responsible for insect hybrid male sterility, and cases involving chemical signalling (Bird et al., 2012).
Figure 1.12 Sympatric speciation in Howea palms. Two species of Howea palm on the tiny and isolated Lord Howe Island off the coast of Australia. Howea forsteriana has straight leaves with drooping leaflets, while H. belmoreana has recurved leaves with ascending leaflets. (H. forsteriana is now one of the world’s most widely traded house plants.) A comprehensive DNA‐based phylogenetic tree indicates that these are sister species with their closest relative, Laccospadix, on the Australian mainland. Molecular dating methods show the two Howea species diverged 1–1.92 million years ago, long after Lord Howe Island was formed by volcanic activity 6.4–6.9 million years ago. H. forsteriana diverged from its sister species (an ancestor of H. belmoreana) by colonising widespread lowland calcarenite deposits. Extensive molecular evidence is consistent with Coyne and Orr’s criteria for sympatric speciation (see earlier). (a) H. forsteriana (green lines) flowers early in the flowering season, with male flowering (solid line) peaking 2 weeks before female receptivity (dashed line); H. belmoreana male and female flowering is synchronous but later in the season. (b) H. forsteriana occurs in soils of higher pH and (c) lower altitude than H. belmoreana.
Source: After Savolainen et al. (2006).
While allopatric speciation is generally accepted to be much more common than sympatric speciation, sympatric lineage divergence due to selection has certainly come of age in the wake of the molecular biology revolution, which has allowed hypotheses that were once untestable to be critically evaluated. Evolutionary ecologists are not so focused now on whether or not sympatric speciation can happen, but rather how often and under what conditions.
APPLICATION 1.3 Conservation significance of hot spots of endemism
Conservationists have to make hard decisions in their quest to preserve biological diversity. Given limited resources, how can the most species be supported at minimum cost? One way is to focus attention on 'biodiversity hot spots' of species that are found nowhere else. Myers et al. (2000) took this approach when mapping the entire globe in terms of exceptional concentrations of endemic species coupled with exceptional loss of habitat (and therefore subject to a greater degree of threat to biodiversity than areas without such habitat loss). Hot‐spot boundaries were set according to the characteristic biotas they contain: examples include island groups such as the Galápagos (Section 1.3.2) and Hawaii (Section 1.4.2), and 'ecological' islands such as the East African Great Lakes (Section 1.3.3) or clearly defined continental units such as the Cape Floristic Province in South Africa. The taxa included in the analysis consisted of vascular plants, mammals, birds, reptiles and amphibians. Figure 1.13 shows 25 identified hot spots that between them contain 133 149 plant species (that is, 44% of the world's plants) and 9645 vertebrate species (35% of the world's total). Or to put it in another way that emphasises their importance, we can say that this set of hot spots provide the sole remaining habitats of 44% of the world's plant species (and 35% of animals).
Figure 1.13 Biodiversity hot spots. Twenty‐five biodiversity hot spots identified because of their exceptional concentrations of endemic species that are undergoing exceptional levels of human induced habitat loss.
Source: From Myers et al. (2000).
The five most prominent hot spots, the tropical Andes, Sundaland, Madagascar, Brazil's Atlantic Forest and the Caribbean, contain 20% of the world's vascular plants and 16% of vertebrate species but together they comprise only 0.4% of the world's surface. Moreover, they are subject to some of the heaviest levels of habitat loss: the Caribbean retains only 11.3% of its primary vegetation, Madagascar 9.9%, Sundaland 7.8% and Brazil's Atlantic Forest 7.5%. There was reasonable congruence between levels of endemism of plants and vertebrates in the hot spots, but note that no invertebrates were included in the analysis. In a geographically more restricted study in South Africa, Bazelet et al. (2016) showed that there was congruence between hot spots of the rather circumscribed diversity of katydids (bush crickets) and the biodiversity hot spots already recognised for much wider groupings, indicating that the conservation of biodiversity hot spots may often also protect non‐target organisms.
Myers et al. (2000) called for a more than 10‐fold increase in annual funding from governmental and international agencies to safeguard these hot spots.
1.4 The role of historical factors in the determination of species distributions
