Ecological Speciation (Oxford Series in Ecology and Evolution)

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We begin by examining the three proposed forms of nonecological speciation: speciation under uniform selection, polyploid speciation, and speciation by genetic drift ( Table 1 in Schluter 2001). We show that ecology is involved in speciation by both uniform selection and polyploidy, and that genetic drift is unlikely to cause speciation unilaterally. Within the “ecological speciation” framework, speciation must sometimes be ecological and at other times, it must not be. Given our review of these cases, we ask the question: When is speciation nonecological? SPECIATION UNDER UNIFORM SELECTION A speciation process in which divergent natural selection drives the evolution of reproductive incompatibility (i.e., isolation) between taxa ( Nosil et al. 2009, p. 145).

Speciation initiated by polyploidy, that is, whole-genome duplication, is observed in a wide variety of organisms, including fish, amphibians, yeast, and plants. Two main mechanisms of polyploid speciation have been recognized; autopolyploids arise from within populations of a single biological species whereas allopolyploids are formed following hybridization between different species ( Ramsey and Schemske 1998). In each case, polyploidy is typically initiated by the production of unreduced gametes, that is, gametes with the somatic chromosome number. The link between sexual selection and sexual isolation presents situations in which genetic drift could conceivably lead to reproductive isolation. However, because female choice and male traits must both drift in concert to produce isolation, it is difficult to imagine conditions under which drift could work alone ( Coyne and Orr 2004). In his influential model of sexual selection, Lande (1981) proposed that female choice for arbitrary male traits can drift along a line of neutral equilibrium. This and subsequent theory has shown that it is possible for sexual isolation to evolve as female preference and male traits drift along this line ( Wu 1985; Uyeda et al. 2009). However, the potential for sexual isolation by drift is drastically reduced if there is a cost associated with female choosiness ( Turelli et al. 2001).If you find that the printed image is unexpectedly faint or dots in the printed image are missing, you may be able to solve these problems by cleaning the print head, which ensures that the nozzles are delivering ink properly. Mayr (1942, 1947) judged ecological factors as the major drivers of speciation. In his classic paper “Ecological factors in speciation,” Mayr (1947) concluded that geographic isolation leads to the formation of segregated populations that experience different ecological conditions, leading to evolutionary divergence. In Animal Species and Evolution, Mayr (1963, p. 556) devoted an entire chapter to the role of ecology in speciation, and began the second paragraph as follows: “An exhaustive treatment of the indicated subject matter would require an entire book, for there is hardly an ecological factor that does not affect speciation directly or indirectly, actually or potentially.” Many other evolutionary biologists have also supported the notion that ecological divergence of populations is typically required for speciation. Simpson (1953, p. 234n) concluded “… speciation, the basic process of radiation, is normally adaptive,” and Grant (1981) provided numerous examples in which ecological factors are the primary isolating barriers between species. An empirical example of the effect of the sequential nature of barriers is illustrated by measures of isolation between Mimulus cardinalis and M. lewisii ( Ramsey et al. 2003). In this study, geographic isolation, pollinator isolation, gametic isolation, and intrinsic postzygotic isolation had strengths of 0.59, 0.99, 0.83, and 0.41 respectively (averaged across the potentially asymmetrical isolation between species for simplification). Although each of these barriers may be considered strong on an individual basis, placing them in the linear sequence reveals that geographic isolation has the highest contribution to total isolation at 0.59 whereas pollinator isolation (0.40), gametic isolation (0.0083), and intrinsic postzygotic (0.00070) exhibit declining relative contributions to the total. In this example, the overwhelming strength of pollinator isolation virtually guarantees that gene flow cannot proceed beyond this barrier, in that very little potential gene flow remains for later acting barriers to prevent. This result has been validated by an extremely low incidence of hybrid seeds found in nature ( Ramsey et al. 2003). Assessing effective and ecogeographic components of geographic isolation. Populations of two taxa represented by X's and O's are separated by a mountain range. Effective geographic isolation is complete, such that taxa X and O experience no gene flow (A). Panels B–D show the outcome of estimating ecogeographic isolation by ecological niche modeling and reciprocal transplants. In (B) ecogeographic isolation is complete; each taxon is adapted to its own environment such that ecological niche modeling and reciprocal transplants show that the taxa would not survive in each other's geographic range. (C) Ecogeographic isolation is absent; the taxa are equally fit in the alternate range. (D) Ecogeographic isolation is incomplete. On average, each species survives and reproduces better in its own environment, but portions of the alternate range are also suitable.

The role of ecology in the establishment of polyploids. Autopolyploidy is presented for simplicity, but equivalent processes function with allopolyploidy. Circles represent topology of a simple two-trait adaptive landscape with darker circles representing trait combinations of higher fitness. A diploid progenitor (2N) sits upon an adaptive peak. There are two potential outcomes of neopolyploid formation: (A) The neopolyploid (neo 4N) could reside at a lower elevation of the same adaptive peak occupied by the progenitor. In this case, the neopolyploid faces both competitive disadvantage and minority cytotype exclusion, and will likely not establish. (B) The neopolyploid could initially reside at the base of a new adaptive peak, and adapt to this new niche (dashed line, neo 4N → established 4N). In this case, speciation can be considered ecological because polyploidy causes an initial change in ecology followed by subsequent adaptation as the neopolyploid climbs the new adaptive peak.Hybrid inviability between M. guttatus populations on and off copper mine tailings is linked to two genes for copper tolerance.

In the past decade, a number of papers have suggested that it is useful to distinguish ecological from nonecological mechanisms to elucidate the role of natural selection in speciation, for example, Schluter (2000, 2001, 2009), Rundle and Nosil (2005), and Nosil et al. (2009). What sets these efforts apart from previous discussion is the proposal of the term, “ecological speciation,” which is defined variously as Maan, M. E. & Seehausen, O. Mechanisms of species divergence through visual adaptation and sexual selection: Perspectives from a cichlid model system. Current Zoology 56, 285–299 (2010). Inland and coastal forms of M. guttatus experience selection for different growth and flowering times resulting in flowering phenology with little overlap. Levin (1975) described this frequency-dependent mating success in polyploid systems as the “minority cytotype disadvantage.” The minority disadvantage experienced by neopolyploids can be overcome in a number of ways, including (1) demographic stochasticity that drives neopolyploids from the minority to the majority cytotype, (2) reduced gene flow via increased self-fertilization or asexual reproduction, (3) migration to a new geographic region so as to eliminate gene flow with progenitors, and (4) the expression of ecological attributes in neopolyploids allowing them to coexist with their progenitors, or to replace them ( Ramsey and Schemske 2002). Unless one or more of these conditions are realized, a neopolyploid will become extinct despite the presence of strong postzygotic barriers.

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Turner, T. L., Hahn, M. W. & Nuzhdin, S. V. Genomic islands of speciation in Anopheles gambiae. PLoS Biology 3, e285 (2005). doi:10.1371/journal.pbio.0030285 Panhuis, T. M. et al. Sexual selection and speciation. Trends in Ecology & Evolution 16, 364–371 (2001). Gently pinch nostrils together, hold for a few seconds and then straighten head to the upright position. Wipe away any excess oil using a clean tissue.

To illustrate this point, consider two allopatric populations experiencing divergent selection. If divergent selection based on habitat differences is strong, habitat isolation will begin to evolve first as effective geographic isolation becomes ecogeographic isolation. Other forms of reproductive isolation, such as mating isolation or intrinsic postzygotic isolation will eventually evolve, and the potential strength of these components will increase with time. It is important to note that the relative contribution of these forms of isolation depends largely on the geographical arrangement of populations in the future. If populations remain allopatric, most barriers (with the exception of ecogeographic isolation) will not be realized in nature. If populations become sympatric in the future, some portion of the potential strength of other barriers could be realized, and their contribution to total reproductive isolation would increase. Functional evolution and development of novel feeding apparatus in parrotfish, pufferfish and other fishesAnadromous and freshwater G. aculeatus experience divergent selection for body size and assortative mating is based on this trait. Divergent pollinator-selected style lengths in Mimulus cardinalis and M. lewisii lead to differentiated pollen tube lengths, reducing the amount of expected hybridization in mixed pollinations.