Ecological Speciation (Oxford Series in Ecology and Evolution)

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Given that most speciation events start with an allopatric phase ( Coyne and Orr 2004), the first reproductive barrier that probably arises in many systems involves adaptation to different habitats. Therefore, without some estimate of ecogeographic isolation, estimates of reproductive isolation will be at best incomplete, and at worst misleading. For example, consider the influential papers on Drosophila by Coyne and Orr (1989, 1997). They collected data from the literature for two components of reproductive isolation, sexual isolation and intrinsic postzygotic isolation. While these barriers are amenable to laboratory measurement, are they relevant to speciation? For the approximately 50% of species pairs in their study that are sympatric, these barriers might contribute to reproductive isolation but they cannot now be regarded as contributing to reproductive isolation in the remaining allopatric species pairs, although this potential isolation could be realized if species become sympatric in the future. Estimating the level of ecogeographic isolation and calculating its relative strength to the forms already measured might change our view of how speciation actually occurs in this important system. 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. Mallet, J. et al. Space, sympatry and speciation. Journal of Evolutionary Biology 22, 2332–2341 (2009). The strength of divergent selection experienced by the allopatric populations could affect which forms of isolation arise and at what rate. For example, if habitats in allopatry are very different, divergent selection is likely to be strong, and ecogeographic isolation will likely arise first as presented above. However, if selection is only weakly divergent or uniform, ecogeographic isolation may be much slower to arise, and other forms of isolation may outpace its evolution. We consider speciation to be ecological when externally imposed selection results in reproductive isolation; therefore, speciation by drift is nonecological. Although factors that influence the magnitude of genetic drift, such as variation in population size or mating success, may often have an ecological basis, this does not imply that speciation mediated by genetic drift is also “ecological.”

Genomic Island: A region of the genome where differentiation between populations is stronger than expected in the absence of divergent selection (stronger than occurs via purely neutral processes such as genetic drift alone). Coyne and Orr (2004) defined reproductive isolation that is favored by selection (i.e., reinforcement) as “direct,” whereas all other forms of isolation arise as “indirect” outcomes of selection. We prefer to use “reinforcement” instead of “direct” and to use “byproduct” when selection does not favor reproductive isolation per se. Within byproduct, direct isolation arises when the trait conferring reproductive isolation is the target of selection, and indirect isolation arises by pleiotropy or linkage. Although the biological mechanisms described by our system are equivalent to the terminology proposed by Coyne and Orr (2004), we feel that our usage of direct and indirect is more intuitive as it is analogous to the way these terms are used in traditional selection experiments. these forms may still be only ... varieties; but we have only to suppose the steps of modification to be more numerous or greater in amount, to convert these forms into species ... thus species are multiplied" (Darwin 1859, p. 120).

Hybrids between divergently migrating populations of Sylvia atricapilla exhibit unfit intermediate migration patterns.

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). No one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and varieties, if he make due allowance for our profound ignorance in regard to the mutual relations of the many beings which live around us.” If you have a plaster of paris (POP) splint on your nose, you should soak some cotton wool in the solution and sniff this. It will avoid getting the splint wet. What if I have been prescribed nasal drops or sprays? 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. 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.If polyploid speciation is nonecological, then postzygotic barriers should be of primary importance. If polyploid speciation is ecological, we expect to find a mix of prezygotic and postzygotic barriers, the latter caused by both extrinsic and intrinsic factors. Testing these alternatives requires estimates of the magnitude of reproductive isolation between neopolyploids and their progenitors for pre- and postzygotic factors. To our knowledge, few such studies are available. Husband and Sabara (2004) examined multiple barriers contributing to reproductive isolation between diploid and tetraploid fireweed ( Chamerion angustifolium, Onagraceae), and found that total isolation between cytotypes was 99.7%. Despite poor seed set in intercytotype crosses and low triploid fertility, prezygotic barriers such as geographic and pollinator isolation accounted for 97.6% of the total reproductive isolation. Thus, in this system, postzygotic factors presently contribute very little to reproductive isolation, but it is not known if postzygotic isolation was of primary importance in the early stages of polyploid establishment. If so, the prezygotic barriers now in place might have evolved because of selection to reduce hybrid formation, that is, reinforcement. This too is essentially unexplored. Divergent natural selection causes low fitness in G. aculeatus benthic–limnetic hybrids, despite the absence of intrinsic postzygotic isolation. Schluter (2001) suggested that “… until recently, neither was there evidence to support ecological speciation, nor had tests been devised to distinguish ecological speciation from other mechanisms that might cause speciation in the wild, such as genetic drift” (Box 1 in Schluter 2001). Rundle and Nosil (2005, p. 336) suggest that the renewed focus on ecological speciation has developed alongside recent efforts for “… a reclassification of speciation models from a scheme of geography (i.e., sympatric vs. allopatric), to one that focuses on mechanisms for the evolution of reproductive isolation …” A DNA-based assessment of Eurasian otter, Lutra lutra, numbers and seasonal movement in the Peak District, UK.

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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. Inland and coastal forms of M. guttatus experience selection for different growth and flowering times resulting in flowering phenology with little overlap. Sexual selection: Differential reproductive success of classes of entities (such as alleles) which differ in some characteristic(s). Whether we view polyploid speciation as ecological or nonecological is largely a semantic argument. If we recognize a polyploid as a new species at the time of its origin, regardless of its ability to persist, then polyploid speciation is nonecological. If instead, polyploids are regarded as new species only if they can establish a self-sustaining population that is reproductively isolated from its progenitor, then polyploid speciation is often ecological. SPECIATION BY GENETIC DRIFT

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 majority of the factors that we have to discuss are environmental, and we might therefore speak of an “ecology of speciation.” However, since we have to include the internal factors (mutability), as well as factors that involve behavior patterns, such as crossability, sexual isolation, pair formation, and the like, it might be preferable to use the broader term, biology. ( Mayr 1942, Chapter IX—The Biology of Speciation, p. 216). virtually all barriers can be considered ecological in the sense that they arise from environmentally imposed selection. ( Coyne and Orr 2004, p. 179) Mimicry in H. cydno and H. melpomene is disrupted in hybrids, causing increased predation on hybrids and reduced mating success. Even among biologists who believe speciation is sometimes nonecological, there is broad agreement that adaptation plays a significant role in most cases ( Templeton 2008). Nevertheless, the mechanisms whereby genetic changes resulting from adaptation contribute to the evolution of reproductive isolation remain poorly understood. Consider the many ways in which even a single adaptive mutation may affect multiple components of reproductive isolation. For example, adaptation to different habitats may lead to the restriction of gene flow and a gene contributing to this divergence may have pleiotropic effects on other traits, and these may cause more isolation. This leads to the question, how do the genetic and phenotypic changes required for speciation arise, and how do they cause complete reproductive isolation?Flo Nozoil can be used in pregnancy and while breastfeeding. Can Flo Nozoil be used with other nasal medications? The primary difficulty lies in the potentially complex relationship between the niche occupied by species, the geographic landscape over which the appropriate ecological conditions exist, and dispersal limitation ( Crispo et al. 2006; Nosil 2008; Price 2008). Geographic ranges are the product of both ecological and historical factors ( Endler 1982; Coyne and Orr 2004; Thorpe et al. 2008), and most speciation events probably begin with an allopatric phase ( Coyne and Orr 2004). Therefore, one cannot assume that all geographic isolation is based upon biological differences between taxa. Dobzhansky (1937, p. 231) recognized this problem in discussing the role of geography in speciation, saying, “… Geographical isolation is therefore on a different plane from any kind of physiological one. This consideration has to be qualified, because the occupation of separate areas by two species may be due not only to the fact that they have developed there, but also to the presence of physiological characteristics that make each species attached to the environment… .” Clearly Dobzhansky appreciated that historical processes leading to allopatry will often give way to ecological processes as populations adapt to different habitats.