Ecological Speciation (Oxford Series in Ecology and Evolution)

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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). A DNA-based assessment of Eurasian otter, Lutra lutra, numbers and seasonal movement in the Peak District, UK.

Nosil, P., Funk, D. J. & Ortíz-Barrientos, D. Divergent selection and heterogeneous genomic divergence. Molecular Ecology 18, 375–402 (2009). 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. Panhuis, T. M. et al. Sexual selection and speciation. Trends in Ecology & Evolution 16, 364–371 (2001). Divergent artificial selection in laboratory populations of D. serrata results in assortative mating.

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We propose that a more fruitful approach is to first identify the isolating barriers that contribute to speciation regardless of their mode of origin, and to then investigate how they evolved. We see two major opportunities. First, despite the historical focus on the geography of speciation, there is presently little appreciation of how a genetically based difference in geographic distribution can itself be a form of reproductive isolation. Second, although many workers recognize the need to study the forms of reproductive isolation that are most “important” to speciation, there are many ways to define importance. To evaluate importance, a method is needed for evaluating the contributions of individual barriers to total reproductive isolation. Geographic Isolation: A Neglected Isolating Barrier 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 The approach of Lowry et al. (2008a) does not eliminate the issue of nonindependence because any correlation in barrier strength for different barriers will still affect the mean. Is it necessary to consider nonindependence when estimating the relative importance of different barriers? When one trait causes multiple forms of isolation, statistical nonindependence does not occur unless the expression of one form of isolation affects the outcome of other forms. Therefore, multiplicative combination of barriers may often be suitable. There may be cases in which the strength of one barrier is moderated by the presence of another. For example, two taxa that reside at different altitudes may be primarily isolated by ecogeography. The timing of reproduction may also differ, but this could be due only to differences in the growing season between the two habitats, and is thus an indirect pleiotropic effect of the genes that contribute to ecogeographic isolation. In these cases, including temporal isolation in the linear sequence of independent barriers may be inappropriate, but it is not known how often this type of nonindependence occurs in nature.

The ecological speciation perspective has rekindled interest in the critical role of ecological factors in speciation, so in this sense it has been extremely valuable. However, natural selection and ecological factors have been at the center of discussions about speciation mechanisms for many decades. Consider the classic studies of Dobzhansky and colleagues on mechanisms of reproductive isolation in Drosophila. Dobzhansky (1951) concluded that gene flow between Drosophila pseudoobscura and D. persimilis was prevented by at least seven different isolating mechanisms, including such ecological factors as differences in habitat, preferred foods, and activity periods. Hiesey et al. (1971) conducted landmark studies on two closely related species of monkeyflowers, Mimulus cardinalis and M. lewisii, and through extensive reciprocal transplant experiments, crossing studies, and physiological observations demonstrated unmistakably that ecology and natural selection were the major factors contributing to speciation. In birds, the extraordinary radiation of Hawaiian honeycreepers from a single common ancestor, with species differentially adapted for feeding on nectar, fruits, seeds, or insects ( Amadon 1950), must surely represent an irrefutable example of divergent natural selection as a major cause of reproductive isolation. The importance of different isolating barriers to speciation remains a topic of considerable debate ( Coyne and Orr 2004). How do we estimate total reproductive isolation and which barriers do we include? For example, there is substantial disagreement about how one should measure and interpret isolation that results from differences in geographic distribution. Are genetically based differences in distribution legitimate isolating barriers, as suggested by Schemske (2000), or does the difficulty in distinguishing historical factors from geographic adaptation require that studies of speciation be restricted to sympatric taxa, as suggested by Coyne and Orr (2004)? Can we study speciation in allopatric taxa, and if so, how? Butlin, R. K., Galindo, J. & Grahame, J. W. Sympatric, parapatric or allopatric: The most important way to classify speciation? Philosophical Transactions of the Royal Society B: Biological Sciences 363, 2997–3007 (2008). While drift may not commonly result in speciation unilaterally, Templeton (2008) argues that it is erroneous to consider speciation a binary drift/selection process. Rather, drift and selection could work simultaneously and/or interact during divergence. A potential example involves the role of chromosomal rearrangements in reproductive isolation. Chromosomal rearrangements, such as inversions, can have an impact on divergence by creating linkage groups of loci involved in multiple forms of reproductive isolation that cannot be disrupted by recombination ( Noor et al. 2001; Rieseberg 2001). Empirical evidence suggests that such chromosomal inversions may contribute to the maintenance of species boundaries despite interspecific gene flow (e.g., Brown et al. 2004). Chromosomal inversions may sometimes be fixed by genetic drift, but the loci within the inversions may be subject to selection. Reproductive isolation could therefore be a product of both the adaptive loci within the inversion and the inversion itself, resulting in speciation that cannot be unambiguously defined as either ecological or nonecological. SEXUAL SELECTION AND SEXUAL CONFLICT Consider the distinction between ephemeral and permanent reproductive barriers. Although the linear sequential model provides guidance for evaluating the importance of contemporary barriers, later acting barriers may deserve attention due to their increased potential for permanence.

Ritchie, M. G. Sexual selection and speciation. Annual Review of Ecology, Evolution, and Systematics 38, 79–102 (2007). Flo Nozoil can be used in pregnancy and while breastfeeding. Can Flo Nozoil be used with other nasal medications?

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. We advocate an approach to the study of speciation first envisioned by Dobzhansky and Mayr in which reproductive isolation is the primary focus. Although these early leaders of our field would have almost certainly embraced new molecular and computational approaches for the study of speciation, the conceptual framework they established is still applicable today. We continue to seek answers to fundamental questions such as: Which forms of reproductive isolation are responsible for speciation? What traits and selective forces are involved? and What is the genetic basis of reproductive isolation? Evolutionary genetics of the phenotypic response to environmental change in the North American red squirrel Nosil, P., Crespi, B. J. & Sandoval, C. P. Host-plant adaptation drives the parallel evolution of reproductive isolation. Nature 417, 440–443 (2002). 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.There is abundant evidence in plants that crosses between ploidy levels are less successful than crosses within ploidy levels, due in large part to a mismatch between the ploidy of the developing embryo and the ploidy of the endosperm ( Ramsey and Schemske 1998). The fertility of progeny produced from between-ploidy crosses is also much reduced due to a high frequency of chromosomal duplications and deficiencies that render gametes inviable ( Ramsey and Schemske 1998; Husband and Sabara 2004). Furthermore, these reproductive barriers are present immediately following hybrid formation; hence, it is reasonable to conclude that such postzygotic isolation might cause speciation without ecological divergence. Poslední představení slibuje nevídaný pohled na lidi, příběhy i tenisky, které stojí za vznikem globálního fenoménu Jumpmana. Seriál je dostupný ve Spojených státech a také mezinárodně na Netflixu.