The Causes of Evolutionary Radiations in Archipelagoes: Passerine Birds in the Lesser Antilles

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The relationship between species richness and the occupation of niche space can provide insight into the processes that shape patterns of biodiversity. For example, if species interactions constrained coexistence, one might expect tendencies toward even spacing within niche space and positive relationships between diversity and total niche volume. I use morphological diversity of passerine birds as a proxy for diet, foraging maneuvers, and foraging substrates and examine the morphological space occupied by regional and local passerine avifaunas. Although independently diversified regional faunas exhibit convergent morphology, species are clustered rather than evenly distributed, the volume of the morphological space is weakly related to number of species per taxonomic family, and morphological volume is unrelated to number of species within both regional avifaunas and local assemblages. These results seemingly contradict patterns expected when species interactions constrain regional or local diversity, and they suggest a larger role for diversification, extinction, and dispersal limitation in shaping species richness.community | niche packing | principal components analysis A lthough species richness varies predictably with latitude and other environmental gradients (1, 2), the mechanisms responsible for geographic patterns in biodiversity remain poorly understood (3, 4). Classic niche-based hypotheses argue that local species richness is related to the variety of available resources, including escape space from enemies (5, 6), which are partitioned among species to reduce interspecific competition and thereby allow coexistence (7-10). Modifications of nichebased theory incorporating, for example, spatial and temporal variation in the environment (11), relax this prediction but have not been assessed by empirical studies. More recent consumerresource models have addressed conditions for coexistence of similar species (e.g., refs. 12 and 13). These involve small numbers of species in simplified environments, although they do contain many basic elements of more complex natural systems. Alternatively, diversification-based hypotheses posit that elevated rates of species production compared with extinction within a region push up steady-state levels of diversity and pack species more tightly into available ecological space, tending to decouple the relationship between species richness and niche space (14-16). Ecologists have not evaluated these hypotheses adequately, in part because of the difficulty of measuring niche space and its partitioning among species (e.g., refs. 17 and 18), although the idea that diversity generally exists in a long-term, resource-influenced equilibrium has increasing support (19,20).Recent analyses of the phylogenetic structure of communities (21, 22) and the distribution of functional traits in communities (23-25) have attempted to address the roles of ecological specialization and species interactions in the assembly of local communities from regional species pools. These studies ask wh...

Section: Methodsmentioning

confidence: 99%

Species richness and morphological diversity of passerine birds

Ricklefs

2012

135

153

“…Many researchers have suggested that speciation might be limited primarily by factors associated with the persistence of incipient species (13)(14)(15). Models of ephemeral speciation propose that it is relatively easy for lineages to form incipient species, but the vast majority of new species do not persist over macroevolutionary timescales (15,19).…”

Section: Groupmentioning

confidence: 99%

“…This has long been recognized by the paleontological community, where "successful" speciation is believed to entail not only the evolution of reproductive isolation but also the persistence of incipient species (13)(14)(15). For example, speciation might be limited primarily by the rate at which lineages form allopatric isolates (6,16) or by the capacity for geographic range expansion (17,18).…”

mentioning

confidence: 99%

Macroevolutionary speciation rates are decoupled from the evolution of intrinsic reproductive isolation in Drosophila and birds

Rabosky

Matute

2013

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125

The rate at which speciation occurs varies greatly among different kinds of organisms and is frequently assumed to result from species-or clade-specific factors that influence the rate at which populations acquire reproductive isolation. This premise leads to a fundamental prediction that has never been tested: Organisms that quickly evolve prezygotic or postzygotic reproductive isolation should have faster rates of speciation than organisms that slowly acquire reproductive isolation. We combined phylogenetic estimates of speciation rates from Drosophila and birds with a method for analyzing interspecific hybridization data to test whether the rate at which individual lineages evolve reproductive isolation predicts their macroevolutionary rate of species formation. We find that some lineages evolve reproductive isolation much more quickly than others, but this variation is decoupled from rates of speciation as measured on phylogenetic trees. For the clades examined here, reproductive isolation-especially intrinsic, postzygotic isolation-does not seem to be the rate-limiting control on macroevolutionary diversification dynamics. These results suggest that factors associated with intrinsic reproductive isolation may have less to do with the tremendous variation in species diversity across the evolutionary tree of life than is generally assumed. A central challenge at the interface between macroevolution and microevolution is to explain the population-level processes that contribute to biological variation in diversification rates and species richness (1). Phylogenetic evidence for biological variation in the rate of species diversification is widespread (2, 3), and numerous studies have now linked specific traits to the dynamics of speciation and extinction as realized over macroevolutionary timescales (2,4). At the population level, a microevolutionary research program on the biology of speciation has focused on the factors that lead to various forms of reproductive isolation (RI) between populations (5, 6). Explaining how and why RI evolves is generally considered to be the central and defining challenge in the study of speciation (2, 7-9), and recent studies have made great progress toward explaining the genetic and ecological basis for various forms of RI (7,8,10).Most microevolutionary research on speciation implicitly assumes that RI is the defining and rate-limiting step in the speciation process (7,8,11), but the evolution of RI need not bear any predictive relationship to rates of species diversification as realized over macroevolutionary timescales (12). This has long been recognized by the paleontological community, where "successful" speciation is believed to entail not only the evolution of reproductive isolation but also the persistence of incipient species (13-15). For example, speciation might be limited primarily by the rate at which lineages form allopatric isolates (6,16) or by the capacity for geographic range expansion (17, 18). Likewise, speciation might be limited more by factors that influe...

“…Suppression of pathogens leading to high population productivity, saturation of niche space, and broad geographic distribution might increase opportunities for population subdivision and species formation. I have argued elsewhere that specialized pathogens might prevent secondary sympatry by sister taxa and reduce diversification within clades (105,106). Suppression of pathogens would reduce this impediment to diversification.…”

Section: )mentioning

confidence: 99%

Evolutionary diversification, coevolution between populations and their antagonists, and the filling of niche space

Ricklefs

2010

217

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The population component of a species' niche corresponds to the distribution of individuals across environments within a region. As evolutionary clades of species diversify, they presumably fill niche space, and, consequently, the rate of increase in species numbers slows. Total niche space and species numbers appear to be relatively stable over long periods, and so an increase in the species richness of one clade must be balanced by decrease in others. However, in several analyses, the total population niche space occupied per clade is independent of the number of species, suggesting that species in more diverse clades overlap more in niche space. This overlap appears to be accommodated by variation in the populations of each species, including their absence, within suitable niche space. I suggest that the uneven filling of niche space results from localized outcomes of the dynamic coevolutionary interactions of populations with their pathogens or other antagonists. Furthermore, I speculate that relationships with pathogens might constrain diversification if pathogen diversity increased with host diversity and resulted in more frequent host switching and emergent disease. Many indirect observations are consistent with these scenarios. However, the postulated influence of pathogens on the filling of niche space and diversification of clades primarily highlights our lack of knowledge concerning the space and time dimensions of coevolutionary interactions and their influence on population distribution and species diversification. mosaic evolution | niche breadth | pathogen E ach species occupies a part of the ecological space available in the environment, usually referred to as its "ecological niche." Although the niche has been difficult to define precisely, most treatments consider conditions of the physical environment, characteristics of resources-whether soil nutrients, plants, or animal prey-and the traits of other interacting species (i.e., competitors, mutualists, predators, and pathogens) as important axes of ecological space that are partitioned among species (1, 2). In this essay, I distinguish individual and population niche space, the former reflecting primarily evolutionary adaptations of individuals to exploit resources and the latter, primarily reflecting demographic processes maintaining populations at a particular place. Of course, the population component of the niche also is influenced by adaptations of individuals to the range of environmental conditions over space within a region.Both components of niche space can be defined for any particular species and also for a set of species, of which those comprising an evolutionary clade descended from a common ancestor are most relevant here. Adaptive radiations of species fill niche space by evolutionary diversification, yet both phylogenetic analyses (3, 4) and paleontological studies (5, 6) suggest that diversification is constrained and that individual clades typically enjoy a limited period of rapid radiation early in their existence. The number...