A comparison of sexual selection versus random selection with respect to extinction and speciation rates using individual based modeling and machine learning

Sexual selection is thought to shape phylogenetic diversity by affecting speciation or extinction rates. However, the net effect of sexual selection on diversification is hard to predict because many of the hypothesized effects on speciation or extinction have opposing signs and uncertain magnitudes. Theoretical work also suggests that the net effect of sexual selection on diversification should depend strongly on ecological factors, though this prediction has seldom been tested. Here, we test whether variation in sexual selection can predict speciation and extinction rates across passerine birds (up to 5812 species, covering most genera) and whether this relationship is mediated by environmental factors. Male-biased sexual selection, and specifically sexual size dimorphism, predicted two of the three measures of speciation rates that we examined. The link we observed between sexual selection and speciation was independent of environmental variability, though species with smaller ranges had higher speciation rates. There was no association between any proxies of sexual selection and extinction rate. Our findings support the view that male-biased sexual selection, as measured by frequent predictors of male-male competition, has shaped diversification in the largest radiation ofbirds.

Is sexual selection beneficial during adaptation to environmental change?

Are polyploids really evolutionary dead-ends (again)? A critical reappraisal of Mayrose etal. ().

At present there are many animal phyla that contain only a few species. The existence of these small phyla can be used to test assumptions about speciation and extinction in multicellular animals.We first model the number of species in a monophyletic clade with a birth and death process that assumes rates of speciation and extinction are constant. If no new phyla have evolved since the Cambrian and speciation and extinction rates for minor phyla are similar to or greater than those estimated from fossils, then our model shows that the probabilities of minor phyla surviving to the present are small. Random variation in extinction and speciation rates does not improve the chances for persistence. If speciation rates exceed extinction rates at the initial radiation of the clade, but before the clade becomes large, speciation rates come to equal extinction rates and both are low, persistence from before the Ordovician up to the present becomes likely. These models show that if speciation and extinction rates are independent of the number of species in a clade, then conditions before the Ordovician strongly influence today's distribution of species among taxa.We also discuss a model in which speciation and extinction rates depend on the number of species in a clade. This alternative model can account for the persistence of phyla with few species to the present and predicts a short duration for phyla that did not exceed a threshold number of species.

Dictated by limited resource availability for land acquisition, a central question in conservation biology is the ability of areas of different size to maintain species diversity. The selected reserves should not only be species rich at the moment, but should also maintain species diversity in the long run. We used two sets of data on vascular plant species in boreal lakes collected in 1933/34 and 1996 to test the relationships between lake area and the extinction, immigration and turnover rates of the species. Moreover, we investigated, whether the number of species in 1933/34 or water connection between lakes was related to extinction, immigration and turnover rates of species. We found that lake area or shoreline length was not correlated with immigration or turnover rate. But extinction rate was slightly negatively correlated with shoreline length. The original number of species was positively related to the number of species extinctions and to the absolute turnover rate in the lakes, which indicates that species richness does not create stability in these communities. Species number was not correlated with immigration rate. Upstream water connections in the lakes did not affect immigration, extinction or turnover rates. We conclude that length of the shoreline is a better measure of suitable area for water plants than the lake area, and that because the correlation between shoreline length and extinction rate was slight, also small lakes can be valuable for conservation.

Most empirical studies support a decline in speciation rates through time, although evidence for constant speciation rates also exists. Declining rates have been explained by invoking niche-filling processes, whereas constant rates have been attributed to non-adaptive processes such as sexual selection, mutation, and dispersal. Trends in speciation rate and the processes underlying it remain unclear, representing a critical information gap in understanding patterns of global diversity. Here we show that the speciation rate is driven by frequency dependent selection. We used a frequency-dependent and DNA sequence-based model of populations and genetic-distance-based speciation, in the absence of adaptation to ecological niches. We tested the frequency-dependent selection mechanism using cichlid fish and Darwin's finches, two classic model systems for which speciation rates and richness data exist. Using negative frequency dependent selection, our model both predicts the declining speciation rate found in cichlid fish and explains their species richness. For groups like the Darwin's finches, in which speciation rates are constant and diversity is lower, the speciation rate is better explained by a model without frequency-dependent selection. Our analysis shows that differences in diversity are driven by larger incipient species abundance (and consequent lower extinction rates) with frequency-dependent selection. These results demonstrate that mutations, genetic-distance-based speciation, sexual and frequency-dependent selection are sufficient not only for promoting rapid proliferation of new species, but also for maintaining the high diversity observed in natural systems.

The integration of Geographic Information System (GIS) methodology within a phylogenetic and statistical framework provides a background against which to evaluate the relationship between biogeographic changes and evolution in the fossil record. A case study based on patterns in Middle and Late Devonian phyllocarids (Crustacea) illustrates the usefulness of this integrated approach. Using a combined approach enhances determination of rates of biodiversity change and the relationship between biogeographic and evolutionary changes. Because the interaction between speciation and extinction rates fundamentally determines biodiversity dynamics, and speciation and extinction rates are influenced by the geographic ranges of component taxa, the relationship between biogeography and evolution is important. Furthermore, GIS makes it possible to quantify paleobiogeographic ranges.Phylogenetic biogeography resolved patterns of both vicariance and geodispersal and revealed that range expansions were more abundant than range contractions in Devonian phyllocarids. In addition, statistical tests on GIS-constrained species ranges and evolutionary-rate data revealed a relationship between increasing species' ranges and increases in both speciation and extinction rates. Extinction rate, however, increased more rapidly than speciation rate in the phyllocarids. The pattern of extinction rate increasing faster than speciation rate in the phyllocarids may illuminate aspects of the Late Devonian biodiversity crisis in particular, and protracted biodiversity crises in general.

GLOBAL VARIATION IN THE DIVERSIFICATION RATE OF PASSERINE BIRDS

Species diversity patterns are governed by complex interactions among biotic and abiotic factors over time and space, but are essentially the result of the diversification dynamics (differential speciation and extinction rates) over the long‐term evolutionary history of a clade. Previous studies have suggested that temporal variation in global temperature drove long‐term diversity changes in Crocodylia, a monophyletic group of large ectothermic organisms. We use a large database of crocodylian fossil occurrences (192 spp.) and body mass estimations, under a taxic approach, to characterize the global diversification dynamics of crocodylians since the Cretaceous, and their correlation with multiple biotic and abiotic factors in a Bayesian framework. The diversification dynamic of crocodylians, which appears to have originated in the Turonian (c. 92.5 Ma), is characterized by several phases with high extinction and speciation rates within a predominantly low long‐term mean rate. Our results reveal long‐term diversification dynamics of Crocodylia to be a highly complex process driven by a combination of biotic and abiotic factors which influenced the speciation and extinction rates in dissimilar ways. Higher crocodylian extinction rates are related to low body mass disparity, indicating selective extinctions of taxa at both ends of the body mass spectrum. Speciation rate slowdowns are noted when the diversity of the clade is high and the warm temperate climatic belt is reduced. Our finding supports the idea that temporal variations of body mass disparity, self‐diversity, and the warm climate belt size provided more direct mechanistic explanations for crocodylian diversification than do proxies of global temperature.

The birth-death model is commonly used to infer speciation and extinction rates by fitting the model to phylogenetic trees with exclusively extant taxa. Recently, it was demonstrated that speciation and extinction rates are not identifiable if the rates are allowed to vary freely over time. The group of birth-death models that have the same likelihood is called a congruence class, and there is no statistical evidence to favor one model over the other. This issue has led researchers to question if and what patterns can reliably be inferred from phylogenies of only extant taxa and whether time-variable birth-death models should be fitted at all. We explore the congruence class in the context of several empirical phylogenies as well as hypothetical scenarios. For these empirical phylogenies, we assume that we inferred the true congruence class. Thus, our conclusions apply to any empirical phylogeny for which we robustly inferred the true congruence class. When we summarize shared patterns in the congruence class, we show that strong directional trends in speciation and extinction rates are shared among most models. Therefore, we conclude that the inference of strong directional trends is robust. Conversely, estimates of constant rates or gentle slopes are not robust and must be treated with caution. Interestingly, the space of valid speciation rates is narrower and more limited in contrast to extinction rates, which are less constrained. These results provide further evidence and insights that speciation rates can be estimated more reliably than extinction rates.

One of the most striking patterns observed among animals is that smaller-bodied taxa are generally much more diverse than larger-bodied taxa. This observation seems to be explained by the mere fact that smaller-bodied taxa tend to have an older evolutionary origin and have therefore had more time to diversify. A few studies, based on the prevailing null model of diversification (i.e. the stochastic constant-rate birth–death model), have suggested that this is indeed the correct explanation, and body-size dependence of speciation and extinction rates does not play a role. However, there are several potential shortcomings to these studies: a suboptimal statistical procedure and a relatively narrow range of body sizes in the analysed data. Here, we present a more coherent statistical approach, maximizing the likelihood of the constant-rate birth–death model with allometric scaling of speciation and extinction rates, given data on extant diversity, clade age and average body size in each clade. We applied our method to a dataset compiled from the literature that includes a wide range of Metazoan taxa (range from midges to elephants). We find that the higher diversity among small animals is indeed, partly, caused by higher clade age. However, it is also partly caused by the body-size dependence of speciation and extinction rates. We find that both the speciation rate and extinction rate decrease with body size such that the net diversification rate is close to 0. Even more interestingly, the allometric scaling exponent of speciation and extinction rates is approximately −0.25, which implies that the per generation speciation and extinction rates are independent of body size. This suggests that the observed relationship between diversity and body size pattern can be explained by clade age alone, but only if clade age is measured in generations rather than years. Thus, we argue that the most parsimonious explanation for the observation that smaller-bodied taxa are more diverse is that their evolutionary clock ticks faster.

Hutchinson and MacArthur (1959) argued that the number of available habitats and thus the number of species should decrease with body size. May (1978) presented an estimate of species number as a function of body length for all terrestrial animals, and he interpreted his data to be more or less in accord with the prediction of Hutchinson and MacArthur (1959; see also Morse et al. 1985). However, recent analysis showed that the smallest organisms are not the most diverse and that global body size distributions among species are humped and right-skewed even on a logscale (Brown and Nicoletto 1991, Blackburn and Gaston 1994a, Barlow 1994; Fig. 1). If a variety of factors acts in a multiplicative way then a lognormal distribution would be expected (May 1975). Thus, the central issue is, why body size distributions deviate from a lognormal shape and show a considerable right-skew (for a review see Blackburn and Gaston 1994b). Based on an energetic definition of fitness, Brown et al. (1993) developed a model which predicts not only the right-skewed shape of the frequency distribution but also an optimal body size. This model simplifies the physiological processes of reproduction and uses scaling functions derived for mammals. It was successful in predicting the optimal body size of mammals and the body size shifts of mammals on islands. However, predictions for other taxa remain obscure because it is unclear whether the scaling functions are valid for other taxa. Another approach to predict a right-skew of body size distributions applies size-biased extinction and speciation rates (Dial and Marzluff 1988, Maurer et al. 1992). There is a complex and scattered literature on speciation rates in correlation to body size (e.g. Bush 1993, Fenchel 1993) with the general conclusion that speciation rates decrease with body size. Thus, the higher speciation rates of small animal species can generate a right-skewed pattern. Maurer et al. (1992), however, conclude that a right-skewed distribution needs size-biased speciation and extinction rates. Until now the relationship between extinction risk and body size is equivocal. In general, population persistence is more likely when fluctuations in numbers are small and the recovery from low numbers is fast. Lawton (1995) notes that these factors can be correlated with body size but not necessarily in ways that act consistently to either promote or reduce the risk of extinction. Pimm et al. (1988) argued that large-bodied organisms are at greater disadvantage in a stochastic environment due to their small growth rates and thus long recovery times from population crashes. This generates a positive relationship between extinction risk and body size (see also Brown and Maurer 1986, Lawton 1989, Blackburn et al. 1990, Gaston and Blackburn 1995). However, Cook and Hanski (1995) found an opposite pattern in shrews. They argued that smallbodied species are more sensitive to environmental fluctuations than large-bodied species (see also Tracy and George 1992). Thus, small-bodied species fluctuate more likely to extinction. Furthermore, in a stable environment large organisms are favoured because they may achieve dominance over resources (Dial and Marzluff 1988). The above discussion shows that the relationship between body size and species' vulnerability to extinction is poorly understood and a unifying approach is badly needed. Using a simulation model we explore how extinction risk of populations in a stochastic environment may depend on body size. We distinguish between two different types of environmental perturbations: fluctuations (frequent, weak perturbations) and catastrophes (rare, strong perturbations). We hypothesize that species respond to environmental fluctuations along different timescales and with different sensitivities, both assumed to be correlated with body size. Catastrophes, however, are special strong perturbations that cause sudden major declines of the population size of all species independent of their biological characteristics and thus independent of body size. We will show that these different types of perturbations translate into a relationship of extinction risk versus body size, which is U-shaped with a skew depending on the actual environmental perturbations and the biological traits of the species. Therefore, even under the assumption of a

The taxonomic evolution of Jurassic and Cretaceous calcareous nannofossil species is described using the following indices: species diversity, rate of speciation, rate of extinction, rate of diversification, rate of turnover, survivorship, and species accretion. The Jurassic prior to the late Oxfordian is characterized by positive diversification rates, that is, rates of speciation exceeded rates of extinction. Highest rates of diversification occurred in the late Lias and early Oxfordian. During the generally regressive latest Jurassic, diversification rates remained low and rates of extinctions exceed rates of speciation. In the early Cretaceous, rates of diversification are positive and peak in the early Valanginian, early Aptian, and middle Albian, after which time rates of extinction generally exceed rates of speciation. Such peaks in rate of evolution coincide with times of increased accumulation of organic carbon in the ocean (“anoxic events”). Peaks in rates of extinction result in very high rates of turnover during times of major regressions, in particular, in the Tithonian and Maastrichtian. Survivorship analyses for three datum planes (74.5, 144, and 160 Ma) show relatively constant extinction rates with some stepping in the older part; they are best explained by a temporally fluctuating abiotic environment causing changes in the probability of extinction. Species accretion curves are also relatively linear with some indication of changing rates of speciation. The coincidences of major changes in evolutionary rates with major paleoceanographic events are indicative of a predominantly abiotic control of nannoplankton evolution. Relationships of evolutionary rates of calcareous nannoplankton with deep ocean ventilation, sea level, and ocean fertility indicates that global tectonic processes are the ultimate causes of evolutionary change.

Rapid diversification is common among herbivorous insects and is often the result of host shifts, leading to the exploitation of novel food sources. This, in turn, is associated with adaptive evolution of female oviposition behavior and larval feeding biology. Although natural selection is the typical driver of such adaptation, the role of sexual selection is less clear. In theory, sexual selection can either accelerate or impede adaptation. To assess the independent effects of natural and sexual selection on the rate of adaptation, we performed a laboratory natural selection experiment in a herbivorous bruchid beetle (Callosobruchus maculatus). We established replicated selection lines where we varied natural (food type) and sexual (mating system) selection in a 2 x 2 orthogonal design, and propagated our lines for 35 generations. In half of the lines, we induced a host shift whereas the other half was kept on the ancestral host. We experimentally enforced monogamy in half of the lines, whereas the other half remained polygamous. The beetles rapidly adapted to the novel host, which primarily involved increased host acceptance by females and an accelerated rate of larval development. We also found that our mating system treatment affected the rate of adaptation, but that this effect was contingent upon food type. As beetles adapted to the novel host, sexual selection reinforced natural selection whereas populations residing close to their adaptive peak (i.e., those using their ancestral host) exhibited higher fitness in the absence of sexual selection. We discuss our findings in light of current sexual selection theory and suggest that the net evolutionary effect of reproductive competition may critically depend on natural selection. Sexual selection may commonly accelerate adaptation under directional natural selection whereas sexual selection, and the associated load brought by sexual conflict, may tend to depress population fitness under stabilizing natural selection.

There is a growing effort to understand how sexual selection varies over space and time under different ecological conditions and how this can maintain variation in sexual traits. An interesting ecological condition is population density, which can either increase or decrease sexual selection depending on the natural history of the species. We examined sexual and natural selection in an undescribed amphipod species (Hyalella sp.) using surveys of two natural populations that increase in density from spring to summer. We also conducted an experiment that directly manipulated density to assess the effects on sexual selection. In the field during spring (low density) and summer (high density), we documented sexual selection on male traits, including body size, gnathopods and antennae. We found that the magnitude and direction of this selection changed from spring to summer and that selection patterns differed between populations. In the experiment, we found no sexual selection occurred at low density, but found significant positive selection on all three male traits at medium and high densities. These results underline the importance of isolating individual ecological factors to determine their role in sexual selection while also documenting patterns in natural populations to understand how selection varies over space and time.