净多样化速率和进化时间对虎耳草目科间物种多样性差异的影响

不同生物类群包含的物种数目常存在巨大差异,这是生态学和生物学研究中普遍观察到的现象。然而,这一现象产生的原因仍然是未解之谜。从宏观进化的角度,进化时间假说和多样化速率假说是两个比较流行的假说。进化时间假说认为类群的演化时间越长,积累的物种丰富度越高;而多样化速率假说认为类群的净多样化速率越快,则其物种丰富度越高。为验证这两个假说,该文以一棵包含1 539个物种化石定年的虎耳草目系统发育树为基础,通过宏观进化分析获取了虎耳草目内15个科的物种形成和灭绝速率,并计算了每个科的平均多样化速率。结果表明：(1)虎耳草目的物种多样化速率有着增加的趋势,并且多样化速率的增加主要出现在温带和高山类群,如茶藨子科、景天科和芍药科等。(2)采用系统发育广义最小二乘模型(PGLS)和线性回归模型(LM)结果表明,虎耳草目15个科的物种丰富度与科的分化时间和科内物种的最近共同祖先年龄都没有显著相关关系,而与净多样化速率显著正相关(R² =0.380,P<0.05)。该研究支持了多样化速率假说,认为不同科的净多样化速率的差异是导致虎耳草目科间物种数目差异的主要原因之一。全球气候变冷...     

ESTIMATING DIVERSIFICATION RATES: HOW USEFUL ARE DIVERGENCE TIMES?

The dynamics of species diversification rates are a key component of macroevolutionary patterns. Although not absolutely necessary, the use of divergence times inferred from sequence data has led to development of more powerful methods for inferring diversification rates. However, it is unclear what impact uncertainty in age estimates have on diversification rate inferences. Here, we quantify these effects using both Bayesian and frequentist methodology. Through simulation, we demonstrate that adding sequence data results in more precise estimates of internal node ages, but a reasonable approximation of these node ages is often sufficient to approach the theoretical minimum variance in speciation rate estimates. We also find that even crude estimates of divergence times increase the power of tests of diversification rate differences between sister clades. Finally, because Bayesian and frequentist methods provided similar assessments of error, novel Bayesian approaches may provide a useful framework for tests of diversification rates in more complex contexts than are addressed here.     

EXPLOSIVE EVOLUTIONARY RADIATIONS: DECREASING SPECIATION OR INCREASING EXTINCTION THROUGH TIME?

A common pattern in time-calibrated molecular phylogenies is a signal of rapid diversification early in the history of a radiation. Because the net rate of diversification is the difference between speciation and extinction rates, such \"explosive-early\" diversification could result either from temporally declining speciation rates or from increasing extinction rates through time. Distinguishing between these alternatives is challenging but important, because these processes likely result from different ecological drivers of diversification. Here we develop a method for estimating speciation and extinction rates that vary continuously through time. By applying this approach to real phylogenies with explosive-early diversification and by modeling features of lineage-accumulation curves under both declining speciation and increasing extinction scenarios, we show that a signal of explosive-early diversification in phylogenies of extant taxa cannot result from increasing extinction and can only be explained by temporally declining speciation rates. Moreover, whenever extinction rates are high, \"explosive early\" patterns become unobservable, because high extinction quickly erases the signature of even large declines in speciation rates. Although extinction may obscure patterns of evolutionary diversification, these results show that decreasing speciation is often distinguishable from increasing extinction in the numerous molecular phylogenies of radiations that retain a preponderance of early lineages.     

Is evolution faster at ecotones? A test using rates and tempo of diet transitions in Neotropical Sigmodontinae (Rodentia,Cricetidae)

We evaluated whether evolution is faster at ecotones as niche shifts may be needed to persist under unstable environment. We mapped diet evolution along the evolutionary history of 350 sigmodontine species. Mapping was used in three new tip‐based metrics of trait evolution – Transition Rates, Stasis Time, and Last Transition Time – which were spatialized at the assemblage level (aTR, aST, aTL). Assemblages were obtained by superimposing range maps on points located at core and ecotone of the 93 South American ecoregions. Using Linear Mixed Models, we tested whether ecotones have species with more changes from the ancestral diet (higher aTR), have maintained the current diet for a shorter time (lower aST), and have more recent transitions to the current diet (lower aLT) than cores. We found lower aTR, and higher aST and aLT at ecotones than at cores. Although ecotones are more heterogeneous, both environmentally and in relation to selection pressures they exert on organisms, ecotone species change little from the ancestral diet as generalist habits are necessary toward feeding in ephemeral environments. The need to incorporate phylogenetic uncertainty in tip‐based metrics was evident from large uncertainty detected. Our study integrates ecology and evolution by analyzing how fast trait evolution is across space.     

Diversity dynamics of Miocene mammals in relation to the history of tectonism and climate

Continental biodiversity gradients result not only from ecological processes, but also from evolutionary and geohistorical processes involving biotic turnover in landscape and climatic history over millions of years. Here, we investigate the evolutionary and historical contributions to the gradient of increasing species richness with topographic complexity. We analysed a dataset of 418 fossil rodent species from western North America spanning 25 to 5 Ma. We compared diversification histories between tectonically active (Intermontane West) and quiescent (Great Plains) regions. Although diversification histories differed between the two regions, species richness, origination rate and extinction rate per million years were not systematically different over the 20 Myr interval. In the tectonically active region, the greatest increase in originations coincided with a Middle Miocene episode of intensified tectonic activity and global warming. During subsequent global cooling, species richness declined in the montane region and increased on the Great Plains. These results suggest that interactions between tectonic activity and climate change stimulate diversification in mammals. The elevational diversity gradient characteristic of modern mammalian faunas was not a persistent feature over geologic time. Rather, the Miocene rodent record suggests that the elevational diversity gradient is a transient feature arising during particular episodes of Earth''s history.     

Diversification rates in a temperate legume clade: Are there “so many species” of Astragalus (Fabaceae)?

Astragalus, the largest genus of flowering plants, contains upwards of 2500 species. Explanations for this exceptional species diversity have pointed to unusual population structure or modes of speciation. Surprisingly, however, three different statistical analyses indicate that diversification rates in Astragalus are not exceptionally high compared to its closest relatives. Instead, rates are high throughout the “Astragalean clade,” a much broader radiation distributed throughout the temperate zone. The increase in diversification rate is associated with the origin and divergence of this clade from common ancestors of it and several much less diverse and more narrowly distributed Asian genera. This suggests that causal factors in the shift toward higher rates of diversification must be due not to factors unique to Astragalus, but to characteristics common to the entire Astragalean clade. However, this larger clade has never been circumscribed in classifications based on morphological data. This raises the possibility that the causes of increased diversification may not be due to morphological innovation, but may instead be related to ecological factors or cryptic physiological or biochemical features.     

Trait-dependent diversification and the impact of palaeontological data on evolutionary hypothesis testing in New World ratsnakes (tribe Lampropeltini)

For studies investigating trait evolution, there are at least two important questions. First, have traits under consideration influenced cladogenesis and extinction in the group? Second, how do fossil data alter inferences about trait evolution or diversification‐rate dynamics? However, relatively few studies have assessed these questions. Here, we use recently developed methods to test for trait‐dependent diversification in the New World colubrid snake tribe Lampropeltini. We also integrate data from fossil taxa into phylogenetic estimation of evolutionary parameters using a simple Monte Carlo randomization test. These analyses suggest that ecological conditions in temperate regions are tied to higher rates of cladogenesis, but that body size is not related to diversification in the group. We also find that the inclusion of fossil taxa alters absolute estimates of size and the rate of size evolution, but not the overall pattern of ecomorphological diversification, as well as estimates of evolutionary rates, particularly extinction.