International Journal of Molecular Evolution and Biodiversity 2024, Vol.14, No.4, 147-161 http://ecoevopublisher.com/index.php/ijmeb 153 7 Case Analysis: Evolutionary Radiation of Mammals 7.1 Phylogenetic relationships and divergence The phylogenetic relationships within mammals have been extensively studied using both molecular and fossil data. Recent advancements in phylogenetic methods, particularly those that do not rely solely on fossils, have significantly enhanced our understanding of mammalian evolution. For instance, a comprehensive phylogeny of rodents, the most diversified mammalian clade, was constructed using a molecular supermatrix of 11 mitochondrial and nuclear genes covering 1 265 species. This study revealed distinct diversification patterns among different rodent clades, such as Myomorpha and Sciuroidea, highlighting the complexity of mammalian phylogenetic relationships (Fabre et al., 2012). Additionally, the use of large comparative sequence data sets has provided robust phylogenetic trees for mammals. For example, Upham et al. (2019) developed a robust evolutionary timescale for all approximately 6,000 extant mammal species by creating a set of reliable trees that capture root-to-tip uncertainties in both topology and divergence times. Their “backbone-and-patch” tree-building approach applied a newly assembled supermatrix of 31 genes to two levels of Bayesian inference (Figure 3). Fossil evidence, combined with molecular data, has been crucial in estimating divergence times within mammals. Molecular phylogenetic analyses calibrated with fossils have provided a time frame for the mammalian radiation, supporting the long-fuse model of diversification. This model suggests that the Cretaceous Terrestrial Revolution and the Cretaceous-Paleogene (KPg) mass extinction played significant roles in opening up ecological niches, thereby promoting mammalian diversification (Meredith et al., 2011). Moreover, the use of relaxed molecular clocks has allowed for more accurate estimates of divergence times. For instance, a study on rodent diversification employed a relaxed molecular clock dating approach, which provided a time framework for speciation events and identified shifts in diversification rates within major rodent clades. These molecular time trees, when compared with the fossil record, suggest that extinction events have led to the loss of diversification signals for many Paleogene nodes, highlighting the interplay between fossil preservation and molecular data in understanding mammalian evolution. 7.2 Adaptive radiations in mammals Adaptive radiations have been a key driver of mammalian diversification. The diversification of placental mammals, for example, has been linked to the end-Cretaceous mass extinction, which created new ecological opportunities. This event is thought to have catalyzed the rapid radiation of various mammalian subclades, including placentals, which diversified from small insectivorous ancestors into a wide range of ecological niches (Grossnickle et al., 2019). Similarly, the radiation of marsupials is another example of adaptive radiation in mammals. Marsupials have undergone significant diversification, particularly in Australia, where they have evolved to occupy a variety of ecological niches. Phylogenetic studies have shown that marsupials and placentals have distinct evolutionary histories, with marsupials experiencing their own unique adaptive radiations (Upham et al., 2019). 7.3 Speciation mechanisms in mammals Speciation in mammals can occur through both allopatric and sympatric mechanisms. Allopatric speciation, where populations are geographically isolated, has been a common mode of speciation in mammals. For instance, the fragmentation of continents during the Cretaceous period likely contributed to the allopatric speciation of placental mammals, as different populations became isolated and evolved independently (Hallström and Janke, 2010). Sympatric speciation, where new species arise within the same geographic area, has also been observed in mammals, although it is less common. Ecological niche differentiation and adaptive traits play crucial roles in sympatric speciation. For example, the diversification of New World monkeys (Platyrrhini) has been linked to ecological niche differentiation, with early phenotypic diversification in body size followed by stasis, suggesting that ecological factors have driven their speciation (Aristide et al., 2015).
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