## 2 Phylogenetic Variations in Cd Accumulation in Plants
It is well documented that the angiosperm ecological incursion is highly driven by environmental circumstances and biotic factors ([PERSON] et al., 2018). [PERSON] et al. [PERSON] et al. (2018) summarized the early records of shoot Cd content in angiosperms and found a variation of 44.3% in shoot Cd content among the 108 angiosperm species that were tested. Up to the present, approximately 20 angiosperm species (\(\sim\)10 families) have already been identified as Cd hyperaccumulators ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018). These Cd accumulators are not randomly distributed among the angiosperm families but mainly belong to the Brassicaaceae, Crassulaceae, and Asteraceae families ([PERSON] et al., 2018; [PERSON] et al., 2018). Two Brassicaceae species, _Arabidopsis halleri_ and _Thluspi caerulescens_, which are the most intensively studied Cd-hyperaccumulating plant species, demonstrate an extraordinary ecotypic variation (ca. 100 > 1000 \(\mu\)g/g among sites and populations) in terms of their Cd accumulation potential ([PERSON] et al., 2018; [PERSON] et al., 2018). Likewise, the Cd hyperaccumulator _Sedum alredii_ exhibits fixed ecotypic differences in Cd sequestration (hyperaccumulating ecotype preferentially accumulates Cd in the pith and cortex, whereas non-hyperaccumulating ecotype restricts Cd to its vascular bundles) ([PERSON] et al., 2018), root-to-shoot translocation (>10 times higher in hyperaccumulating ecotype than in non-hyperaccumulating ecotype) ([PERSON] et al., 2018), and shoot Cd concentration (7.5-1090 mg/kg among different populations) ([PERSON] et al., 2018). These results indicate the local adaptation of the above species to natural habitats ([PERSON] et al., 2018) or, probably, to the anthropogenic metal pollution ([PERSON] et al., 2018). Taken together, plants, particularly angiosperms, have developed widespread adaptations to their habitat, with contrasting Cd contents throughout the course of their evolution ([PERSON] et al., 2018; [PERSON] et al., 2018). It has been proposed that the trait of Cd accumulation is broadly distributed over the plant phylogeny and has evolved independently many times and under selection ([PERSON] et al., 2018). Similar convergent patterns of evolution in angiosperms have also been observed in terms of the accumulation of other metals like selenium (Se) and nickel (Ni) ([PERSON] et al., 2018; [PERSON] et al., 2018). Hence, the question regarding which selection pressures (for example, biotic and abiotic environmental factors as well as physiological processes) favor the evolution of Cd accumulation in plants arises ([PERSON] et al., 2018). It is well known that the emergence of the Cd hyperaccumulation trait in some plant species like _Arabidopsis halleri_, _Thluspi caerulescens_, and _Sedum alredii_ coincided with the appearance of anthropogenic metal-polluted sites in mining regions ([PERSON] et al., 2018; [PERSON] et al., 2018; [PERSON] et al., 2018). Therefore, it is believed that the colonization of plant species in heavy metal-contaminated soils resulting from human activities represents a relatively recent event in their evolutionary history ([PERSON] et al., 2018). However, a comparison between _Arabidopsis halleri_ (Cd accumulator) and _Arabidopsis lyrate_ (non Cd accumulator) suggests that these two close relatives of the model species _Arabidopsis_ thaliana diverged about 5 Mya or earlier, and the trait of Cd hyperaccumulation had evolved well in _Arabidopsis halleri_ long before anthropogenic activities fostered the spread of Cd-polluted areas ([PERSON] et al., 2018). Therefore, Cd accumulation may have evolved in calamine outcrops prior to metal pollution by mining activities or in nonmetalliferous soils prior to the colonization of metalliferous soils and an increase in metal tolerance [57]. The other hypotheses proposed on the selective factors in Cd accumulation by plants include protection against herbivores or pathogens, allelopathy, and positive physiology effect [10].