Most desert plants and animals initially evolved from ancestors in moister habitats, an evolution that occurred mostly independently on each continent. Despite this phylogenetic divergence, a high degree of similarity of body shape and life form exists among the floras and faunas of different desert regions. Since desert environments are defined by their water limitation and have similar landscapes worldwide, it is not surprising that many organisms show convergent evolution and are morphologically and functionally alike. Similar pressures of natural selection have resulted in similar life forms. In fact, many of these analogous species groups became textbook examples of evolutionary convergence:
• Stem and leaf succulence is found in nonrelated plant taxa: cacti in New World, milkweeds and Euphorbia species in the Old World (however, this form is lacking in Australia).
• Bipedal locomotion is found in unrelated small rodent groups: jerboa (family Dipodidae) in the Old World, kangaroo rats (family Heteromyidae) in the New World.
• Bipedal locomotion is shared in a few larger mammals: African springhare (genus Pedetes), desert-living kangaroos (Macropodidae) in Australia.
• North American horned lizards (genus Phrynosoma) and the Australian thorny devil, the unrelated agamid lizard Moloch horridus, share similar grotesque spiny body armors. This has been explained as an adaptive suit that facilitates their need of having a large body due to their specialization on ants. Ants as eusocial insects present a clumped however low digestible source of food (formic acid, chitin). Both lizard groups are in need of a larger digestive system and therefore large bodies that in turn makes them slow moving and in need of protection.
Many adaptations that are discussed in the following sections are typical for all desert regions of the world. A combination of these traits creates the 'typical' desert life form that to some extent is similar worldwide.
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