|
The succulents provide the most dramatic example of
evolutionary convergence. The different deserts of the world
have been isolated from one another since the origin of the flowering
plants, with only limited opportunities for migration among different
areas. Hence, succulents have evolved independently in many different
families in different deserts.
Desert regions are characterized by low precipitation and
often by high day time temperatures during at least part of the
year. The present deserts of the world are of relatively recent
origin, and their plants and animals have been selected from
those found in the bordering areas. Annual plants are better
represented, both in number and kind, in the deserts and semiarid
regions of the world than anywhere else. The relatively few perennial
herbs that grow in the desert are often bulbous and dormant for
much of the year. Most of the taller plants are either succulent
or have small leaves that are either leathery or are shed during
an unfavorable season. Many of the succulent species in this
biome have adopted CAM (Crassulacean acid metabolism) photosynthesis
and absorb carbon dioxide at night. Perhaps the most magnificent
examples of convergent evolution are shown by the succulents
of the deserts of the world. The one raised in the cactus deserts
of Arizona, however, the convergance of leaf types in the many
families of deciduous forest herbs and trees might seem just
as bizarre and noteworthy!
|
