Evolution: convergent and divergent
Evolution encompasses two significant processes: convergent and divergent evolution. Convergent evolution occurs when unrelated organisms from different lineages develop similar traits or adaptations in response to comparable environmental challenges, despite being geographically distant. A prime example of this is seen in the evolution of succulents in both the cactus family (Cactaceae) of the Americas and the euphorbs (Euphorbiaceae) of South Africa, where both have adapted to arid conditions by developing water-storing stems and reduced leaves.
In contrast, divergent evolution involves species that share a common ancestor evolving into various forms adapted to different environments. This is notably illustrated by the species in the Galápagos Islands, where limited competition allowed for adaptive radiation, leading to the emergence of numerous endemic plant species. A classic case in divergent evolution is the domestication of the common plant Brassica oleracea, which has yielded distinct cultivars such as broccoli and cabbage, all derived from a single ancestral species.
Together, convergent and divergent evolution highlight the dynamic adaptability of life forms in response to environmental pressures, illustrating the intricate pathways through which species evolve over time.
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Evolution: convergent and divergent
Categories: Ecosystems; evolution; paleobotany
Convergent Evolution
Convergent evolution occurs when organisms from different evolutionary lineages evolve similar adaptations to similar environmental conditions. This can happen even when the organisms are widely separated geographically. A classic example of convergent evolution occurred with Cactaceae, the cactus family, of the Americas and with the euphorbs, or Euphorbiaceae, the spurge family of South Africa, both of which have evolved succulent (water-storing) stems in response to desert conditions.
The most primitive cacti are vinelike, tropical plants of the genus Pereskia. These cacti, which grow on the islands of the West Indies and in tropical Central and South America, have somewhat woody stems and broad, flat leaves. As deserts developed in North and South America, members of the cactus family began to undergo selection for features that were adaptive to hotter, dryer conditions.
The stems became greatly enlarged and succulent as extensive water-storage tissues formed in the pith or cortex. The leaves became much reduced. In some cactus species, such as the common prickly pear (Opuntia), the leaves are small, cylindrical pegs that shrivel and fall off after a month or so of growth. In most cacti, only the leaf base forms and remains as a small hump of tissue associated with an axillary bud. In some cacti this hump is enlarged and is known as a tubercle. Axillary buds in cacti are highly specialized and are known as areoles. The “leaves” of an areole are reduced to one or more spines. Particularly in columnar cacti, the areoles are arranged in longitudinal rows along a multiple-ridged stem.
With the possible exception of the genus Rhipsalis, which has one species reported to occur naturally in Africa, all cacti are native to the Americas. As deserts formed in Africa, Eurasia, and Australia, different plant families evolved adaptations similar to those in cacti. The most notable examples are the candelabra euphorbs of South Africa. Desert-dwelling members of the Euphorbiaceae frequently have succulent, ridged, cylindrical stems resembling those of cacti. The leaves are typically reduced in size and are present only during the rainy season. They are arranged in rows along each of several ridges of the stem. Associated with each leaf are one or two spines. As a result, when the leaves shrivel and fall off during the dry season, a spiny, cactuslike stem remains.
The succulent euphorbs of Africa take on all of the forms characteristic of American cacti, from pincushions and barrels to branched and unbranched columns. Other plant families that show convergence with the cacti, in having succulent stems or leaves, are the stem succulents of the milkweed family, Asclepiadaceae; sunflower family, Asteridaceae; stonecrop family, Crassulaceae; purslane family, Portulacaceae; grape family, Vitaceae; leaf succulents of the ice plant family, Aizoaceae; daffodil family, Amaryllidaceae; pineapple family, Bromeliaceae; geranium family, Geraniaceae; and lily family, Liliaceae.
Divergent Evolution
Some of the most famous examples of divergent evolution have occurred in the Galápagos Islands. The Galápagos comprise fourteen volcanic islands located about 600 miles west of South America. A total of 543 species of vascular plants are found on the islands, 231 of which are endemic, found nowhere else on earth. Seeds of various species arrived on the islands by floating in the air or on the water or being carried by birds or humans.
With few competitors and many different open habitats, variant forms of each species could adapt to specific conditions, a process known as adaptive radiation. Those forms of a species best suited to each particular habitat were continually selected for and produced progeny in that habitat. Over time, this natural selection resulted in multiple new species sharing the same ancestor. The best examples of divergent evolution in the Galápagos have occurred in the Cactaceae and Euphorbiaceae. Eighteen species and variety of cacti are found on the islands, and all are endemic. Of the twenty-seven species and varieties of euphorbs, twenty are endemic.
An interesting example of the outcome of divergent evolution can be seen in the artificial selection of different cultivars (cultivated varieties) in the genus Brassica. The scrubby Eurasian weed colewort (Brassica oleracea) is the ancestor of broccoli, brussels sprouts, cabbage, cauliflower, kale, and kohlrabi (rutabaga). All of these vegetables are considered to belong to the same species, but since the origin of agriculture, each has been selected for a specific form that is now recognized as a distinct crop.
Bibliography
Bowman, Robert I., Margaret Berson, and Alan E. Leviton. Patterns of Evolution in Galápagos Organisms. San Francisco: California Academy of Sciences, 1983. A series of symposium papers, including one on the flora of the Galápagos Islands.
Darwin, Charles. “Journal and Remarks: 1832-1836.” In Narrative of the Surveying Voyages of His Majesty’s Ships “Adventure” and “Beagle” Between the Years 1826 and 1836: Describing Their Examination of the Southern Shores of South America, and the “Beagle’s” Circumnavigation of the Globe, by Robert Fitzroy. Vol. 3. Reprint. New York: AMS Press, 1966.
Harris, James G., and Melinda Woolf Harris. Plant Identification Terminology: An Illustrated Glossary. Spring Lake, Utah: Spring Lake, 1994. Clear line drawings illustrate specific terminology used to identify and describe plants.
Uno, Gordon, Richard Storey, and Randy Moore. Principles of Botany . New York: McGraw-Hill, 2001. A good introduction to all aspects of plant biology.