Evolution of Plants, History of

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Evolution of Plants, History of

The conception that living organisms are changing or mutable originated in the thoughts of Empedocles, an ancient Greek philosopher (c. 490-430 B. C. E.). He drew on the theories of Greek philosophers preceding him to suggest a unitary view of the world consisting of the four elements of earth, air, fire, and water, which interacted with each other according to the principles of love (attraction) and strife (repulsion). Organisms arose from varying combinations of the elements under the action of either love or strife. Thus organic (living) entities were thought to arise from inorganic (nonliving) materials, which then became adapted through a process of selection to their environment. Empedocles not only applied this theory to the origin of plants, but stated that they were the first living organisms.

From Aristotle to Lamarck

This evolutionary worldview was put on hold by the theories of the great Greek philosopher Aristotle (384-322 B. C. E.) and by his famous student Theophrastus (c. 372-287 B. C. E.), who is regarded as the father of scientific botany. They believed in a fixed or static universe, which saw all plants and animals as falling into discrete types or kinds organized in a well-defined, hierarchical scheme from lower to higher organisms. Aristotle called this organization of life, the scala naturae, or the ladder of creation. This view dominated natural philosophy and reached its fullest expression with the work of the great Swedish taxonomist, Carolus Linnaeus (1707-1778). His reform of the taxonomic system was built on the idea that organisms such as plants fall into well-defined types. The Linnaean system follows this typological or essentialistic approach to the natural world by adhering to the notion of the ideal type. This is seen in the pivotal taxonomic use of the type specimen , the technical taxonomic term for the first species of a new group described.

The return to a changing vision of life accompanied the new geological theories of the eighteenth century. These new theories were fueled by new fossil discoveries and the establishment of the view that fossils are organic in nature; that is, that they were remnants of once-living organisms. Plant fossils figured prominently in this. Geological theories were also informed by the revolutionary beliefs associated with the Enlightenment , especially the idea of progress. This idea implied a directionality to history and suggested that progress was in the natural order of things. It also suggested that the world, and Earth, were much older than previously thought, especially as revealed by Biblical scripture. Combined with developments in the comparatively new morphology of both plants and animals and the staggering diversity of new specimens of plants and animals flooding Europe from the voyages of discovery, the new geological theories began to uphold the view that Earth changed in a slow, gradual manner. Instead of accepting the view of unique, one-time, or catastrophic events—such as the flood of Noah—the view that uniform geological processes were responsible for creating Earth, or uniformitarianism, began to dominate geology. Soon, naturalists began to challenge the notion of the fixity of species as they realized that Earth itself was undergoing constant but uniform change.

The French naturalist Comte Georges-Louis Leclerc de Buffon (1707-1788), a contemporary of Linnaeus, was one of the first to suggest a transmutationist theory for living organisms. He made a famous speculative statement applying his belief in a constantly changing Earth to living organisms in volume three of his great compendium of the natural history of Earth, Histoire naturelle (1749-67). This statement paved the way for the first coherent transmutationist theory formulated by Jean Baptiste Lamarck (1744-1829), a botanist of some repute, and the first person to use the word "biology." This theory was clearly formulated in his Philosophie zoologique (1809). Despite its zoological title, the book drew on insights Lamarck had gleaned from his botanical background to explain the phenomenon of adaptation in all living organisms. According to Lamarck, favorable adaptations were originated by the effect of the environment acting directly on the organism. Lamarck therefore mistakenly thought that the environment directly induces permanent change in the genetic composition of organisms. This was what he meant by the "inheritance of acquired characters." The phenomenon of use and disuse, also associated with Lamarck, stated that prolonged use of an organ led to its modification, and disuse led to its elimination. Although Lamarck's theory was widely discussed and became especially popular in his native France, it did not provide a mechanism for how the environment induced such permanent modification, nor did it provide good scientific evidence.

Darwin, Mendel, and the Evolutionary Synthesis

These inadequacies were addressed by the individual who is most closely associated with the theory of organic evolution, Charles Darwin (1809-1882). Darwin did not explicitly reject Lamarck's explanation for adaptation, but instead suggested another mechanism he called natural selection. This was the process by which organisms with favorable variations survived to reproduce those favorable variations. Given enough time, he argued, subsequent generations would depart from the parental type under the action of selection until they formed a new species. This principle of divergence supported and strengthened what Darwin formally called his "theory of descent with modification." This theory was set forth in On the Origin of Species (1859). Darwin drew heavily on examples from the distribution of plants and on knowledge of plant breeding to formulate his theory. In the last twenty years of his life, he studied the phenomenon of adaptation in plants such as orchids, which had evolved spectacular contrivances by which to attract pollinators such as bees. Plants were in fact to provide Darwin with some of the best evidence in support of his theory.

Darwin's theory of evolution transformed understanding of the origins of all life on planet Earth. Botanists such as Joseph Dalton Hooker (1817-1911) in England and Asa Gray (1810-1888) in the United States grew to accept and apply Darwinian evolution to the plant kingdom and to promote the theory further. Despite its success, a considerable number of botanists continued to uphold Lamarckian notions well into the twentieth century. One reason for this is that it is especially hard to distinguish between genotypical variation (variation due to genetics), and phenotypical variation (variation as the result of a direct response to the environment). Unlike animals, which have closed developmental systems, plants have open or indeterminate developmental systems that permit them to continue to "grow" and generate new tissue in regions such as the shoot and root. Plants are thus able to demonstrate the phenomenon of phenotypic plasticity, the ability to adapt readily to new environments and to generate especially complex variation patterns that appear to support Lamarckian inheritance.

It took the work of the geneticists such as Gregor Mendel (1822-1884), who formulated the modern theory of heredity, and Wilhelm Johansen (1857-1927), who first drew the distinction between phenotype and geno-type, to begin to understand more complex aspects of plant evolution, such as plant speciation . It was not until 1950, however, that botanists were able to finally integrate Darwinian natural selection theory with Mendelian genetics, and finally dispelled notions of Lamarckian inheritance. Modern ideas of plant evolution and the science that is framed by the subject, plant evolutionary biology, appeared with the publication of Variation and Evolution in Plants (1950), by the American botanist George Ledyard Stebbins Jr. (1906-2000). With its appearance, botany and plant evolution are generally thought to have become part of the historical event termed "the evolutionary synthesis." This event finally saw the establishment of Darwinian evolution by means of natural selection synthesized with Mendelian genetics. It remains the overarching theoretical framework for explaining the evolution of plants.

see also Darwin, Charles; Evolution of Plants; Hooker, Joseph Dalton; Mendel, Gregor.

Vassiliki Betty Smocovitis