Plant evolution: historical perspective
Plant evolution: historical perspective explores the development of plant life through various scientific theories and discoveries over centuries. Early ideas, such as those proposed by Anaximander and Lucretius, hinted at evolutionary concepts like adaptation and natural selection. However, for much of history, the idea of fixed species dominated scientific thought, with Plato and Aristotle promoting the notion of immutable essences, leading to the static "scale of being" that characterized early biological classification.
The paradigm began to shift with contributions from figures like Jean-Baptiste Lamarck, who suggested that species could adapt through acquired traits, although his ideas did not gain widespread acceptance. Charles Darwin's groundbreaking work in the 19th century introduced the concept of branching evolution and natural selection, demonstrating how species evolve through adaptive changes influenced by their environments. Darwin's theories, solidified in his seminal work "On the Origin of Species," laid the foundation for modern evolutionary biology.
In the early 20th century, the integration of Mendelian genetics with Darwinian concepts led to the modern synthesis of evolutionary theory, which emphasizes that evolution is a complex, non-linear process. This synthesis acknowledges that all species share a common ancestor and undergo adaptive radiation, resulting in diverse life forms shaped by historical evolutionary processes. Understanding plant evolution is crucial for comprehending the development and functionality of biological systems, as evolution fundamentally influences the diversity and complexity of life on Earth.
Plant evolution: historical perspective
Categories: Classification and systematics; evolution; history of plant science; paleobotany
The concept of evolution has ancient roots. Anaximander suggested in the sixth century b.c.e. that life had originated in the seas and that humans had evolved from fish. Empedocles (c. 450 b.c.e.) and Lucretius (c. 96-55 b.c.e.), in a sense, grasped the concepts of adaptation and natural selection. They taught that bodies had originally formed from the random combination of parts, but that only harmoniously functioning combinations could survive and reproduce. Lucretius even said that the mythical centaur, half horse and half human, could never have existed because the human teeth and stomach would be incapable of chewing and digesting the kind of grassy food needed to nourish the horse’s body.
Early Biological Theory
For two thousand years, however, evolution was considered an impossibility. The theory of forms (also called his theory of ideas) proposed by Plato (c. 428-348 b.c.e.) gave rise to the notion that each species had an unchanging “essence” incapable of evolutionary change. As a result, most scientists from Aristotle (384-322 b.c.e.) to Carolus Linnaeus (1707-1778) insisted upon the immutability of species.
Many of these scientists tried to arrange all species in a single linear sequence known as the scale of being (also called the great chain of being or scala naturae), a concept supported well into the nineteenth century by many philosophers and theologians as well. The sequence in this scale of being was usually interpreted as a static “ladder of perfection” in God’s creation, arranged from higher to lower forms. The scale had to be continuous, for any gap would detract from the perfection of God’s creation. Much exploration was devoted to searching for missing links in the chain, but it was generally agreed that the entire system was static and incapable of evolutionary change. Pierre-Louis Moreau de Maupertuis and Jean-Baptiste Lamarck (1744-1829) were among the scientists who tried to reinterpret the scale of being as an evolutionary sequence, but this single-sequence idea was later replaced by the concept of branching evolution proposed by Charles Darwin (1809-1882). Georges Cuvier (1769-1832) finally showed that the major groups of animals had such strikingly different anatomical structures that no possible scale of being could connect them all; the idea of a scale of being lost most of its scientific support as a result.
The theory that new biological species could arise from changes in existing species was not readily accepted at first. Linnaeus and other classical biologists emphasized the immutability of species under the Platonic-Aristotelian concept of essentialism. Those who believed in the concept of evolution realized that no such idea could gain acceptance until a suitable mechanism of evolution could be found. Many possible mechanisms were therefore proposed. Étienne Geoffroy Saint-Hilaire (1805-1861) proposed that the environment directly induced physiological changes, which he thought would be inherited, a theory now known as Geoffroyism. Lamarck proposed that there was an overall linear ascent of the scale of being but that organisms could also adapt to local environments by voluntary exercise, which would strengthen the organs used; unused organs would deteriorate. He thought that the characteristics acquired by use and disuse would be passed on to later generations, but the inheritance of acquired characteristics was later disproved. Central to both these explanations was the concept of adaptation, or the possession by organisms of characteristics that suit them to their environments or to their ways of life. In eighteenth century England, the Reverend William Paley (1743-1805) and his numerous scientific supporters believed that such adaptations could be explained only by the action of an omnipotent, benevolent God. In criticizing Lamarck, the supporters of Paley pointed out that birds migrated toward warmer climates before winter set in and that the heart of the human fetus had features that anticipated the changes of function that take place at birth. No amount of use and disuse could explain these cases of anticipation, they claimed; only an omniscient God who could foretell future events could have designed things with their future utility in mind.
Darwin’s Theory
The nineteenth century witnessed a number of books asserting that living species had evolved from earlier ones. Before 1859, these works were often more geological than biological in content. Most successful among them was the anonymously published Vestiges of the Natural History of Creation (1844), written by Robert Chambers (1802-1871). Books of this genre sold well but contained many flaws. They proposed no mechanism to account for evolutionary change. They supported the outmoded concept of a scale of being, often as a single sequence of evolutionary “progress.” In geology, they supported the outmoded theory of catastrophism, an idea that the history of the earth had been characterized by great cataclysmic upheavals. From 1830 on, however, that theory was being replaced by the modern theory of uniformitarianism, championed by Charles Lyell (1797-1875). Charles Darwin read these books and knew their faults, especially their lack of a mechanism that was compatible with Lyell’s geology. In his own work, Darwin carefully tried to avoid the shortcomings of these books.
Darwin brought about the greatest revolution in biological thought by proposing both a theory of branching evolution and a mechanism of natural selection to explain how it occurred. Much of Darwin’s evidence was gathered during his voyage around the world aboard HMS Beagle between 1831 and 1836. Darwin’s stop in the Galápagos Islands and his study of tortoises and finchlike birds on these islands is usually credited with convincing him that evolution was a branching process and that adaptation to local environments was an essential part of the evolutionary process. Adaptation, he later concluded, came about through natural selection, a process that killed the maladapted variations and allowed only the well-adapted ones to survive and pass on their hereditary traits. After returning to England from his voyage, Darwin raised pigeons, consulted with various animal breeders about changes in domestic breeds, and investigated other phenomena that later enabled him to demonstrate natural selection and its power to produce evolutionary change.
Darwin delayed the publication of his book for seventeen years after he wrote his first manuscript version. He might have waited even longer, except that his hand was forced. From the East Indies, another British scientist, Alfred Russel Wallace (1823-1913), had written a description of an identical theory and submitted it to Darwin for his comments. Darwin showed Wallace’s letter to Lyell, who urged that both Darwin’s and Wallace’s contributions be published, along with documented evidence showing that both had arrived at the same ideas independently. Darwin’s great book, On the Origin of Species by Means of Natural Selection, was published in 1859, and it quickly won most of the scientific community to a support of the concept of branching evolution. In his later years, Darwin also published The Descent of Man and Selection in Relation to Sex (1871), in which he outlined his theory of sexual selection. According to this theory, the agent that determines the composition of the next generation may often be the opposite sex. An organism may be well adapted to live, but unless it can mate and leave offspring, it will not contribute to the next or to future generations.
After Darwin
In the early 1900’s, the rise of Mendelian genetics (named for botanistGregor Mendel, 1822-1884) initially resulted in challenges to Darwinism. Hugo de Vries (1848-1935) proposed that evolution occurred by random mutations, which were not necessarily adaptive. This idea was subsequently rejected, and Mendelian genetics was reconciled with Darwinism during the period from 1930 to 1942. According to this modern synthetic theory of evolution, mutations initially occur at random, but natural selection eliminates most of them and alters the proportions among those that survive. Over many generations, the accumulation of heritable traits produces the kind of adaptive change that Darwin and others had described. The process of branching evolution through speciation is also an important part of the modern synthesis.
The branching of the evolutionary tree has resulted in the proliferation of species from the common ancestor of each group, a process called adaptive radiation. Ultimately, all species are believed to have descended from a single common ancestor. Because of the branching nature of the evolutionary process, no one evolutionary sequence can be singled out as representing any overall trend; rather, there have been different trends in different groups. Evolution is also an opportunistic process, in the sense that it follows the path of least resistance in each case. Instead of moving in straight lines toward a predetermined goal, evolving lineages often trace meandering or circuitous paths in which each change represents a momentary increase in adaptation. Species that cannot adapt to changing conditions die out and become extinct.
Evolutionary biology is itself the context into which all the other biological sciences fit. Other biologists, including physiologists and molecular biologists, study how certain processes work, but it is evolutionists who study the reasons why these processes came to work in one way and not another. Organisms and their cells are built one way and not another because their structures have evolved in a particular direction and can only be explained as the result of an evolutionary process. Not only does each biological system need to function properly, but it also must have been able to achieve its present method of functioning as the result of a long, historical, evolutionary process in which a previous method of functioning changed into the present one. If there were two or more ways of accomplishing the same result, a particular species used one of them because its ancestors were more easily capable of evolving this one method than another.
Bibliography
Bowler, Peter J. Evolution: The History of an Idea. Rev. ed. Berkeley: University of California Press, 1989. A comprehensive, fascinating account of the history of evolutionary theories introduces specialist and nonspecialist alike to one of the most potent scientific ideas of modern times. This new edition is updated in its content and includes an augmented bibliography that offers an unparalleled guide to further reading.
‗‗‗‗‗‗‗‗‗‗‗‗. Life’s Splendid Drama: Evolutionary Biology and the Reconstruction of Life’s Ancestry, 1860-1940. Chicago: University of Chicago Press, 1996. Histories of the Darwinian revolution have paid far more attention to theoretical debates and have largely ignored the researchers who struggled to comprehend the deeper evolutionary significance of fossil bones and the structures of living animals. Bowler recovers some of this lost history in a definitive account of evolutionary morphology and its relationships with paleontology and biogeography.
Brandon, Robert N. Concepts and Methods in Evolutionary Biology. New York: Cambridge University Press, 1996. Robert Brandon is one of the most important and influential of contemporary philosophers of biology. This collection of his recent essays covers all the traditional topics in the philosophy of evolutionary biology and could serve as an introduction to the field.
Grant, Verne. The Evolutionary Process: A Critical Study of Evolutionary Theory. 2d ed. New York: Columbia University Press, 1991. A comprehensive study of the field of evolution, giving full time to critiques of the theory. Concentrates on general principals rather than specific examples.
Minkoff, Eli C. Evolutionary Biology. Reading, Mass.: Addison-Wesley, 1983. A comprehensive general textbook on evolutionary biology, including its historical aspects. The history of evolutionary theories and the mechanisms of the evolutionary process are described in detail. Many examples are given, and the book is profusely illustrated; it assumes no prior knowledge. Contains an excellent bibliography, arranged by topic.
Zimmer, Carl. Evolution: The Triumph of an Idea. New York: HarperCollins, 2001. A companion to the PBS series of the same title, this book presents both the history of the idea of evolution and the science that supports it. A thoroughly up-to-date presentation of the field at the dawn of the twenty-first century. Abundantly illustrated and written for a general audience.