Генетика и происхождение видов

1937 book by Theodosius Dobzhansky

Genetics and the Origin of Species is a 1937 book by the Ukrainian American evolutionary biologist Theodosius Dobzhansky. It is regarded as one of the most important works of modern synthesis and was one of the earliest. The book popularized the work of population genetics to other biologists and influenced their appreciation for the genetic basis of evolution. In his book, Dobzhansky applied the theoretical work of Sewall Wright (1889–1988) to the study of natural populations, allowing him to address evolutionary problems in a novel way during his time. Dobzhansky implements theories of mutation, natural selection, and speciation throughout his book to explain the habits of populations and the resulting effects on their genetic behavior. The book explains evolution in depth as a process over time that accounts for the diversity of all life on Earth. The study of evolution was present, but greatly neglected at the time. Dobzhansky illustrates that evolution regarding the origin and nature of species during this time in history was deemed mysterious, but had expanding potential for progress to be made in its field.

In Darwin's theory of natural selection, more organisms are produced than can survive. Some have variations that give them a competitive advantage, and they have the best chance of surviving and procreating. The main element lacking in the theory was any mechanism that would allow organisms to pass on these favorable variations. Lacking such a mechanism, the theory of evolution faced competition from theories such as neo Lamarckism, in which the environment acted directly on organisms, changing their structures. Darwin did not know that the monk Gregor Mendel was already working on experiments that would explain inheritance in terms of units of heredity that we now call genes. When Mendelian genetics was rediscovered by several scientists, it initially increased the confusion. The Dutch botanist Hugo de Vries developed a theory called mutationism in which most variations were inconsequential and could not lead to species change. Instead, new species were formed by large mutations.

The book began as a series of lectures at Columbia University in October and November 1936. Dobzhansky decided to attend the summer 1936 meeting of the Genetics Society of America in Woods Hole, Massachusetts. When the geneticist Leslie Dunn learned of this, he invited Dobzhansky to give a series of lectures at Columbia. He sent the invitation in April, and within a month he had suggested that the lectures be a springboard for writing a general treatise on evolutionary genetics. Dobzhansky was enthusiastic, and in May he wrote back, proposing the title "Genetics and the Origin of Species". He envisioned two parts – "part I containing data on the sources of evolutionary change, and part II containing a discussion of their interactions leading to race and species formation." After the lectures (for which he received $500), Dobzhansky returned to Pasadena, and in December Columbia University Press accepted his proposal for a book. The pace of writing was helped by a riding accident in February 1937 in which he crushed his knee, rendering him immobile; and by April he was able to send a manuscript to Dunn for his perusal.

Genetics and the Origin of Species has two main points to make. The first is that speciation is a genuine problem that must be explained by the theory of evolution. In nature, there is not a single population of organisms separated from each other by small variations. Instead, the natural world is divided into species, each with its own limited range of variability. The second point is that all the variations can be explained by the principles of genetics. The 1937 edition was divided into nine chapters, whose contents are described below.

The first chapter is a short outline of the main points of the book: Evolutionary theory must account for variations on the level of the individual and also that of populations and species. It must explain how reproductive isolation can occur. And the goal is to explain all this using genetic principles that can be verified in the laboratory. (Humans are diploid, having one set each from the mother and father.) He discusses case histories such as that of Raphanobrassica, a hybrid between the radish and the cabbage. This is an example of "cataclysmic" speciation, an exception to his general rule that speciation is a slow process.

An isolating mechanism prevents members of two different species from successfully producing offspring. Dobzhansky invented the term, Although previous authors had recognized the importance of isolation, their reasons were entirely different from his. George Romanes thought that isolation was a cause of change, whereas Dobzhansky sees two competing effects. Isolation reduces the rate of formation of harmful gene combinations, but it also restricts the scope of genetic variation. Thus, a species remains near a given peak in the fitness landscape and does not find new adaptive peaks.

Through his experiments, Dobzhansky discovers that mutation of genes leads to evolution within a specific species. Adaptations play a large role in genetic drift, and it is known that genes and mutations influence this genetic drift in a particular environment. Mutations can result from external influences within the surrounding environment, especially if an organism inhabits an area with harsh living conditions. An organism can adapt to its environment in order to better suit its own needs. When an organism successfully adapts, it has higher survival and reproduction rates. Therefore, there is a higher chance that its genes will be passed on to its offspring. Specific genes and alleles are then passed on to future generations to continue the trend of modern evolution that Dobzhansky presents in the book. Dobzhansky said that natural mutation, aided by variation, can lead to change when acted upon by natural selection. Mutations were thought to be relatively rare and other variations were even considered to be harmful. Since an organism's overall genetic make up was the result of natural selection, with damaging mutations weeded out, wild populations were assumed to have very few mutations. As a result, evolution was said to be a relatively slow process. One of Dobzhansky's major contributions in this book was to show that this view of slow moving evolution was incorrect. While analyzing chromosome structure in wild populations of Drosophila pseudoobscura, Dobzhansky discovered a surprising amount of undiscovered variability. These variations could not be observed in the outward appearance of the individual organisms. Dobzhansky suggested that the preservation of extensive variation would allow populations to evolve rapidly as environmental conditions change. This book was a landmark in the evolutionary synthesis, as it presented the union of Mendelian genetics and Darwinian theory. In Genetics and the Origin of Species, polyploidy is considered as a type of mutation. Polyploid cells have a chromosome number that is more than twice the haploid number. The effects of polyploidy between two different species causes hybridization and even greater evolution. It was not long before the synthesis was broadened to include paleontology, systematics and botany in a series of notable books: Systematics and the Origin of Species (1942) by Ernst Mayr; Tempo and Mode in Evolution (1944) by George Gaylord Simpson; and Variation and Evolution in Plants (1950) by G. Ledyard Stebbins. The emerging synthesis was called the evolutionary synthesis by Julian Huxley in his book, Evolution: The Modern Synthesis. In 1947, a diverse collection of biologists met at a symposium in Princeton and declared their acceptance of this synthesis. However, it was not yet complete. Developmental biologists did not accept that the theory explained their observations, and it was not until the 1970s and 1980s that molecular biology bridged the gap. Also, for decades there was disagreement over whether the unit of selection was the gene or the individual as a whole. In 1974 all the living founders of the modern synthesis (with the exception of Simpson and Bernhard Rensch) met with historians of biology in a conference to evaluate their work. All acknowledged Genetics and the Origin of Species as the direct instigator of all the work that followed.

Dobzhansky was plagued by a form of leukemia in his later years, but he remained vigorously active until the day before his death on December 18, 1975. Sixty years after its publication, the National Academy of Sciences commissioned a book entitled Genetics and the Origin of Species: From Darwin to Molecular Biology 60 Years After Dobzhansky. It was also included in the 1990 edition of Great Books of the Western World, placing it with Charles Darwin's two great works The Origin of Species and The Descent of Man. A third edition was published in 1951.