Natural Behavior. Burton A. Weiss
previous year before the Linnaean society in London along with Wallace, who offered similar concepts. The competition with Wallace finally led Darwin to publish. His work was so well anticipated that it sold out on the very first day.
Darwin had enormous impact on life science because he presented not only the idea that the evolution of life had occurred, but he also explained the process with the mechanism of natural selection. In addition, his wide travel experience enabled him to support his theory with an unequaled and unassailable array of accurate examples. The timing also favored Darwin’s ideas about evolution. In 1860, Pasteur demonstrated conclusively that organisms were responsible for spoilage and fermentation, and that spontaneous generation of life did not occur. Virchow reformulated the earlier postulates of Schleiden and Schwann, that cells were the basic unit of life, and that cells came from other cells. The cell theory, coupled with Pasteur’s discoveries and Darwin’s work, sparked a major revolution in life science. For the first time, both the units of life, the cells, and the explanation of the origin and continuity of life were known.
But the heralds were not trumpeting. Pasteur was rejected. Not until later in the century, when called upon by the Russian Czar to stop an anthrax epidemic, would Pasteur’s concepts be accepted. The cell theory was ignored until 1895, when Verworn revitalized it. Darwin’s ideas became distorted into cliches like “struggle for existence” and “survival of the fittest.” These phrases were wrongly employed to emphasize the survival of individuals, rather than the population principle of differential reproduction of the species. Spencer, and other Social Darwinists, usurped these cliches to excuse the inequities of the very rich and very poor of the rising industrial society. Thus, the wealthy were seen as the fittest and the poor were viewed as the unfit. Life scientists, horrified by the political distortion of their work, abandoned the pursuit of general explanations of life, like evolution. Again, laws describe the behavior of populations, not individuals.
For the next six decades evolution was taboo and reductionism reigned. Müller’s followers tried to use his laws of nerve function to explain all behavior. Loeb joined the movement with his idea of taxis, the approach or avoidance of physical stimuli. He attempted to explain all behavior as the physics of taxis. Thus, a cockroach ran into darkness because of negative photo taxis. A resurgence of Lamarckian ideas, which focused on the individual, had prevailed.
Part of the difficulty was the fact that Darwin’s work was lacking an important piece necessary to complete the puzzle. Darwin understood natural selection. He did not grasp the significance of sex as the mechanism for generating and preserving the population differences on which selection operates. Darwin believed that sex was a means of obscuring population variance. He thought offspring would be intermediate compared to parental extremes. Thus, a tall and a short parent should have a medium-size child. Darwin’s ideas on natural selection were remarkable considering the paucity of genetic knowledge at the time.
In the 20th century, the, link that converted evolutionary theory to fact appeared. Mendel in 1866 had found the basic laws of genetics stating that parental characteristics do not blend in progeny. He found that the combination of various parental characteristics in the offspring followed the laws of statistical probability, and that dominant parental characteristics mask recessive ones in the next generation. Mendel’s work, published in the Proceedings of the Brünn Natural Science Society (1866), was ignored until rediscovered independently in 1900 by Correns, deVries, and von Tschermak-Seysenegg. In 1903, Sutton realized that the chromosomes life scientists had been watching divide and reproduce through microscopes for years were the carriers of the genetic characteristics. In 1909 Nilsson-Ehle extended the understanding of qualitative gene characters, like red or white, to quantitative features having multiple genes, like the degree of redness. Chetverikov, Haldane, Fisher, and Wright in the late 1920’s and early 1930’s provided the mathematical basis for selection. Fisher (1930) synthesized evolution and genetics by indicating that genetic fitness was related to population variance.
Finally, life scientists knew the three features by which sex contributed to evolution. First, sexuality was a device for generating variability by rapidly creating various, and even new, parental genetic combinations in offspring. Sexual combinations were fast, compared to the slow accumulation of new features in asexual populations. Variation is also enhanced by dominant genes shielding, retaining, and preserving the variety of recessive characteristics, even, if they were fatal by themselves. Variation is important because it provides the resilience in the population to adjust to changes in selection. Second, generation of new genes by mutation, discovered by deVries, is a random occurrence with no regard for any needs of the organism, unlike Lamarck’s purposive acquired characteristics. Third, the unit of evolutionary change is not the individual, as Lamarck thought, but the population’s total pool of genes. To repeat, as with dice tosses, laws describe the behavior of populations, not of individuals. Sex itself is an illustration of the last point because sexuality, while important to individuals, has no adaptive significance to the individual. Males are no more or less adaptive than females. If they were, that would mean selection could operate separately on males or on females and eliminate one sex. That is not possible! The species, as a whole, is the unit of selection. Sex is a mechanism that has been favored because of its adaptive value of contributing to variation in the population, the real unit upon which selection operates.
Sex has proved so adaptive that the mechanism has appeared in numerous species in different individual forms. The male differentiating system (XY) in humans, the female differentiating system (WZ) in birds, and the haploid male (developing by parthenogenesis) and diploid female bees are all different. Thus, genetically, a human male is more like an avian female. However, mathematically, the sexual systems serve the same purpose of generating variation for the species. All sexual systems function alike to provide a source of variation in the population.
Some theories about the origin of sex suggest that early organisms were asexual like modern protists. With a functional method of reproduction already in existence, sex would have needed some purpose other than reproduction to displace the asexual technique. Indeed, enhancing variation in a population was just such an advantage. Sexuality became favored in many organisms, because it served to accelerate evolutionary rates by increasing variation. However, in some species sex promoted too much genetic variation. Thus, to slow down their evolutionary rate and adjust to the environment, those species abandoned sex by becoming hermaphroditic.
An alternative sequence could have been that the sexual mechanism was originally an exchange of genetic material between equivalent organisms like the conjugation of similar individual Paramecium. Specialization of sexual organs in multicellular species led to hermaphroditism. Interchange of genetic material was still between equivalent individuals employing self fertilization to adjust evolutionary rates, like many species of worms today. Sexual specialization of individuals into separate males and females then occurred through secondary loss of the organs of the opposite sex by hermaphrodites. Genetic interchange can be accomplished effectively by either hermaphrodites or separate sexes. Separate sexes trade away redundancy to gain efficient reproductive specialization in individuals, but do not fundamentally alter the sexual mechanism. In this view, abandonment of the sexual mechanism has not happened, except partially, in some species like those in worm phyla which have returned to or retained asexual fission. Asexual fission by-passes the sexual functions of hermaphroditic reproduction and thereby adjusts the rate of genetic exchange.
In either theory, sexual forms were generated by having one switch with two positions. Thus, in humans, the switch is the hormone testosterone. Testosterone presence in early development means male, while testosterone absence means female. Humans are not basically female with males needing an additional hormone, rather one hormone switch simply determines development. Two switches, one for each sex, are not required. Natural selection has a tendency to be parsimonious.
Modern Evolution
Modern life science recognizes the fact that evolution depends on genetic, phenotypic (constitutional), and ecological opportunity. But the combination of opportunities in the right place and time is dependent upon probabilistic chance. Therefore, evolution is a matter of probability. The probabilistic nature of evolution makes it a one-way sequence and precludes an exact repetition of the process. If everything could be restarted, the same opportunities would not be likely to recur, especially in the