George C. Williams

PLEIOTROPY, NATURAL SELECTION, AND THE EVOLUTION OF SENESCENCE

form the much simpler task of merely maintaining what is already formed. It is true, of course, that some parts of organisms do literally wear out. Human teeth, for instance, show wear similar to that of any tool subjected to friction, but this wear is no more a part of senescence than is the wearing away of replaceable epidermal cells. The senescence of human teeth consists not of their wearing out but of their lack of replacement when worn out. August Weismann (1891) was the first biologist of the evolutionary era to advance a theory of senescence. He believed that organisms must inevitably show a decline analogous to that of mechanical devices, but that, in addition, there was a specific death-mechanism designed by natural selection to eliminate the old, and therefore wornout, members of a population. He did not clearly indicate how such a mechanism could be produced by natural selection. He was likewise dubious about the exact nature of the deathmechanism, but indicated that it might involve a specific limitation on the number of divisions that somatic cells might undergo. Weismanns theory is subject to a number of criticisms, the most forceful of which are: 1) The fallacy of identifying senescence with mechanical wear, 2) the extreme rarity, in natural populations, of individuals that would be old enough to die of the postulated death-mechanism, 3) the failure of several decades of gerontological research to uncover any deathmechanism, and 4) the difficulties involved in visualizing how such a feature could be produced by natural selection.

Natural selection : domains, levels, and challenges.

A philosophical position The gene as a unit of selection Clade selection and macroevolution Levels of selection among interactors Optimization and kindred concepts Historicity and constraint Diversity within and between clades Some recent controversies Statis Other challenges and anomalies Bibliography Appendix.

Why we get sick : the new science of Darwinian medicine

Using numerous examples drawn from many scientific disciplines, however, Hess does make a very strong case that past scientific developments have been strongly affected by cultural norms and power considerations. Hess further uses these examples to convincingly argue that the history of science has not been shaped solely by white males of European descent. He illustrates that individuals from numerous cultures and races, many of whom were in fact female, contributed significantly to major developments in many scientific enterprises. These contributions frequently have been either minimized or completely ignored due to the cultural emphasis of extolling the virtues of those in power, which typically have been Anglo-Saxon-Protestant white males.

The Dawn of Darwinian Medicine

While evolution by natural selection has long been a foundation for biomedical science, it has recently gained new power to explain many aspects of disease. This progress results largely from the disciplined application of what has been called the adaptationist program. We show that this increasingly significant research paradigm can predict otherwise unsuspected facets of human biology, and that it provides new insights into the causes of medical disorders, such as those discussed below: 1. Infection. Signs and symptoms of the host-parasite contest can be categorized according to whether they represent adaptations or costs for host or parasite. Some host adaptations may have contributed to fitness in the Stone Age but are obsolete today. Others, such as fever and iron sequestration, have been incorrectly considered harmful. Pathogens, with their large populations and many generations in a single host, can evolve very rapidly. Acquisition of resistance to antibiotics is one example. Another is the recently demonstrated tendency to change virulence levels in predictable ways in response to changed conditions imposed incidentally by human activities. 2. Injuries and toxins. Mechanical injuries or stressful wear and tear are conceptually simpler than infectious diseases because they are not contests between conflicting interests. Plant-herbivore contests may often underlie chemical injury from the defensive secondary compounds of plant tissues. Nausea in pregnancy, and allergy, may be adaptations against such toxins. 3. Genetic factors. Common genetic diseases often result from genes maintained by other beneficial effects in historically normal environments. The diseases of aging are especially likely to be associated with early benefits. 4. Abnormal environments. Human biology is designed for Stone Age conditions. Modern environments may cause many diseases-for example, deficiency syndromes such as scurvy and rickets, the effects of excess consumption of normally scarce nutrients such as fat and salt, developmental diseases such as myopia, and psychological reactions to novel environments. The substantial benefits of evolutionary studies of disease will be realized only if they become central to medical curricula, an advance that may at first require the establishment of one or more research centers dedicated to the further development of Darwinian medicine.

The lek paradox is not resolved

Abstract In a population at equilibrium in a stable environment under natural selection there can be very little heritability of fitness. In some species, for example, those with leks, females exhibit strong preferences for certain characteristics in their mates, though all they obtain are his genes and these cannot contribute much to the fitness of their offspring. In paternal species, where there are often clear economic benefits in obtaining good mates, mate choice mechanisms do not seem so highly developed. This is the lek paradox. We examine three forces, mutational load, migrational load, and two kinds of linkage disequilibrium, which might generate enough heritability to maintain a mating convention in a nonpaternal species, and we evaluate the strength of the selective forces they generate. Only under rather extreme conditions can these forces be of more than negligible strength.

Why reproduce sexually

Abstract There is reason to believe that intense selection, such that only a small minority at the top of the fitness distribution has any appreciable chance of survival, can sometimes give sexual reproduction an immediate (one-life-cycle) advantage over asexual. The advantage must be great enough to balance the 50% loss of genetic material in meiosis.One model shows the advantage to be frequency-dependent in life cycles in which there are several asexual generations and one sexual. The observed frequency of sexual reproduction in such a life cycle is explained as an evolutionary equilibrium by this model. In another model the optimum frequency of asexual reproduction drops to zero as fecundity and competition increase. This explains the exclusively sexual reproduction of such fecund organism as elms and oysters. Once lost, asexual reproduction may be difficult to evolve secondarily. This explains the presence of such exclusively sexual, low-fecundity organisms as the higher vertebrates.

GENETIC DIFFERENTIATION WITHOUT ISOLATION IN THE AMERICAN EEL, ANGUILLA ROSTRATA. II. TEMPORAL STABILITY OF GEOGRAPHIC PATTERNS

In an earlier report (Williams et al., 1973), we examined the problem of genetic isolation among differentiated stocks in the American eel, Anguilla rostrata. This species has an unusual life cycle, with a breeding area in the tropical Atlantic and subsequent extended larval dispersal (about 1 yr) to the shores of the Americas from Surinam to Greenland. Because of the supposed universal panmixia of this species, genetic isolation cannot exist

Population Genetics of North Atlantic Catadromous Eels (Anguilla)

The genetics of catadromous eels of the North Atlantic basin is of special interest because of uncertainties in the taxonomic status of the stocks found on opposite sides of the ocean, because their extraordinary life cycle makes them a source of information otherwise difficult to obtain on evolutionary processes, and because of uncertainties in genotypeenvironment interactions in the determination of sex.

THE TITHONUS ERROR IN MODERN GERONTOLOGY

Tithonus asked Aurora for eternal life, when he meant eternal youth. Modern gerontological research makes the same mistake in its preoccupation with death, as if it were a programmed event in an organisms life history. Gerontology ought instead to investigate senescence, the decreasing effectiveness of mechanisms by which adult organisms avoid death or loss of fitness. Such studies should measure rates of decline in a diversity of adaptations and compare them within and between individuals and relate these rates and their correlations to genetic and environmental factors. The death of a studied organism must necessarily end its usefulness in providing valuable data. It is of little scientific significance.

Interannual fluctuations in the density of sand lance, Ammodytes americanus, larvae in Long Island Sound, 1951-1983

Enumeration data from over 2,300 ichthyoplankton samples collected during 17 yr, spanning a 32-yr interval (1951–1983), were compiled to determine interannual variations in density of sand lance larvae. Years of relatively high densities were noted during the winters of 1965–1966 and 1978–1979 and low densities in 1971–1974. A regular increase in numbers during the late 1970’s and the peak in 1978–1979 coincided with increases in population size found throughout the coastal northwest Atlantic Ocean. Densities in Long Island Sound began to decline in 1980 and this continued through 1983. In contrast, densities throughout coastal Atlantic areas during the 1980’s remained at least as high as they were 1976–1978. Interannual fluctuations in density of sand lance larvae could be partially explained by water temperatures in December. Warm Decembers were associated with low larval densities.