Symbiogenesis,natural selection,and the dynamic Earth

One century ago, Constantin S. Mereschkowsky introduced the symbiogenesis theory for the origin of chloroplasts from ancient cyanobacteria which was later supplemented by Ivan E. Wallin’s proposal that mitochondria evolved from once free-living bacteria. Today, this Mereschkowsky–Wallin principle of symbiogenesis, which is also known as the serial primary endosymbiosis theory, explains the evolutionary origin of eukaryotic cells and hence the emergence of all eukaryotes (protists, fungi, animals and plants). In 1858, the concept of natural selection was described independently by Charles Darwin and Alfred R. Wallace. In the same year, Antonio Snider-Pellegrini proposed the idea of shifting continents, which was later expanded by Alfred Wegener, who published his theory of continental drift eight decades ago. Today, directional selection is accepted as the major cause of adaptive evolution within natural populations of micro- and macro-organisms and the theory of the dynamic Earth (plate tectonics) is well supported. In this article, I combine the processes and principles of symbiogenesis, natural selection and the dynamic Earth and propose an integrative ‘synade-model’ of macroevolution which takes into account organisms from all five Kingdoms of life.     

Modularity, noise, and natural selection

Most biological systems are formed by component parts that are to some degree interrelated. Groups of parts that are more associated among themselves and are relatively autonomous from others are called modules. One of the consequences of modularity is that biological systems usually present an unequal distribution of the genetic variation among traits. Estimating the covariance matrix that describes these systems is a difficult problem due to a number of factors such as poor sample sizes and measurement errors. We show that this problem will be exacerbated whenever matrix inversion is required, as in directional selection reconstruction analysis. We explore the consequences of varying degrees of modularity and signal-to-noise ratio on selection reconstruction. We then present and test the efficiency of available methods for controlling noise in matrix estimates. In our simulations, controlling matrices for noise vastly improves the reconstruction of selection gradients. We also perform an analysis of selection gradients reconstruction over a New World Monkeys skull database to illustrate the impact of noise on such analyses. Noise-controlled estimates render far more plausible interpretations that are in full agreement with previous results.     

Productivity,relevance and natural selection

Recent papers by a number of philosophers have been concerned with the question of whether natural selection is a causal process, and if it is, whether the causes of selection are properties of individuals or properties of populations. I shall argue that much confusion in this debate arises because of a failure to distinguish between causal productivity and causal relevance. Causal productivity is a relation that holds between events connected via continuous causal processes, while causal relevance is a relationship that can hold between a variety of different kinds of facts and the events that counterfactually depend upon them. I shall argue that the productive character of natural selection derives from the aggregation of individual processes in which organisms live, reproduce and die. At the same time, a causal explanation of the distribution of traits will necessarily appeal both to causally relevant properties of individuals and to causally relevant properties that exist only at the level of the population.

Darwin and sexual selection: One hundred years of misunderstanding

Darwin's book on the Descent of Man and Selection in Relation to Sex (1871) is often viewed as the continuation of TheOrigin of Species published 12 years earlier (1859), both because of the implicit parallelism between natural selection and sexual selection, and because Darwin himself presents the book as developing a subject (man) which he intentionally omitted in the Origin. But the Descent can also be viewed as the continuation of his book on Variation published three years earlier (1868). Firstly because Darwin's hypothesis of pangenesis links the selection process to the origin of variation through use and disuse, an idea underlying his speculations on the origin of moral sense in humans. Second because like the action of the horticulturist on his domestic crops, sexual selection exerted by one sex on the other sex can develop fancy traits that are not easily accounted for by their utility to the selected organism itself, such as artistic taste, pride, courage, and the morphological differences between human populations. These traits are difficult to reconcile with pangenesis. They add up to other contradictions of the book possibly resulting from Darwin's erroneous inference about the mechanism of inheritance, like those on the determination of sex-ratio, or the confusion between individual adaptation and the advantage to the species. These inconsistencies inaugurate a weakening of the Darwinian message, which will last 50 years after his death. They contributed to the neglect of sexual selection for a century. Darwin however maintained a logical distinction between evolutionary mechanisms and hereditary mechanisms, and an epistemological distinction between evolutionary theory and Pangenesis hypothesis. In the modern context of Mendelian genetics, Darwin's sexual selection retrospectively appears as luminous an idea in its pure principle as natural selection, even though the mechanisms governing the evolution of sexual choice in animals remain largely unresolved.     

Ocean waves, nearshore ecology, and natural selection

Although they are subjected to one of the most stressful physical environments on earth, wave-swept rocky shores support a highly diverse community of plants and animals. The surprising presence of such diversity amidst severe environmental adversity provides a unique opportunity for exploration of the role of extreme water flows in community ecology and natural selection. Methods are described by which the maximal water velocity and acceleration can be predicted for a site on the shore, and from these values maximal hydrodynamic forces are calculated. These forces can limit the range and foraging activity of some species, and can determine the rate of disturbance in others, but in general, wave-swept organisms have surprisingly high factors of safety. This apparent over-design can help to explain the diversity of forms present on wave-swept shores, and provides examples of how mechanics can limit the ability of natural selection to guide specialization. Although flow itself may commonly be prohibited from selecting for optima in morphology, it nonetheless continues to play a potentially important role in evolution by providing a mechanism for breaking or dislodging individuals that have been selected by other means.     

Variation, natural selection, and information content--a simulation

In Neo-Darwinism, variation and natural selection are the two evolutionary mechanisms that propel biological evolution. Variation implies changes in the gene pool of a population, enlarging the genetic variability from which natural selection can choose. But in the absence of natural selection, variation causes dissipation and randomization. Natural selection, in contrast, constrains this variability by decreasing the survival and fertility of the less-adapted organisms. The objective of this study is to propose a highly simplified simulation of variation and natural selection, and to relate the observed evolutionary changes in a population to its information content. The model involves an imaginary population of individuals. A quantifiable character allows the individuals to be categorized into bins. The distribution of bins (a histogram) was assumed to be Gaussian. The content of each bin was calculated after one to twelve cycles, each cycle spanning N generations (N being undefined). In a first study, selection was simulated in the absence of variation. This was modeled by assuming a differential fertility factor F that increased linearly from the lower bins (F<1.00) to the higher bins (F>1.00). The fertility factor was applied as a multiplication factor during each cycle. Several ranges of fertility were investigated. The resulting histograms became skewed to the right. In a second study, variation was simulated in the absence of selection. This was modeled by assuming that during each cycle each bin lost a fixed percentage of its content (variation factor Y) to its two adjacent bins. The resulting histograms became broader and flatter, while retaining their bilateral symmetry. Different values of Y were monitored. In a third study, various values of F and Y were combined. Our model allows the straightforward application of Shannon's equation and the calculation of a Shannon-entropy (SE) values for each histogram. Natural selection was, thus, shown to result in a progressive decrease in SE as a function of F. In other words, natural selection, when acting alone, progressively increased the information content of the population. In contrast, variation resulted in a progressive increase in SE as a function of Y. In other words, variation acting alone progressively decreased the information content of a population. When both factors, F and Y, were applied simultaneously, their relative weight determined the progressive change in SE.     

Maladaptation and natural selection

The transformations George Williams initiated in evolutionary biology seem so blindingly obvious in retrospect that they spur the question of why he saw what no one else did. While most humans are prone to see only what theory predicts, Williams sees in bold relief whatever does not fit. Not an adaptationist or an anti-adaptationist, Williams is better described as a maladaptionist. The challenge of finding evolutionary explanations for apparent maladaptations has been overlooked with casualness akin to that once typical for group selection. Suboptimal traits tend to be dismissed as illustrations of the weakness and stochastic nature of selection compared with mutation and drift. A closer look suggests that such constraints are only one of six possible kinds of explanations for apparently suboptimal designs: mismatch, coevolution, tradeoffs, constraints, reproductive advantage at the expense of the individual, and defenses that are aversive but useful Medicine has asked proximate questions at every possible level but has only begun to ask evolutionary questions about why bodies are vulnerable to disease. Considering all six possible evolutionary reasons for apparently suboptimal traits will spur progress not only in medicine but also more generally in biology. 'Williams Vision" may not yield a net benefit to the possessor, but it is invaluable for the species.     

Sexual selection, natural selection and copulatory patterns in male primates.

Complex copulatory patterns, involving multiple brief intromissions or prolonged single intromissions (PI) occur most frequently among primate species in which females mate with a number of partners (multimale and dispersed mating systems). However, the PI pattern is also confined almost exclusively to arboreal primates - to either smaller-bodied, cryptic, nocturnal species or much larger diurnal forms. Predation pressures may have limited the evolution of the PI pattern, particularly where small-bodied diurnal primates or terrestrial forms are concerned. The available evidence indicates that both sexual selection and natural selection may have influenced the evolution of copulatory patterns.     

Grandmothering and natural selection

Humans are unique among primates in that women regularly outlive their reproductive period by decades. The grandmother hypothesis proposes that natural selection increased the length of the human post-menopausal period—and, thus, extended longevity—as a result of the inclusive fitness benefits of grandmothering. However, it has yet to be demonstrated that the inclusive fitness benefits associated with grandmothering are large enough to warrant this explanation. Here, we show that the inclusive fitness benefits are too small to affect the evolution of longevity under a wide range of conditions in simulated populations. This is due in large part to the relatively weak selection that applies to women near or beyond the end of their reproductive period. However, we find that grandmothers can facilitate the evolution of a shorter reproductive period when their help decreases the weaning age of their matrilineal grandchildren. Because selection favours a shorter reproductive period in the presence of shorter interbirth intervals, this finding holds true for any form of allocare that helps mothers resume cycling more quickly. We conclude that while grandmothering is unlikely to explain human-like longevity, allocare could have played an important role in shaping other unique aspects of human life history, such as a later age at first birth and a shorter female reproductive period.     

Humanity and natural selection

In some modern biological images of the human species human action is reduced to a consequence of natural selection, that is, to a tendency to maximize fitness. The precise nature and scope of the theory of natural selection are, however, undecided; yet in evolutionary interpretations of human society neodarwinism is often treated as a dogma, and natural selection sometimes becomes a transcendental force. There are instances of changes in gene frequencies, in human as well as other populations, that conform with neodarwinian (“socio-biological”) presumptions; but it is not known to what extent the evolution of the human (or other) species has been due to such changes. There is still less ground for explaining the diversity of human societies in this way.The image-makers also make illegitimate use of the comparative method: familiar features of human conduct, such as favoring kin and reciprocation, are used in accounts of animal behavior, and are then rediscovered among human beings; during this two-way transfer, the meanings of words are changed; altruism, egoism, and deceit lose their moral content, and the complex human idea of kin is reduced to a measure of genetical similarity.The intention of neodarwinists is to reveal a human nature determined by evolutionary processes, but one of the most important distinctive features of our species is the plasticity of our behavior, attitudes, and intentions. Moreover, if neodarwinian premises are accepted, to speak of intention is misleading, and there are no independent criteria by which neodarwinian (or any other) arguments may be judged: all one does is regulated by the need to maximise one's inclusive fitness. Hence much writing in this field wavers between an uncompromising reduction of human action to considerations of population genetics, on the one hand, and a recognition that there are other kinds of authentic knowledge about human beings, on the other. Among the latter is historical knowledge.The neodarwinian images of humanity emphasize human depravity. In their misanthropy they reflect the outlook of conservative pessimists who have influenced European thought for two and a half millennia, and whose views imply that most attempts to improve the human condition are against nature and so must fail.An alternative, which corresponds to the facts of everyday life and of history, is that human beings are capable of rejecting what is conventionally held to be inevitable, and of determining their destiny by conscious, deliberate action.