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.

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.     

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.     

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.     

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.     

Why male butterflies are non-mimetic: natural selection,sexual selection,group selection,modification and sieving*

Thomas Belt suggested that the frequent limitation of mimicry in butterflies to the female resulted from sexual selection. Because female butterflies store sperm they can be fully fertile after only one mating; the reproductive success of a male is proportional to the number of times he mates. Sexual selection is therefore much stronger in males than females, with selection coefficients being greater by a small multiple of the number of times a female is courted during her life (long-lived species) or of the reciprocal of the female mortality rate between courtships (short-lived species). As butterflies of both sexes respond to colour when courting, sexual selection resists colour changes especially strongly in males. As a result, genes conferring new mimetic colour patterns can often become established in a butterfly population much more readily if their expression is initially limited to females; when the population size of a Batesian mimic, its model, and its predator fluctuates, such sex-limited genes have an enhanced probability of ultimate fixation in the population, and a reduced chance of loss; this effect is accentuated by the selection of modifiers which improve the mimicry. When the establishment of unimodal mimicry (expressed in both sexes) is favoured in a Batesian mimic, the gene tends to rise to an equilibrium frequency at which modifiers suppressing the expression of the mimicry only in males and'modifiers enhancing the mimicry only in females are favoured. The outcome is female-limited mimicry, or unimodal mimicry with better mimicry in the females, the males either retaining some of their sexual colour or the selective behaviour of the females becoming altered. In a Muellerian mimic there is no such equilibrium and selection ultimately favours expression of mimicry in both sexes and an appropriate alteration in the courtship responses. Hence Muellerian mimicry is seldom female-limited. Exceptional cases appear to result from the sexes flying in separate habitats. The genetical evidence in Papilio and Heliconius favours initial limitation of expression over subsequent modification as the usual basis for female-limited mimicry. Other explanations of female-limited mimicry can be found wanting in various ways; a higher predation rate on females could produce sex-limitation, but is probably not a strong factor. But the greater variability of the female in Lepidoptera may indicate lesser developmental stability, which could result in greater penetrance of mutants in the female, and hence account for the initial female-limitation. At very high densities of a mimetic species which has no non-mimetic form, mimicry tends to deteriorate more rapidly in a unimodal than in an otherwise identical sex-limited species. Although by itself this would equally favour male-limitation, and hence cannot explain the predominance of female-limitation, this effect may over evolutionary time be causing a slight increase in the proportion of sex-limited species among mimics. The stability of some mimetic polymorphisms is investigated by linear approximation: in some instances a stable equilibrium can be changed into an oscillating equilibrium by changes in the population size.     

Sexual selection, natural selection and the evolution of dimorphic coloration and ornamentation in agamid lizards

Both sexual selection and natural selection can influence the form of dimorphism in secondary sexual traits. Here, we used a comparative approach to examine the relative roles of sexual selection and natural selection in the evolution of sexually dimorphic coloration (dichromatism) and ornamentation in agamid lizards. Sexual dimorphism in head and body size were used as indirect indicators of sexual selection, and habitat type (openness) as an index of natural selection. We examined separately the dichromatism of body regions "exposed to" and "concealed from" visual predators, because these body regions are likely to be subject to different selection pressures. Dichromatism of "exposed" body regions was significantly associated with habitat type: males were typically more conspicuously coloured than females in closed habitats. By contrast, dichromatism of "concealed" body regions and ornament dimorphism were positively associated with sexual size dimorphism (SSD). When we examined male and female ornamentation separately, however, both were positively associated with habitat openness in addition to snout-vent length and head SSD. These results suggest that natural selection constrains the evolution of elaborate ornamentation in both sexes as well as sexual dichromatism of body regions exposed to visual predators. By contrast, dichromatism of "concealed" body regions and degree of ornament dimorphism appear to be driven to a greater degree by sexual selection.     

Symbiogenesis: the hidden face of Constantin Merezhkowsky

Constantin Merezhkowsky is celebrated today for his theory of symbiogenesis, postulated in the early decades of the twentieth century, particularly that chloroplasts were symbiotic cyanophytes (cyanobacteria). While biologists point singularly to what they see as his heroic achievement, its neglect and subsequent rediscovery, we introduce a broader and much more complex perspective on his science, his troubled life and career. We present a view of Merezhkowsky as zoologist, anthropologist, botanist, philosopher, and novelist. We explain the genesis of his theory of the origin of chloroplasts and of nucleus and cytoplasm as symbionts, as well as his depiction of the geo-chemical context of the origins and early evolution of life on earth. We also disclose his sordid social and political activities, his eugenics and racist writings, his paedophilia, and his metaphysics. Finally, we describe the context of his elaborate suicide in 1921.     

Sexual selection and natural selection in bird speciation

The role of sexual selection in speciation is investigated, addressing two main issues. First, how do sexually selected traits become species recognition traits? Theory and empirical evidence suggest that female preferences often do not evolve as a correlated response to evolution of male traits. This implies that, contrary to runaway (Fisherian) models of sexual selection, premating isolation will not arise as an automatic side effect of divergence between populations in sexually selected traits. I evaluate premating isolating mechanisms in one group, the birds. In this group premating isolation is often a consequence of sexual imprinting, whereby young birds learn features of their parents and use these features in mate choice. Song, morphology and plumage are known recognition cues. I conclude that perhaps the main role for sexual selection in speciation is in generating differences between populations in traits. Sexual imprinting then leads to these traits being used as species recognition mechanisms. The second issue addressed in this paper is the role of sexual selection in adaptive radiation, again concentrating on birds. Ecological differences between species include large differences in size, which may in themselves be sufficient for species recognition, and differences in habitat, which seem to evolve frequently and at all stages of an adaptive radiation. Differences in habitat often cause song and plumage patterns to evolve as a result of sexual selection for efficient communication. Therefore sexual selection is likely to have an important role in generating premating isolating mechanisms throughout an adaptive radiation. It is also possible that sexual selection, by creating more allopatric species, creates more opportunity for ecological divergence to occur. The limited available evidence does not support this idea. A role for sexual selection in accelerating ecological diversification has yet to be demonstrated.     

Complexity,adaptive complexity and the Creative View of natural selection

In this paper, I respond to arguments proposed by Brunnander in this journal issue concerning my position regarding the Creative View of natural selection (Razeto-Barry & Frick, 2011). Brunnander argues that (i) the Creative View we defend does not serve to answer William Paley’s question because (ii) Paley’s question is “why there are complex things rather than simple ones” and (iii) natural selection cannot answer this question. Brunnander’s arguments for (iii) defend a Non-creative View of natural selection (sensu Razeto-Barry & Frick, 2011). Here I claim that Brunnander’s arguments for (iii) are mistaken and I also argue that even accepting (iii) we do not have to accept (i), given that statement (ii) is historically and conceptually flawed. Thus here I analyze Paley’s question from a historical point of view and from a contemporary perspective in a quest for the potential conceptual relevance of Paley’s question today. In this vein I argue that from a contemporary point of view statement (iii) may be correct but for different reasons than those adduced by Brunnander.     

Sexual dichromatism in frogs: natural selection, sexual selection and unexpected diversity

Sexual dichromatism, a form of sexual dimorphism in which males and females differ in colour, is widespread in animals but has been predominantly studied in birds, fishes and butterflies. Moreover, although there are several proposed evolutionary mechanisms for sexual dichromatism in vertebrates, few studies have examined this phenomenon outside the context of sexual selection. Here, we describe unexpectedly high diversity of sexual dichromatism in frogs and create a comparative framework to guide future analyses of the evolution of these sexual colour differences. We review what is known about evolution of colour dimorphism in frogs, highlight alternative mechanisms that may contribute to the evolution of sexual colour differences, and compare them to mechanisms active in other major groups of vertebrates. In frogs, sexual dichromatism can be dynamic (temporary colour change in males) or ontogenetic (permanent colour change in males or females). The degree and the duration of sexual colour differences vary greatly across lineages, and we do not detect phylogenetic signal in the distribution of this trait, therefore frogs provide an opportunity to investigate the roles of natural and sexual selection across multiple independent derivations of sexual dichromatism.     

Community assembly,natural selection and maximum entropy models

The papers in this Forum discussion debate various aspects of my maximum entropy model of community assembly. The questions raised centre around (1) the possible mechanisms generating the patterns predicted by my maxent model of community assembly, and (2) the appropriate statistical methods for testing the patterns. Here I briefly explain the proposed mechanistic basis of the model: natural selection occurring between individuals of different species. If trait differences are linked to differential demographic probabilities (i.e. fitness differences) then natural selection will constrain the average trait values found in the community and such average (‘community‐aggregated’) traits will then possess information that is translated into the maximum entropy probabilities. If community assembly is strictly neutral then the maxent model will have no predictive ability. This also justifies the null model, and the permutation test, proposed by Roxburgh and Mokany.     

Adaptation and natural selection for the masses

Plan and Purpose in Nature by George C. Williams Weidenfeld and Nicholson Press, 1996. ￡11.99 hbk (viii +191 pages) ISBN 0 297 81646 2.