John Cohen

Decline in sperm counts: an artefact of changed reference range of “normal”?

Abstract Objective: To investigate a reported fall in sperm counts during 1940–90 in relation to the reduced lower reference value of “normal” during the same period by assuming the null hypothesis that no change had occurred in the probability distribution of the sperm concentration. Design: Analysis by using various mathematical models of the probaility distribution of sperm concentration together with experimental data which supported a model employing a logarithmic distribution. Subjects: 235 men presenting for stimulated in vitro fertilisation at Midland Fertility Services, Aldridge, in 1992 together with samples of 20 ejaculates from each of five men attending the same centre during 1992–3. Results: The effect of the change in lower reference value for the “normal” sperm concentration (from 60x109 to 20x109/l) depended on the probability distribution of the concentration in the population. If that distribution was normal or uniform, then very little of the reported decline was a consequence of the change in lower reference value. If it was heavily skewed, then most or all of the reported decline may have been a consequence of that change. The limited experimental data available indicate that the distribution was heavily skewed. Conclusions: Depending on the actual distribution of sperm concentration in the population, the reported decline in concentration may have been accounted for entirely or in part by the change in lower reference value. The original evidence does not support the hypothesis that the sperm count declined significantly between 1940 and 1990.

From exaptation to radical niche construction in biological and technological complex systems

Biological adaptation assumes the evolution of structures toward better functions. Yet, the roots of adaptive trajectories usually entail subverted—perverted—structures, derived from a different function: what Gould and Vrba called “exaptation.” Generally, this derivation is regarded as contingent or serendipitous, but it also may have regularities, if not rules, in both biological evolution and technological innovation. On the basis of biological examples and examples from the history of technology, the authors demonstrate the centrality of exaptation for a modern understanding of niche, selection, and environment. In some cases, biological understanding illuminates technical exaptation. Thus, the driver of exaptation is not simply chance matching of function and form; it depends on particular, permissive contexts.

Symmetry-Breaking as an Origin of Species

A central problem in evolutionary biology is the occurrence in the fossil record of new species of organisms. Darwin’s view, inThe Origin of Species,was that speciation is the result of gradual accumulations of changes in body-plan and behaviour. Mayr asked why gene-flow failed to prevent speciation, and his answer was the classical allopatric theory in which a small founder population becomes geographically isolated and evolves independently of the main group. An alternative class of mechanisms, sympatric speciation, assumes that no such isolation occurs. These mechanisms overcome the stabilising effect of gene-flow by invoking selection effects, for example sexual selection and assortative mating. We interpret sympatric speciation as a form of symmetry-breaking bifurcation, and model it by a system of nonlinear ODEs that is ‘all-to-all coupled’, that is, equivariant under the action of the symmetric group SN. We show that such bifurcations can be interpreted as speciation events in which the dominant long-term behaviour is divergence into two species. Generically this divergence occurs by jump bifurcation - `punctuated equilibrium’ in the terminology of evolutionary biology. Despite the discontinuity of such a bifurcation, mean phenotypes change smoothly during such a speciation event. So, arguably, do mean-field genotypes related to continous characters.Our viewpoint is that speciation is driven by natural selection acting on organisms, with the role of the genes being secondary: to ensure plasticity of phenotypes. This view is supported, for example, by the evolutionary history of African lake cichlids, where over 400 species (with less genetic diversity than humans) have arisen over a period of perhaps 200,000 years. Sympatric speciation of the kind we discuss is invisible to classical mean-field genetics, because mean-field genotypes vary smoothly.Our methods include numerical simulations and analytic techniques from equivariant bifurcation theory. We focus on two main models: the generic cubic-order truncation of a symmetry-breaking bifurcation in an SN-equivariant system of ODEs, and the BirdSym system introduced by Elmhirst in which the biological interpretation of variables is more explicit.We relate our conclusions to field observations of various organisms, including Darwins finches. We also offer a biological interpretation of our models, in which speciation is represented as an emergent property of a complex system of entities at the organism level. We briefly review questions about selection at the level of groups or species in the light of this interpretation

Why are there simple rules in a complicated universe

Abstract The role of science is to seek simplicity in a complex world. Nature exhibits many remarkable regularities and patterns, and science works by assuming that these patterns arise from the regularities and patterns of the underlying ‘laws of nature’ that govern them. This is a comfortable picture, which encourages a view of the relation between laws and their consequences—between cause and effect, ie simple rules imply simple behaviour, therefore complicated behaviour must arise from complicated rules. This article questions whether chaos and complexity theories hold the answer to the mysteries of emergent phenomena such as mind, consciousness, biological form and social structure.

Where are the dolphins

Interest in extraterrestrial life has tended to focus on a search for extrasolar planets similar to the Earth. But what of forms of intelligent life that are very different from those found on Earth? Some features of life will not be peculiar to our planet, and alien life will resemble ours in such universals. But if intelligent, non-humanoid aliens exist, where might they be? Would they wish to visit Earth and would we know if they did?

Subjective probability, gambling and intelligence.

WE do not make a general statement to the effect that subjective probabilities in all situations diminish with increase in age, but merely refer to the subjective probability of winning the uncertain prizes in our particular experiments. Secondly, we do not speak of subjective estimates of probability but of ‘subjective probability’. That is, our subjects do not make any estimates of probabilities. Indeed, these are explicitly stated to them. What they do is make a choice or express a preference; and this choice or preference is perhaps influenced by their subjective assessment of the value of the prize—the point which Prof. Heath emphasizes—as well as by how likely they think they are of getting it. We have no indication that our subjects distinguish between these two components in making their choice. We referred in particular to the second component but do not wish to deny the possible significance of the first. Indeed, we explicitly stated in the last sentence of our communication that the subjective probability on which the preference is based is affected by the value of the prizes offered. On the other hand, if the explanation rested only on the supposed reduction with age in the “imagined appetite for sweets”, we should also have to conclude that this appetite increases with intelligence.

How does complexity develop

A great variety of answers springs to mind in response to this question. They range from the simple idea that multiplying simple processes results in more complex causality through the “chaotic” divergence of interactive systems, so that the space between the piano keys comes to include new keys, to recursive systems that change their parameters each time around and indeed to recursive systems that change their rules as they evolve. This is not only a philosophical question, interesting for itself but without application to the everyday world. We are surrounded by systems becoming more complex, from embryos and ecosystems to industrial processes and international law.

What does the human Y chromosome sequence suggest about diploidy

Abstract The Y human chromosome has many ancient genes whose fidelity seems to have been preserved by tandem sequences and palindromic ‘hairpins’, compared/repaired by ‘gene conversion’. That a primary function of recombination machinery is DNA repair has been suggested, and rejected, several times; this new evidence is very persuasive. The process, better called gene conservation than gene conversion, could operate in all diploid organisms, accounting for the retention of long gene sequences without ‘informational meltdown’ (concerted evolution). It resembles rocket‐science computer‐redundancy error‐checking, comparison of three or four sequences, not just two. If recognition of errors in ‘converted’ sequences can be followed by either repair or rejection, the rejection option can account for the vast wastage of meiotic products. The repair option might be used in Drosophila oocytes and even zygotic nuclei, possibly other oocytes, ancient asexual lineages such as mycorrhizal fungi, perhaps the Y itself. Both evolutionary stasis (conservatism) and development and deployment of complex developmental modules can be understood in these terms so both the evolution of biodiversity and the practice of systematics may have these mechanisms as their bases. The main individual‐fitness and evolutionary advantages of diploidy were not primarily cloaking of recessive al‐leles, or allelic recombination and Mendelism, but conserving long DNA sequences.

Figments of reality: We Wanted to Have a Chapter on Free Will, but We Decided not to, so Here It Is

A senior Royal Air Force officer had organised an official reunion for World War II veterans, all in full dress uniform, covered in medals and ribbons, aged about seventy. The highlight of the event was a fly-past of restored aircraft - Spitfires, Lancaster bombers, and so on - and he stood in front of the veterans to watch them. Suddenly, sensing something odd, he turned round - to find that the veterans had disappeared. Then he realised that they were all lying flat on the grass. The explanation? A Fokker (a WWII German fighter) had roared across the field, flying low … ‘It would be very singular,’ wrote Voltaire, ‘that all nature, all the planets, should obey eternal laws, and that there should be a little animal, five feet high, who, in contempt of these laws, could act as he pleased, solely according to his caprice.’ It is an eloquent statement of the problem of free will, and it is the place where our figments run slap up against reality, like the proverbial irresistible force meeting the immovable object. We have a distinct, overwhelming impression that we have a free choice concerning the actions that we take: free, that is, subject to the evident constraints of physical law. We cannot choose to float into the air, for example. Yet there is absolutely nothing in the inorganic world that possesses that kind of freedom.

Taxing the rat farms: pollution in context

Abstract Both authors are at the University of Warwick, UK, in the Department of Biological Sciences and Mathematics Institute, respectively. Their most recent book Collapse of Chaos was stated to be, in a review in Nature , the best place to start for an understanding of science in the next century.