Darwin and deduction

One of the most remarkable but insufficiently noted features of Charles Darwin's conception of evolution is that its logical implications are still being worked out. I am not merely claiming that experimental and observation studies continue to make use of and bear on Darwinian ideas and principles. I am calling attention to the fact that after almost a century and a half, new deductions are still being teased out of his very fertile axioms of descent with modification and natural selection.

Neil S. Greenspan

The apparent simplicity of the phrase "survival of the fittest" (which was coined by Herbert Spencer and was not used in the early editions of Darwin's On the Origin of Species) hides more than it reveals. Far from being tautological, the underlying insight has vast implications that have, in some instances, taken so long to realize because of the series of logical steps required for their derivation. Even some of the more readily derivable conclusions have escaped relatively informed and sophisticated individuals.Consider the admission, within the past six months, by the Harvard psychology professor and evolution advocate, Steve Pinker. On the web site edge.org, Pinker revealed that he only recently came to accept that human evolution did not stop thousands of years ago. It is welcome news that Pinker has reached this understanding, but it is a bit surprising that he would have entertained his prior view at all. Highly persuasive evidence, which did not in fact exist, should have been necessary to conclude that human evolution had reached a standstill.If organisms of any species harbor phenotypic variations that are capable of being inherited, and these phenotypic variations influence the numbers of descendants an organism is likely to leave, then the distribution of phenotypes (and genotypes) in the population will very likely change over time. In other words, the population will evolve. Thus, heritable variation that influences reproductive success is necessary and sufficient for evolution to occur unless the selective "forces" acting on the population happen to be perfectly balanced, which is highly unlikely. So the continuation of human evolution should be anyone's default assumption given that, as Pinker might have known, humans exhibit substantial genetic variation that could plausibly influence reproductive success.In fact, there are numerous experimental reports on the allelic diversity at individual human genetic loci, as well as studies offering evidence of selection for or against alleles at particular loci, such as the lactase and Duffy blood group loci. Furthermore, recent studies designed to exploit the vast number of genetic markers now defined for the human genome, along with relatively new and sophisticated methods to detect selection, have provided evidence of selection at numerous human loci, and have, in one case, suggested that the pace of human evolution has accelerated in the past 40,000 years, in parallel with greater population size.

Darwin's connection to deductive reasoning has other notable aspects. On reading his famous tome some years ago I was struck by the overwhelming evidence of his formidable logical powers. Others have preferred to see Darwin's achievement more in inductive terms, but Darwin clearly worked out the deductive consequences of his concept of natural selection for numerous features of the living world, such as the spatial and temporal distributions of organisms and species. For instance, Darwin presciently saw the implications of competitive mechanisms for inter-species relations such that fluctuations in the numbers of one species might influence the numbers of a second species with which there are no direct interactions. He specifically suggested, for example, that the number of felines in an area might ultimately affect the frequency of a particular flower, of no significance to the felines, by virtue of a chain of influences: felines on mice, mice on bees, and bees on flowers.Darwin, who I think can reasonably be regarded as the greatest applied logician since Euclid, introduced into biology the notion of categories for which membership was not all-or-none. Categories with absolute criteria for inclusion had been the norm in mathematics and logic for two millennia, but with Darwin's emphasis on evolving lineages, the intellectual community was given a powerful new tool. Thus, two humans (or members of any other species) do not have to share an identical list of attributes to qualify for membership in the species. Only some attributes (and a variable subset at that) must be shared. Astonishingly, even today, many who regard themselves as Darwinians (or neo-Darwinians) continue to construct and use biological categories that have more in common with those of ancient Greek thought than of Darwin's. For example, in my own field of immunology, it is common to distinguish between putatively innate and putatively adaptive immune mechanisms as if there were an absolutely obvious boundary between the two. Yet the actual mechanisms show little respect for this distinction, as when antibodies (adaptive) mediate immunity by interacting with phagocytes (innate). One of the most dramatic examples of the limitations of rigid categories in biology is the very notion of "species," for which there is still neither a consensus definition nor one that applies equally well to all types of organisms. Perhaps that is why the eminent population geneticist and evolutionary biologist, Richard Lewontin, once remarked that "Biology remains in many ways obdurately Platonic."Neil Greenspan mail@the-scientist.comNeil Greenspan is a professor of pathology at the Case Western Reserve University School of Medicine. His research, clinical, and teaching interests relate to immunology and allied fields and are informed by his study of evolution. Greenspan and about fifteen Case faculty colleagues have organized a Celebration of Darwin and Evolution spanning the 2008-2009 academic year. Details can be found at: www.case.edu/darwin/.