When philosophers of physics explore the nature of chance, they usually look to quantum mechanics. When philosophers of biology explore the nature of chance, they usually look to microevolutionary phenomena, such as mutation or random drift. What has been largely overlooked is the role of chance in macroevolution. The stochastic models of paleobiology employ conceptions of chance that are similar to those at the microevolutionary level, yet different from the conceptions of chance often associated with quantum mechanics and Laplacean determinism.

In a recent paper, William K. Goosens objects to the arguments I set out some time ago attacking the logical empiricist analysis of reduction as applied to genetics. In these works I did not argue against the claim that Mendelian genetics was being reduced to molecular biology. Nor did I conclude, as Goosens asserts, that in the case of genetics, “reduction is insignificant”. To the contrary, I repeatedly stated that, “given our pre-analytic intuitions about reduction,” the reduction of Mendelian to (...) molecular genetics “is a case of reduction, a paradigm case”. And in agreement with Kenneth Schaffner I argued that reduction “in some very important, pre-analytic sense has taken place and is taking place in genetics”. The target of my objections to reduction in genetics was not “reduction” in some pre-analytic sense. Nor was it the various explications which have appeared since. Rather it was the notion of theory reduction set out a generation ago by such logical empiricists as Ernest Nagel, The particular version of this analysis which I chose to attack was that presented by Schaffner ; for Schaffner's later views, see his. I chose Schaffner's explication to attack because I thought it was the best of its kind and because it was the only defense of the logical empiricist analysis at the time which used genetics as one of its chief examples. Contrary to Goosens' assumption, in criticizing Schaffner's explication, I did not thereby “endorse” it. (shrink)

Among the liveliest disputes in evolutionary biology today are disputes concerning the role of chance in evolution--more specifically, disputes concerning the relative evolutionary importance of natural selection vs. so-called "random drift". The following discussion is an attempt to sort out some of the broad issues involved in those disputes. In the first half of this paper, I try to explain the differences between evolution by natural selection and evolution by random drift. On some common construals of "natural selection", those two (...) modes of evolution are completely indistinguishable. Even on a proper construal of "natural selection", it is difficult to distinguish between the "improbable results of natural selection" and evolution by random drift. In the second half of this paper, I discuss the variety of positions taken by evolutionists with respect to the evolutionary importance of random drift vs. natural selection. I will then consider the variety of issues in question in terms of a conceptual distinction often used to describe the rise of probabilistic thinking in the sciences. I will argue, in particular, that what is going on here is not, as might appear at first sight, just another dispute about the desirability of "stochastic" vs. "deterministic" theories. Modern evolutionists do not argue so much about whether evolution is stochastic, but about how stochastic it is. (shrink)

A belief common among philosophers and biologists alike is that Mendelian genetics has been or is in the process of being reduced to molecular genetics, in the sense of formal theory reduction current in the literature. The purpose of this paper is to show that there are numerous empirical and conceptual difficulties which stand in the way of establishing a systematic inferential relation between Mendelian and molecular genetics. These difficulties, however, have little to do with the traditional objections which have (...) been raised to reduction. (shrink)

The functions and expression pattern of urea cycle enzymes have undergone considerable changes during the course of evolution. Sequence analyses shows that urea cycle enzymes from mammals are homologous to microbial enzymes of the arginine‐metabolic pathway. Recently, an unexpected relationship was found between argininosuccinate lyase (EC 4.3.2.1), the fourth enzyme of the cycle, and δ‐crystallin, a lens structural protein of birds and reptiles.

Plant MADS‐box genes encode transcriptional regulators that are critical for a number of developmental processes. In the angiosperms (the flowering plants), these include the specification of floral organ identities, flowering time and fruit development. It appears that the MADS box gene family has undergone considerable gene duplication and sequence divergence within the angiosperms. Here I discuss the possibility that these events have allowed the recruitment of these genes to new developmental pathways in particular angiosperm lineages. Recent analyses of sequence changes, (...) expression patterns and, in a few cases, gene function are beginning to provide tantalizing evidence for deciphering when and how such genetic diversification has led to particular morphological innovations. In the future, comparative studies of large numbers of species will be required to assess the extent of such variation as well as to fully understand the mechanisms by which evolution of these developmental regulators has played a role in shaping new morphologies. BioEssays 25:637–646, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Despite decades of research, morphogenesis along the various body axes remains one of the major mysteries in developmental biology. A milestone in the field was the realisation that a set of closely related regulators, called Hox genes, specifies the identity of body segments along the anterior–posterior (AP) axis in most animals. Hox genes have been highly conserved throughout metazoan evolution and code for homeodomain‐containing transcription factors. Thus, they exert their function mainly through activation or repression of downstream genes. However, while (...) much is known about Hox gene structure and molecular function, only a few target genes have been identified and studied in detail. Our knowledge of Hox downstream genes is therefore far from complete and consequently Hox‐controlled morphogenesis is still poorly understood. Genome‐wide approaches have facilitated the identification of large numbers of Hox downstream genes both in Drosophila and vertebrates, and represent a crucial step towards a comprehensive understanding of how Hox proteins drive morphological diversification. In this review, we focus on the role of Hox genes in shaping segmental morphologies along the AP axis in Drosophila, discuss some of the conclusions drawn from analyses of large target gene sets and highlight methods that could be used to gain a more thorough understanding of Hox molecular function. In addition, the mechanisms of Hox target gene regulation are considered with special emphasis on recent findings and their implications for Hox protein specificity in the context of the whole organism. BioEssays 30:965–979, 2008. © 2008 Wiley Periodicals, Inc. (shrink)