Students Of Evolution Research Articles

PDB40: The Protein Data Bank celebrates its 40th birthday

PDB40: The Protein Data Bank celebrates its 40th birthday

Within‐generation bet hedging: a seductive explanation?

Bet hedging occurs when a single genotype shows a variety of phenotypes in the same environment, and each phenotype is successful only when the particular circumstances to which it is adapted occur. The time scale of between‐generation bet hedging ensures that all individuals with a given phenotype suffer the same fate – circumstances such as drought exert homogenous pressure on all members of a population. Under within‐generation bet hedging, however, individuals with the same phenotype are subject to heterogeneous selection pressure – predation, for example, will affect some individuals but not others. An important consequence of this difference is that conditions favoring the evolution of within‐generation bet hedging are very restricted. While a single lineage may realize increased fitness via within‐generation bet hedging, this fitness advantage varies inversely with population size and becomes vanishingly small at even modest population sizes. Although several reviews and analyses have highlighted the differences between these two types of bet hedging, confusion persists regarding their respective definitions and evolutionary justification. Bet hedging is a seductive explanation because most students of evolution are trained to focus on costs and benefits at the individual level, and tend to seek adaptive explanations for individual traits. Although this focus is often successful, it leads us astray in the case of within‐generation bet hedging. Only by assessing the fitness effects of a trait in the context of whole populations can one accurately identify traits that can and cannot be favored by within‐generation bet hedging.

Evolution and Ecological Value of Duplicate Genes

The existence of duplications of parts of chromosomes and of specific genetic loci was inferred in Drosophila by Bridges (39) and Sturtevant (280) early in this cen­ tury. In his address as the retiring president of the American Society of Naturalists in 1947, Metz (201 ), however, could point to only a few specific instances of duplicate bands after cytological studies on the polytene chromosomes from Dipt­ eran insects. Stephens, in a 195 1 review of the significance of duplications in evolu­ tion, concluded that there was not a single proven case in which two gene products could be homologously related (273). These and other students of evolution [see (273) for references] were certainly aware that new genetic material must come from duplications of previously existing genes or chromosomes. Metz even surmised correctly that some duplicate genes might be unstable and that heterochromatin may contain repeated chromosomal parts. Nevertheless, before the advent of molecular biology, essentially only speculation about the extent and the �elective importance of duplicate genes was available. Now, however, while there is still no shortage of speculation about this phenomenon and its evolutionary meaning, there are many firmly documented cases of duplicate genes, including those in which the members of the gene pair are at several different levels of divergence. Kimura & Ohta (172) now list gene duplication as one of five principles of molecular evolu­ tion, and, in addition, Ohno (213) has written a book stressing its importance during the evolution of the major vertebrate classes. This article reviews some of the information from molecular biology which, in the last fifteen years, has effected this change. Soon after the identification of DNA as the genetic material, the extent of the evolutionary increase in the number of genes per organism became evident. The amount of DNA per genome (expressed as picograms or nucleotide pairs) ranges over eight orders of magnitude, from certain simple viruses to some algae, amphibi-

Types of group selection.

THE terms group selection and intergroup selection1,2 have been used in referring to a variety of phenomena including recently the evolution of behavioural social systems involved in population control3,4. Since the phenomenon of group selection promises to receive greater attention from students of evolution in the future, I feel that a clarification of terms at this time will help to prevent confusion. The “groups” under consideration may be quite different from each other and the action of natural selection on them will depend on the nature of the group.

A QUANTITATIVE METHOD FOR COMPARING THE KARYOTYPES OF RELATED SPECIES

In many groups of organisms comparison of chromosome morphology is, or should be, an important tool of taxonomists and students of evolution. Objectivity of comparison is attained under three conditions. Firstly, where hybridization between taxa is possible, studies of the chromosome pairing at meiosis may reveal homologies. Secondly, comparisons may be based on patterns of longitudinal differentiation of chromosomes. Because minute detail is observable in only a few tissues like dipteran salivary glands or Trillium root tips, this basis of comparison is of restricted use. The third condition, which is the only one possible in most groups of organisms, is comparison of lengths of chromosome arms. The object of this communication is to show how this can be made quantitative and subject to normal statistical methods. The most useful measure of length is the mean per cent length, i.e., the mean value, obtained from a number of nuclei, of the length of a chromosome arm expressed as a percentage of the total length of all chromosome arms in the same nucleus. This metric has been used by Rothfels and Siminovitch (1958) and by several other authors. The difficulties of karyotype analysis using mean per cent length have been ably discussed by Patau (1960) and by Moore and Gregory (1963). Where these difficulties can be overcome, i.e., where chromosome morphology is quite distinct so that visual pairing of homologues within a cell is most unlikely to be wrong, mean per cent length could be used for comparing the karyotypes of two species. This comparison could be direct only if the two species had the same total amounts of chromosomal material, i.e., the same total absolute lengths at the same degree of contraction of their chromosomes. This would not always be so, and comparisons based on actual measurement of length would be unreliable because the degrees of contraction of the chromosomes and of distortion during slide preparation would be likely to vary considerably and not necessarily at random between the species being compared. The basic assumption of the method reported here is that, in an interspecific comparison, the ratio of total lengths will be the same as the ratio of amounts of DNA per chromosome complement. If this is true then LA, the mean per cent length of a certain chromosome arm in species A, can be compared directly with LBDBIDA where LB is the mean per cent length of a supposedly homologous chromosome arm in species B, and DB and DA are, respectively, the amounts of DNA per chromosome complement in species A and B. This assumption implies that no differences in degrees of polyteny occur between the somatic chromosomes being compared. However, such a situation should be detectable. In the examples which follow DBIDA has been measured microspectrophotometrically using Feulgen-stained mammalian leucocytes which had been grown in vitro by a modification of the method of Moorhead et al. (1960). After the usual colchicine and hypotonic citrate treatment the cells were fixed in acetic alcohol. One drop of species A cells in fixative was placed 34 inch from one end of a microscope slide (subbed as for stripping film autoradiography) and one drop of species B cells 34 inch from the other end. With care the two drops did not mingle and were allowed to dry. Slides were hydrolyzed for ten minutes in N HCl at 60? C., precautions being

Axenic Seedlings of Drosera

CARNIVORY in higher plants fascinated Darwin1 and later students of evolution, yet a century-long debate2 on the extent of Drosera's dependence on, and specialization for, insect prey remains inconclusive. A technique for obtaining axenic D. intermedia (a sundew) is reported here—a step forward in analysing its biochemical capabilities.

Some Concepts of Hybridization and Intergradation in Wild Populations of Birds

A PERSISTENT conception among a considerable group of taxonomists holds that intergradation and hybridization are contrasting phenomena and that the terms should be used as antonyms. They regard an intergrading situation as denoting involvement of two races and hybridization as involving two species. Accordingly, all one needs to do is to find out which phenomenon is displayed where two forms come in contact, and the troublesome borderline cases between race status and species status are thereby correctly indicated and classified. This chain of reasoning I regard as fallacious, for it is an oversimplification of the factors and it does not clearly emphasize the best criteria for species. This incorrect reasoning seems to be an outgrowth of a view now abandoned by nearly all geneticists and students of evolution that species possess one sort of character and races another, or in other words, that species display qualitative differences and races quantitative differences which blend or The truth of the matter is that both types of characters may differentiate races from races and species from species and that there is no sharp distinction between these kinds of characters anyway. Intergradation in its typical manifestation in continental areas indicates two things: (1) blending inheritance of the characters involved, and (2) frequent, if not free, interbreeding of individuals. Hybridization indicates: (1) non-blending or alternate type of character manifestation, usually with few genetic determiners involved in any one character, and (2) free, or partial, or at least a little, interbreeding. Since the kind of inheritance and the kind of character expression, whether blending or of discontinuous-alternate type, are not decisive criteria of species, what do these situations tell us about the important matter of freedom of interbreeding and subsequent fertility? One implies freedom or something close to it, the other may involve a state of freedom but also a state of rare, sporadic interbreeding. The two situations are thus not clearly contrasted in this important matter. It is well to look into the exact meaning of the words and intergrade. To hybridize is to cross or interbreed organisms that are different, whether of varieties, races, species, or genera. How different must they be? The difference may be a matter of eye color, or only of bristle size or number in Drosophila. It may involve black or white rats of the same species, or short and tall corn plants. When, in wild populations, we can see that hybridization must have taken

Population Dynamics and Adaptive Evolution of Animals

PROBLEMS of fluctuation in numbers in animal populations are of direct interest both to those concerned with the exploitation of natural animal resourced (fish, game, fur-bearing animate) and to students of evolution. Theoretical research on these problems is visualized by the author mainly as investigation of facts illustrating the Darwinian theory of the struggle for existence. Acoording to him the overwhelming importance of the struggle for existence is overlooked by the majority of theoretical biologists, and the accotints for the spread of such rdealistic theories as hologenesns, Lamarckism, nomogenesis, etc., while a tendency to ignore Darwinian ideas is characteristic of 'bourgeois science'. Such a view of Western biology may cause some surprise, as it is scarcely justified by the modern trends in its development, which appear to have remained largely unknown to the author. In fact, when discussing evolutionary theories later in the book, he directs his polemics against O. Abel and Beurlen, neither of whom can be called o very modern, or typical of Western science. Population Dynamics and Adaptive Evolution of Animals By S. A. Sewertzoff. (In Russian.) Pp. 316. (Leningrad: Academy of Sciences U.S.S.R. Moscow, 1941.) 11 roubles.

Evolution, and the Age and Area Hypothesis

DR. WILLIS'S assumption that new genera and new species may arise directly by mutation is rather startling to most students of evolution. He supports his contention, chiefly, by the observation that the frequency distribution of genera containing 1, 2, 3 species follows a regular, hollow curve, with monotypic genera the most frequent. Mr. Yule (Phil. Trans. Roy. Soc. B. 403) has shown that assuming (1) that species give new species by chance at an irregular rate, constant on the average and the same for all species, and (2) that species give new genera in the same way, by mutation, then the frequency distribution of size of genus will approximate closely to that observed in Nature; the latter being such that log. number of species plotted against log. number of genera gives practically a straight line. That all genera arise directly by mutation is implied throughout, since they are all supposed to start as monotypes. Finally, Mr. Yule concludes that viable specific mutations probably do not occur, in all the flowering plants over the whole earth, more often than about once in thirty years; hence that our failure to observe them cannot disprove their occurrence. This conclusion is disquieting; and we clearly cannot accept this mechanism if we can otherwise explain the evidence adduced for it.

Versuch einer philosophischen Selektionstheorie

THE object of this essay is to place the theory of selection on a purely deductive and abstract basis as distinguished from the concrete form in which it is made familiar to modern evolutionists through the writings of Darwin and Wallace. From the philosophical point of view, it is certainly desirable that we should realise that the particular kind of selection which is operative in species transformation, or in the production of artificial races, is only one phase of selection in the abstract, and any attempt to make our ideas on this subject more exact will be welcome to philosophical students of evolution. It is, in fact, somewhat remarkable that, while all working naturalists have now accepted the doctrine of evolution in one form br another, comparatively few have attempted to examine into the philosophical basis of the principles of selection. Dr. Unbehaun's discussion of the subject, if not exhaustive, is at any rate very suggestive, and as a contribution to a much neglected aspect of the philosophy of evolution the work may be safely recommended to English biologists. Versuch einer philosophischen Selektionstheorie. Von Dr. Johannes Unbehaun Gotha. Pp. 150. (Jena: Gustav Fischer, 1896.)