Large Number Of Characters Research Articles

VARIATION WITHIN POPULATIONS AND CLASSIFICATION

SummaryVariation was measured in samples of populations of species from four plant families including tropical, temperate and cosmopolitan taxa. Populations of species of Asclepiadaceae, Bromeliaceae, Leguminosae and Umbelliferae were sampled. Employing a simple application of numerical phenetics and tested character lists, estimates of within‐group overall variation were made. The coefficients of phenetic variation (CPV) are compared among the various groups. The groups considered are populations as well as taxa at several levels in the classification hierarchy, although in all cases, individuals sampled randomly from putative populations are the operational taxonomic units (OTUs). A small number of individuals is employed with a large number of characters. This application of numerical phenetics serves both alpha taxonomic and biosystematic goals and an OTU x OTU matrix of distance values is the basis for obtaining estimates of a population's variability. The phenetic distances between OTUs are the variates. Distinct trends of within‐group variation are consistent as they were revealed in the four families. Intrapopulational coefficient of phenetic variation for populations of the tropical family Bromeliaceae is estimated as nearly double that for the other three families. The intraspecific variation as portrayed by CPV in the Asclepiadaceae and Leguminosae is about half that for the tropical family, Bromeliaceae. As sampled a clear distinction is evidenced in the change in variation between various taxonomic ranks for the families. The difference in CPV between the intrapopulational and intraspecific levels for the Leguminosae and Bromeliaceae is nearly double the difference between these two ranks for the Asclepiadaceae. Results show that the method has descriptive and heuristic value for systematics.

An optimal recognition algorithm for some sequences of patterns

1. The recognition of a line of text containing a large number of characters without gaps in the presence of noise can be performed by a method of comparison with standard characters, in spite of the extremely large number of standard lines. 2. For an exact solution of the problem of finding the standard line giving the greatest similarity (by correlation) with the patterns to be recognized, a comparatively small amount of storage and computation is required.

THE QUANTIFICATION OF SOME PLANT‐TAXA CIRCUMSCRIPTIONS

Intrataxon variability is estimated by the technique of numerical taxonomy. Taxa which had been determined by orthodox taxonomic methods are subjected to analyses by numerical phenetics. Comparisons are made of the amounts of cohesion among the members of different taxa in the Bromeliaceae at several taxonomic levels by computing the degrees of similarities between members of the same taxon and of different taxa. Results of these comparisons with several taxa reveal that the variability of a taxon in terms of phenetic distance computed with a large number of characters does not necessarily agree with a subjective appraisal of variability. The method facilitated difficult taxonomic determinations and provided an estimate of the relative evolutionary ages of different taxonomic groups.

A Numerical Taxonomic Analysis of the Peripsocidae of the Oriental Region and the Pacific Basin

The relationships of 83 species of psocids of the family Peripsocidae (Insecta: Psocoptera) were analysed numerico-taxonomically, using alternative methods of processing. The results of using different sets of adult morphological characters and different combinations of them were compared. The effect of admitting a large number of species with incomplete data (known from one sex only) into the system was investigated. Certain modifications of the accepted classification of the family were suggested. Numerical taxonomy was developed to provide an objective and repeatable method of evaluating the phenetic affinity between taxonomic units (see Sokal and Sneath, 1963). The method rests on the hypothesis that there are no special groups of genes the effects of which are confined to single mQrphological regions, and that a random sample of the genotype is best obtained by sampling many and various characters. A large number of characters is used and each character is considered to be of equal weight, and the larger the number of characters used, the more stable is the value of the overall similarity coefficient, up to an asymptote which will vary for each analysis. Rohlf (1963) used numerical taxonomic methods to compare the classification of Aedes using only adult characters with that using only larval characters and found a definite though low correlation between the resulting classifications. In 1963 Sokal and Michener (see Rohlf, 1963) compared the results of classifications based on female and male, and on head and non-head characters, and found similar values of correlation. In the study reported here species with incomplete data (known from one sex only) are admitted to the analysis by degrees, and the effect of these admissions on the classification based on individuals with information on both sexes is noted. Also, different sets of characters are omitted in different analyses and the results compared. Alternate methods in the processing of data are used, in order to find the combination best suited for the classification of Peripsocidae. MATERIAL AND METHODS General.-Eighty-three species of the family Peripsocidae representing three of the five genera were examined, using numerical taxonomic methods. Previously described species of which examples were unobtainable were not included in the study, due to reason which will be obvious later. A preliminary survey was run to find some 60 characters that vary among species (Sokal and Sneath, 1963). These characters were then carefully studied and their characteristic expression or dimensions recorded, males and females separately. Whenever possible, five of each sex (selected at random) were used. This figure represented over 30% of the sample in hand in 75% of the species studied. In the present study 146 characters were used, comprising 52 asexual characters, 52 female charters and 42 male characters. The distribution of these characters over the animal was as follows: 14 head characters, 45 thoracic characters and 87 abdominal characters. Of these, 90 are descriptive, 16 metric, 20 ratios, and 20 meristic. A histogram of these characters and the number thereof applicable to Ectopsocus and Peripsocus is provided in Fig. 1; the species, their code numbers, and the number of individuals used in this study are listed in Table 1. A description of characters in some detail can be found in the manuscript of a thesis by the junior author submitted for the degree of Ph.D. at the University of Hong Kong. Analyses of the distribution of the number of states for the various types of

Genetics: the centre of science.

It is not often possible to pinpoint the origin of a whole new branch of science accurately in time and place, because, as Isaac Newton said, there are usually so many precursors on whose shoulders the successor stands and is thereby able to see further than they. But genetics is an exception, for it owes its origin to one man, Gregor Johann Mendel, who expounded its basic principles at Brno on 8 February and 8 March 1865. If a precursor is a man who, at an earlier date, makes a discovery which his successor is able to expand into a general principle of universal validity, Mendel had no precursors. There were not wanting breeders who hybridized plants: Joseph Gottlieb Koelreuter, Carl Friedrich von Gaertner, and William Herbert, to mention only the chief names, but what they were studying was not really basic genetics at all. They wanted to know if sterility in a hybrid is the fault of the pollen-parent or the seed-parent, whether either parent could be held responsible for the characters of different specified regions of the plant, or which had prepotency over the characters of the hybrid. The parent races that they chose for their crossing experiments were either different species, or varieties differing in large numbers of characters, and the results which they obtained were chaotic, inconstant, and contradictory, and led to no general principles at all. This was the difference between previous attempts to study heredity, and the Mendelian revolution that resulted in genetics.

Intra-sexual selection in Drosophila

SINCE Darwin first wrote on the subject in 1871, sexual selection has been generally accepted as one of the basic facts of biology. The evidence in its favour seems, however, to be mainly circumstantial. Its existence has usually been inferred from sex differences depending on what are called secondary sexual characters which are supposed to have arisen as results of that selection. Such an approach has its dangers, and Huxley (1938) has made important criticisms of the original concept of sexual selection. He has shown that a large number of characters which have been attributed to sexual selection are unconnected with competition for mates. This is particularly the case in monogamous birds which offer some of the most striking examples of secondary sexual differences. In the first place monogamy, at least when the sexes are numerically equal, is the mating system least likely to develop sexual selection. In the second place, and more important, observations on bird behaviour have shown that much of the display of birds occurs after pairing, when competition must have ceased. Such sexual differences are concerned, either with inducing the female to copulate, or with maintaining the association of the sexes as long as it is necessary for the rearing of the young. Huxley therefore introduced the term epigamic to apply to characters which increased the fertility of a given mating and therefore had a selective value for the species as a whole. Epigamic selection includes the major part of what Darwin meant by sexual selection. It also includes selection for characters to which Darwin did not refer, such as the structure of copulatory organs, sex differences in frequency of crossing over, and the XY mechanism. It is only a special case of natural selection as generally understood. What remains of Darwinian sexual selection has been called intra-sexual selection, which denotes that it involves competition between members of one sex for mates. It can only indirectly affect the survival of the species and then is often deleterious (e.g. the cumbersome antlers of the stag). There is not invariably, however, a clear distinction between epigamic and intrasexual selection. In a promiscuous species like Drosophila pairing and copulation are synchronous. Courtship behaviour determines the number of mates and therefore enters into intra-sexual selection.

Genetics Since 1910

THE state of genetical knowledge in 1910 can be learnt from Bateson's “Mendel's Principles of Heredity”, of which the first two editions were published in 1909 and 1913. The principles which Mendel had shown to hold for seven contrasted pairs of characters in Pisum had been extended to a very large number of characters in many plant and animal species, including man. The main additions to these principles were the pure line, multiple allelomorphism, partial linkage between genes, epistasy, and one of the four types of sex linkage now known. The latter discovery, along with cytological work, had made the genetics of sex determination fairly clear. At least one case of extranuclear inheritance had been recorded.

The Age of Australopithecus

THE little fossil ape skull that was found at Taungs five years ago is, in the opinion of many, the most important fossil ever discovered. It is manifestly the remains of an anthropoid ape somewhat allied to the chimpanzee, and of about the same size. But it differs from both it and the gorilla in a large number of characters, and in almost all these characters it resembles man. It thus seems highly probable that it is very near to the ape from which man sprang, and possibly a representative of the very genus. One difficulty has been our ignorance of the age of the cave deposit. The bone breccia found in most caves has proved to be of Pleistocene age, and if the Taungs cave is also of this period, then Australopithecus would be too recent to be a possible human ancestor, as man is known to have existed in the Pliocene.

A Revision of the Genus “Portunus” (A. Milne-Edwards, Bell, etc.)

1. The characters of the species of “Portunus” are redescribed in detail and comparatively.2. On the basis of the characters studied an attempt is made to determine the relationships of the constituent species, both within the genus and with allied genera, P. corrugatus and P. arcuatus, being regarded as the more primitive forms.3. P. marmoreus and P. holsatus are separated by a large number of characters.4. The status of various foreign, fossil and doubtful species is discussed.5. An account is given of the present state of knowledge regarding the distribution of the species.6. The synonymy of the generic name is considered at length.