N the inorphologic identification of chromosomes, the location of the I centromere is the most useful landmark, and one which is characterized by great constancy. It would seem that not much could be added to the definitions by E. B. WILSON (1928) of the locations on the chromosome of the centrornere or, in the terminology of that time, the spindle attachment: “Attachment of the chromosome to the spindle is commonly limited to a small area, and is of two general types, namely: (1) terminnl or telomitic and (2) non-ferminal or atelomitic, being in the former case at one end, and in the latter at some other point or points. Non-terminal attachment may be at the middle point (median) or at an intermediate point (submedian, sub-terminal). All gradations exist between these various cases;” (I.c., p. 130-131). In the acconipanying picture (l.c., Fig. 56, p. 132), here reprinted as Fig. l., the four locations of median, submedian, subterminal and terminal are represented, and, in addition, “lateral”, which corresponds to the modern term “diffuse centromere”. Nevertheless, at the present time, with the immense increase in research activity in mammalian cytology, the terminology of the centromeric position has become burdened by much obscurity and confusion. One cause of confusion is that different authors, and even the same author on different occasions, have used the terms median, submedian etc. with great amplitude, and it is often difficult to know in a specific case what each term signifies. Another cause of confusion is that a set of terms for chromosomes with specific centromeric positions, such as metacentric, acrocentric, telocentric, have come into wide usage without being clearly defined in relation to the positional terms median, submedian, subterminal and terminal. During the spring of 1963 the present writers exchanged epistolary

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1601-5223.1964.tb01953.x

Nomenclature for centromeric position on chromosomes

SUMMARY
1According to ‘Gause's hypothesis’ a corollary of the process of evolution by natural selection is that in a community at equilibrium every species must occupy a different niche. Many botanists have found this idea improbable because they have ignored the processes of regeneration in plant communities.
2Most plant communities are longer-lived than their constituent individual plants. When an individual dies, it may or may not be replaced by an individual of the same species. It is this replacement stage which is all-important to the argument presented.
3Several mechanisms not involving regeneration also contribute to the maintenance of species-richness:


(a). differences in life-form coupled with the inability of larger plants to exhaust or cut off all resources, also the development of dependence-relationships,
(b) differences in phenology coupled with tolerance of suppression,
(c) fluctuations in the environment coupled with relatively small differences in competitive ability between many species,
(d) the ability of certain species-pairs to form stable mixtures because of a balance of intraspecific competition against interspecific competition,
(e) the production of substances more toxic to the producer-species than to the other species,
(f) differences in the primary limiting mineral nutrients or pore-sizes in the soil for neighbouring plants of different soecies, and
(g) differences in the competitive abilities of species dependent on their physiological age coupled with the uneven-age structure of many populations.


4The mechanisms listed above do not go far to explain the indefinite persistence in mixture of the many species in the most species-rich communities known.
5In contrast there seem to be almost limitless possibilities for differences between species in their requirements for regeneration, i.e. the replacement of the individual plants of one generation by those of the next. This idea is illustrated for tree species and it is emphasized that foresters were the first by a wide margin to appreciate its importance.
6The processes involved in the successful invasion of a gap by a given plant species and some characters of the gap that may be important are summarized in Table 2.
7The definition of a plant's niche requires recognition of four components:


(a) the habitat niche,
(b) the life-form niche,
(c) the phenological niche, and
(d) the regeneration niche.


8A brief account is given of the patterns of regeneration in different kinds of plant community to provide a background for studies of differentiation in the regeneration niche.
9All stages in the regeneration-cycle are potentially important and examples of differentiation between species are given for each of the following stages:


(a) Production of viable seed (including the sub-stages of flowering, pollination and seed-set),
(b) dispersal, in space and time,
(c) germination,
(d) establishment, and
(e) further development of the immature plant.


10In the concluding discussion emphasis is placed on the following themes:


(a) the kinds of work needed in future to prove or disprove that differentiation in the regeneration niche is the major explanation of the maintenance of species-richness in plant communities,
(b) the relation of the present thesis to published ideas on the origin of phenological spread,
(c) the relevance of the present thesis to the discussion on the presence of continua in vegetation,
(d) the co-incidence of the present thesis and the emerging ideas of evolutionists about differentiation of angiosperm taxa, and
(e) the importance of regeneration-studies for conservation.

The maintenance of species-richness in plant communities: the importance of the regeneration niche

1Department of Genetics, University of Georgia, Athens, Georgia 30602; 2NMFS/ CZES, Genetics, 2725 Montlake Boulevard East, Seattle, Washington 98112; 3Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 29801; ~Department of Microbiology and Immunology, School of Medicine, University of California, Los Angeles, California 90024; -SSchool f Veterinary Medicine, Virginia Tech University, Blacksburg, Virginia 24046

/pdf/intraspecific-phylogeography-the-mitochondrial-dna-bridge-5e2au5n1qt.pdf

Intraspecific Phylogeography: The Mitochondrial DNA Bridge Between Population Genetics and Systematics

The harmful effects of close inbreeding have been noticed for many centuries (34, 35, 165). With the rise of Mendelian genetics, it was realized that the main genetic consequence of inbreeding is homozygosis (165, Ch. 2). Two main theories were early proposed to account for inbreeding depression and its converse, heterosis (the increase in vigor observed in an F1 between two inbred lines). These are the overdominance and partial dominance hypotheses, discussed in more detail below. Research into this question has continued up to the present, and this is one of the topics that we discuss. Darwin (35, 36) was the first to point out that the evident adaptations of many plants for ensuring outcrossing could be understood in terms of the selective advantage of avoiding inbreeding depression. We review the evidence that the evolution of breeding systems of animals and plants has been significantly influenced by the occurrence of inbreeding depression. In order to do this, we consider the contemporary genetic theory of inbreeding depression and heterosis, and the experimental data concerning the strength of inbreeding depression. We emphasize data and theory relevant to natural, rather than domesticated, populations as we are chiefly concerned to evaluate the evolutionary significance of inbreeding depression. We do not attempt to give a complete bibliography of this very extensive field but try to concentrate on what seem to be the most significant findings in relation to this aim.

https://www.jstor.org/stable/pdf/2097132.pdf

Inbreeding depression and its evolutionary consequences

Publisher Summary This chapter focuses on evolutionary significance of phenotypic plasticity in plants. The expression of an individual genotype is modified by its environment. The amount by which it can be modified is termed its plasticity. This plasticity can be either morphological or physiological; these are interrelated. The plasticity of a character is related to the general pattern of its development, and apart from this, that plasticity is a general property of the whole genotype. Plasticity of a character appears to be specific for that character, specific in relation to particular environmental influences, specific in direction, under genetic control not necessarily related to heterozygosity, and able to be radically altered by selection. Because plants are static organisms, plasticity is of marked adaptive value in a great number of situations. Examples of all these situations in plant species are discussed. They indicate that adaptation by plasticity is a widespread and important phenomenon in plants and has evolved differently in different species. The mechanisms involved in plasticity are varied. At one extreme, the character shows a continuous range of modification dependent on the intensity of the environmental stimulus. At the other, the character shows only two discrete modifications. The stimulus causing these modifications may be direct or indirect. The mechanisms found can be related to the particular environmental situation involved.

Evolutionary Significance of Phenotypic Plasticity in Plants

Among the approximately 130 species ofHibbertia found in Australia, there are tall shrubs, low or trailing shrubs and vines bearing a diversity of leaves as to shape and venation pattern. Flowers are solitary, in leafy cymes or in false spikes, and display various gradual and abrupt transitions from vegetative to reproductive appendages. In the androecium, stamen number is highly variable both between and within species. Some sections have radial symmetry, others bilateral symmetry of the androecium and gynoecium. Follicle number varies from 10 to 1. Basic chromosome numbers of n = 4, 5, 8, 9, 10, 12 and 13 have been found in various sections, and occasional higher numbers, up to n = 64, indicate the presence of polyploidy. Habitats vary from tropical savanna through rain forest margins, wet and dry sclerophyll forests, heaths, sphagnum swamps, and mallee scrub to desert margins. The principal center of diversity is southwestern Australia, less diverse centers are in southeastern and northern Australia. With respect to leaf size, structure and venation; floral symmetry; and chromosome numbers; the diversity found among the species ofHibbertia exceeds that found in all but a few genera of Angiosperms, and is greater than that in any other exclusively woody genus. Nevertheless, individual species are relatively constant with respect to both morphology and ecological preferences.

Species diversity, ecology and evolution in a primitive Angiosperm genus: Hibbertia ( Dilleniaceae )

Edward J. Klekowski, Jr., Hans Mohr,* and Nina Kazarinova-Fukshansky* Botany Department University of Massachusetts Amherst, MA 01003 U.S.A. *Biologisches Institut II Universitat Freiburg Schanzlestrasse 1 D-7800 Freiburg im Breisgau Federal Republic of Germany Since in plants Weismann's doctrine of the separation of soma and germ is invalid, somatic mutations may occur in cell lines which in turn give rise to meiocytes. Although some mutations are adaptive, the majority of mutations are disadvantageous and ultimately reduce the fitness of a ramet (or genet). Many anatomical and developmental characteristics may reduce the mutation frequency within ramets. Apical meristems may influence mutation frequency by either reducing the mutation rate or enhancing diplontic selection. The former is possible in meristems with permanent apical initials (structured meristems) through nonrandom DNA strand segregation and in meristems with impermanent apical initials (stochastic meristems) which have a meristeme d'attente. Diplontic selection is maximized in apical meristems in which the initials and derivative cells are least determined ontogenetically, thus allowing maximum intercellular competition (stochastic meristems). In spite of these mechanisms, the individual ramet meristems in long-lived genets still may be expected to diverge genetically through the fixation of mutations in the small cell pools within these meristems by mechanisms similar to Muller's Ratchet. The continued accumulation of disadvantageous mutations within

Genetics, Development, and Evolution

In recent years, several evolutionists have challenged the synthetic theory as developed 30 to 40 years ago. In recent papers (Stebbins and Ayala, 1981; Stebbins, 1982c) I stated my opinion that while change is always necessary, we need not cast aside the theory that was developed on the basis of a half century of research, and is essentially a synthesis of Darwin's insight, coupled with the experimental research of leading geneticists, as well as of the more progressive systematists and paleontologists. What we need, rather, is a shift in emphasis, .and a new basis for collaboration. Scientists who are observing evolution in action must understand the viewpoint of those who, largely through the analysis of the fossil record, are charting the course of evolution over millions of years of time, and vice versa.

G. Ledyard Stebbins

Papers

Species diversity, ecology and evolution in a primitive Angiosperm genus: Hibbertia ( Dilleniaceae )

Genetics, Development, and Evolution

Perspectives in evolutionary theory.

Grass Systematics and Evolution: Past, present and future

Classification of flowering plants