Closely related species often differ karyotypically due to the fixation of chromosomal translocations and inversions which are generally deleterious when heterozygous but have normal fitnesses when homozygous (White, 1973; Muller, 1954, 1956). The fixation of such chromosomal rearrangements requires small population sizes, because they are selected against when in the minority but favored when in the majority, and hence they can become established in a local area only through random genetic drift. Naturalists have long known that most species exist in subdivided populations where each individual has a limited number of potential mates; and recent biogeographic studies, especially on islands, have emphasized the high turnover rates of local populations by extinction and colonization (Simberloff, 1974; Spight, 1974). The present paper develops population genetic models to determine how the fixation rates of negatively heterotic chromosomal rearrangements are influenced by the spontaneous occurrence rates, strengths of selection against heterozygotes, the effective size of demes (or local populations), interdeme migration rates, local extinction and colonization, and other factors. Data on chromosomal rearrangements concerning spontaneous occurrence, selection coefficients, and fixation rates in phylogeny are then used to draw inferences about the breeding structure of species during longterm evolution, particularly the effective size of demes. The effective size of a population through time, Ne, is the number of breed-

Effective deme sizes during long-term evolution estimated from rates of chromosomal rearrangement.

The neo-Darwinian synthesis that resulted from the integration of Mendelian genetics into evolutionary theory has dominated evolutionary biology for the last 30 to 40 years, due largely to its agreement with a huge body of experimental and observational data. The classic works representative of this school of thought come from the fields of genetics (Fisher, 1930; Wright, 1931; Haldane, 1932; Dobzhansky, 1937; Muller, 1940), development (de Beer, 1940), zoology, (Huxley, 1942; Mayr, 1942; Rensch, 1959), botany (Stebbins, 1950), and paleontology (Simpson, 1944, 1953). These authors broadly agree with Darwin's view that most important evolutionary changes at the level of the visible phenotype, as revealed by paleontological and systematic studies, have resulted from natural selection acting on variation within populations. This variation is ultimately due to mutations that arise at random with respect to the direction of selection. A role for evolutionary forces other than selection and mutation, such as random genetic drift, is of course recognized by neo-Darwinists, but selection is regarded as the main guiding force of phenotypic evolution. Within this general framework, there has been plenty of room for vigorous disagreements about the relative importance of different processes, such as the Wright-Fisher debate on the significance of random genetic drift. Recently, however, certain elements of neo-Darwinism have been sharply challenged by advocates of the "punctuated equilibria" theory of evolution (Eldredge and Gould, 1972; Gould, 1977, 1980; Stanley, 1975, 1979; Gould and Eldredge, 1977). Indeed, Gould (1980) states:

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1982.tb05068.x

A neo-darwinian commentary on macroevolution.

A partial molecular linkage map of the Musa acuminata diploid genome is presented. This map is based on 58 RFLP, four isozyme and 28 RAPD markers segregating in an F2 population of 92 individuals. A total of 90 loci was detected, 77 of which were placed on 15 linkage groups while 13 segregated independently. Segregation distortions were shown by 36% of all loci, mostly favoring the male parent. Chromosome structural rearrangements were believed to be one of the main causes of these distortions. The use of genetic linkage data to further the genetic and evolutionary knowledge of the genus Musa, as well as to help improve the design of breeding strategies, is discussed.

A molecular marker-based linkage map of diploid bananas (Musa acuminata)

The statistical analysis of 95 cases of reciprocal translocations involving autosomes detected among about 10,000 patients studied with the R-banding technique gives the following information: 1. An excess of break points exists for chromosome arms 4p,9p, 10q, 21q, and 22q and a deficiency for 1p, 2p, and 6q. Furthermore, there are relatively more break points in the small arms than in the large arms, when the translocation is ascertained through an unbalanced translocation carrier. Except for chromosome 22, an ascertainment bias explain this non random distribution. 2. An excess of telomeric break points exists in all cases of translocations ascertained through unbalanced carriers, and an excess of centromeric break point exists in the case of 3:1 and 1:3 segregations only. These excesses are also explained by an ascertainment bias. 3. The break points are located usually at the junction of the bands (interfaces). 4. The size of the chromosomal imbalance varies in the ascertainment classes. It is very large in cases ascertained through balanced carriers (at least one break point is far from the telomere), large in cases ascertained through abortion, and relatively moderate in cases ascertained through unbalanced translocation carriers (at least one break point is juxta telomeric). 5. An excess of balanced reciprocal translocations exists in our sample of mentally retarded and malformed children (position effect?). 6. An excess of balanced reciprocal translocations (not involving chromosome 21) exists among the trisomics 21 and their parents (interchromosomal effect?). 7. A large excess of maternal transmission exists in cases of 3:1 segregation of reciprocal translocation.

Systematic analysis of 95 reciprocal translocations of autosomes

Publisher Summary The purpose of this chapter is to discuss the origin and cytogenetics rice. Rice and wheat are the most important food plants of man. The chapter reviews that the importance of rice is much more than the production figures would indicate. About 90% of the rice is produced and consumed in South and Southeast Asia. It forms the staple food of 54% of all humans, against 34% of wheat and only 4% of maize. Compared to its importance, much less is known about the genetics of rice than of other crop plants. The chapter also discusses that the cultivated rice's of the world belong to two species, sativa, and glaberrima. Commonly known as Asiatic rices, sativa is the species cultivated throughout the rice-growing areas of the world. However, it is in South and Southeast Asia, where most of the rice is grown, that both sativa and wild rice's occur in great profusion and variability.

Origin and cytogenetics of rice

The foregoing survey of the literature shows that translocations are of wide-spread occurrence in plants. Most of them have been found in the angiosperms: in 49 genera of dicots and 47 genera of monocots; but some have been reported in the gymnosperms (in two genera), also in the fungi. Studies at meiosis will undoubtedly reveal translocations in many other species, either in occasional plants or in different groups isolated geographically. In certain cases they seem to have played a part in the evolution of present karyotypes. In a number of cases, the presence of translocations has not been definitely established, but they are one possible explanation of the observed configurations. The survey reveals the need for additional detailed studies of interchanges. What accounts for directed segregation in the interchange rings in certain species? For each of the factors which has been suggested there is one or more species which does not behave in the predicted manner. Experimentally produced interchanges in other species having these features would be useful; also in species having directed segregation of the sex chromosomes. A discussion of how they have been and how they might be used as tools in the investigation of other cytological and cytogenetic problems and also certain applications to practical breeding have been included.

C. R. Burnham

Papers

Chromosomal interchanges in plants