To test the hypothesis that population subdivision into small demes promotes both rapid speciation and evolutionary changes in gene arrangement by inbreeding and *drift, we estimated rates of speciation and rates of chromosomal evolution in 225 genera of vertebrates. Rates of speciation were estimated by considering the number of living species in each genus and the fossil record of each genus as well as information about extinction rates. Speciation rate was strongly correlated with rate of chromosomal evolution and average rates of speciation in lower vertebrate genera were one-fifth those in mammalian genera. Genera with high karotypic diversity and rapid speciation rates may generally have small effective population size (Ne), whereas large Ne values may be associated with karyotypically uniform genera and slow rates of speciation. Speciation and chromosomal evolution seem fastest in those genera with species organized into clans or harems (e.g., some primates and horses) or with limited adult vagility and juvenile dispersal, patchy distribution, and strong individual territoriality (e.g., some rodents). This is consistent with the above hypothesis regarding the evolutionary importance of demes. Among evolutionary biologists there is a widespread acceptance of the hypothesis that small populations are essential for rapid speciation (1-7). According to this hypothesis, new species may arise from populations initially founded by a small number of individuals isolated on the periphery of the range of the parental species-the founder principle (3-5)-or in small demes maintained by social structuring and ecology (7,8). Widespread also is the idea that small populations are essential for chromosomal evolution, and some authors suggest that the two processes-speciation and chromosomal evolution-may be causally related (7)(8)(9)(10)(11)(12). It is now possible to test these ideas quantitatively because methods have been developed recently for estimating both rates of speciation (13,14) and rates of evolutionary change in gene arrangement (8,15). Unfortunately, few estimates of effective population size are available. Hence, it is not easy to make a direct examination of the relationship between population size and rates of evolution. It is possible, however, to examine quantitatively the relationship between rate of speciation and rate of chromosomal evolution. If both processes are dependent on the occurrence of small demes, the two rates should be correlated. Such a correlation was found in higher plants (14).The present article deals with vertebrates. Rates of chromosomal evolution vary greatly in these animals, being an order of magnitude higher in most mammals than in most other vertebrates (8,15). It was important, therefore, to estimate rates of speciation in a wide variety of vertebrates. The results of such estimates are reported below and compared with the results of an improved method for estimating rate of chromosomal evolution.

To test the hypothesis that the evolution of organisms is dependent to a large degree on gene rearrangement, we devised a way of estimating rates of evolutionary change in karyotype. This non-biochemical method is based on consideration of chromosomal variability within taxonomic groups having a fossil record. The results show that chromosomal evolution has been faster in placental mammals than in other vertebrates or molluscs. This finding is consistent with published evidence that placentals have also been evolving unusually fast in anatomy and way of life. However, the structural genes of placentals seem not to have experienced accelerated evolution. Possibly, therefore, anatomical evolution may be facilitated by gene rearrangement.To explain how placentals achieved this high rate of chromosomal evolution, we consider the process by which a new gene arrangement becomes fixed and spreads.

The in-house examinations were of relatively low quality. The quality of examination questions can be significantly improved by providing question writers with formal training.