Chromosomal Evolution and Speciation Revisited

Abstract

Many descriptive models of chromosomal evolution have been formulated over the past two decades. We review the assumptions and predictions of these models and consider some pertinent evidence. One model of adaptive divergence, the canalization model, is difficult to test, and where predictions are explicitly stated, empirical data generally do not support the model. However, few adequate data sets are available to test this hypothesis, and it deserves further study. Several process-oriented models of chromosomal speciation are reviewed with respect to genetic isolating mechanisms, the fitness components of individual rearrangements, and the permissible population structures. From the last we attempt to estimate qualitatively the expected genetic correlates of each model to maximize its testability. Theoretical and empirical inadequacies are discussed and suggestions are made for improvement. Both phyletic and speciation models of chromosomal evolution have often been based on broad comparative studies of large taxonomic groups, but more precise testing will require multidisciplinary populationlevel studies of organisms hypothesized to be undergoing some form of chromosomally-mediated speciation or adaptive radiation.

Multivalents resulting from monobrachial homologies within a hybrid zone in Dichroplus pratensis (Acrididae): meiotic orientation and segregation

“…Chromosomal differences between taxa have long been recognized but the role of chromosomal rearrangements in evolution is controversial. Opinions range from an almost total rejection of any role for the chromosome to the more extreme positions that have been put forward by Matthey, 1966;White, 1968White, , 1969White, , 1970White, , 1973White, , 1975White, , 1978bWhite, , 1982Mayr, 1970Mayr, , 1982Bush, 1975;Bush et al, 1977;Bickham & Baker, 1979;Hewitt, 1979Hewitt, , 1985John, 1981John, , 1983Templeton, 1981;Kirig, 1982Kirig, , 1985Kirig, , 1987Patton & Sherwood, 1983;Reig, 1984;Camacho, 1985;Lande, 1985;Baker & Bickham, 1986;Sites & Moritz, 1987;Bidau, 1988Bidau, , 1990Nachman & Myers, 1989. Some biologists, however, have maintained a judicious and critical view by analysing available data in an impartial way (John, 1981;Patton & Sherwood, 1983;Sites & Moritz, 1987).…”

“…analysed although its importance has been emphasized (John, 1981(John, , 1983Sites & Moritz, 1987). Robert- tion between taxa and have been thoroughly studied in 228 C. J. BIDAU mammals and grasshoppers although their role as postmating barriers to gene flow is debatable.…”

“…Robert- tion between taxa and have been thoroughly studied in 228 C. J. BIDAU mammals and grasshoppers although their role as postmating barriers to gene flow is debatable. Some authors attach great importance to them (Capanna, 1982;White, 1978b) but others have challenged this (Baker & Bickham, 1986;Sites & Moritz, 1987;Searle, 1988). The ability of a fusion to generate reproductive isolation depends on the chromosomes involved and their meiotic behaviour in the heterozygotes (Ford & Evans, 1973;.…”

“…Several chromosomal speciation models have been proposed Sites & Moritz, 1987) and some rest upon differences for single rearrangements (White, 1968(White, , 1982King, 1981) while others, rest upon for multiple ones (HaIl, 1983;White, 1978b). Robertsonian variation is readily identifiable and therefore received special attention in this respect (John & Hewitt, 1968;John & Freeman, 1975;John, 1981John, , 1983Searle, 1988).…”

“…In addition, the role of monobrachial homologies in the establishment of postzygotic reproductive isolation and evolutionary divergence, is the subject of speculation, discussion and model-making (Capanna, 1982;Capanna et al, 1985;Baker & Bickham, 1986;Searle, 1986aSearle, , 1988Sites & Moritz, 1987;Bidau, 1988Bidau, , 1990Redi & Capanna, 1988). The results presented in this paper are discussed in relation to the present knowledge of multivalent behaviour.…”