[Role of chromosomes in evolution: a new interpretation].

The number of possible meiotic segregations of paternal and maternal chromosomes is 2N (N = haploid chromosome number). In eutherian Mammals, where large variations of N exist, 2N may be very different in related species, genes, or families. For instance, in Cercopithecidae, species with the highest value (N = 36) make 2(15) = 32,768 more gametic chromosome combinations than those with the lowest value (N = 21). It is also shown that the number of chiasmata varies from species to species and is proportional to the haploid number of arms NF/2 of the karyotype. Thus, increase or decrease of both N and NF/2, often concomitant, both increase or decrease genetic mixing. The chromosomal phylogeny of Primates, Carnivora and Rodentia shows that pure dichotomic evolution is rare, at contrast with populational (non dichotomic) evolution which seems most frequent. In these Mammals, the few examples of dichotomic evolution are observed in groups with low values of N and NF/2. It is concluded that dichotomic evolution, which should be favoured by the transmission of groups of linked mutant alleles, is prevented, in most instances, by the high recombination rate in karyotypes with high values of N and NF/2. Thus the mode of speciation would depend on karyotype modifications by long term effects on population genetics, in addition to the immediate effects of gametic barriers due to chromosomal rearrangements in the heterozygous state.