Do you have any references of the cases you describe for mammalia?
Missed that earlier. First two non-
Sorex examples. The presence of complex chromosomal variations for the second example,
Mus musculus, is relatively well known and far more has been written about it than just the two references I mention here:
* Pieczarka, de Souza Barros, de Faria Jr, & Nagamachi (1993). Aotus from the southwestern Amazon region is geographically and chromosomally intermediate between A. azarae boliviensis and A. infulatus. Primates 34(2): 197-204.
* Castiglia & Capanna (1999). Contact zones between chromosomal races of Mus musculus domesticus. 1. Temporal analysis of a hybrid zone between the CD chromosomal race (2n=22) and populations with the standard karyotype. Heredity 83(3): 319–326.
* Solano, Castiglia & Capanna (2009). Chromosomal evolution of the house mouse, Mus musculus domesticus, in the Aeolian Archipelago (Sicily, Italy). Biological Journal of the Linnean Society 96 (1): 194-202.
The situation in
Sorex is highly complex, where there are both so-called "chromosomal races" and even monotypic species with major variations in their chromosomes. The most extreme is
S. araneus where the diploid number varies from 2n = 20-33! Numerous articles + at least one Ph.D. Thesis dealing with this have been written - indeed, few other mammalian groups have had their chromosomes analysed to the same extend as the widespread Palearctic
Sorex and below are just a sample of articles. Note that the first deals with another species (
S. tundrensis; 2n = 31-41), but there are yet other species with known chromosomal variations (e.g.
S. trowbridgii; 2n = 31-42), and several species from this genus are still rather poorly known (i.e. they may display some chromosomal variations, but at this point it remains unknown).
* Lukáĉová, Zima & Volobouev (1996). Karyotypic variation in Sorex tundrensis (Soricidae, Insectivora). Hereditas 125(2-3): 233-238.
* Polyakov, Chadova, Rodionova, Panov, Dobrotvorsky, Searle & Borodin (1997). Novosibirsk revisited 24 years on: chromosome polymorphism in the Novosibirsk population of the common shrew Sorex araneus L. Heredity 79(2), 172–177.
* Narain & Fredga (1997). Meiosis and fertility in common shrews, Sorex araneus from a chromosomal hybrid zone in central Sweden. Cytogenetics and cell genetics 78(3-4): 253-259.
* Fedyk & Chętnicki (2007). Preferential segregation of metacentric chromosomes in simple Robertsonian heterozygotes of Sorex araneus. Heredity 99(5): 545–552.
* Fredga & Narain (2008). The complex hybrid zone between the Abisko and Sidensjö chromosome races of Sorex araneus in Sweden. Biological Journal of the Linnean Society 70(2): 285-307.
* Ratkiewicz, Fedyk, Banaszek, Gielly, Chetnicki, Jadwiszczak & Taberlet (2002). The evolutionary history of the two karyotypic groups of the common shrew, Sorex araneus, in Poland. Heredity 88(4): 235–242.
* Borodin, Karamysheva, Belonogova, Torgasheva, Rubtsov, & Searle (2008). Recombination Map of the Common Shrew, Sorex araneus (Eulipotyphla, Mammalia). Genetics 178(2): 621–632.
* Andersson, Narain, Tegelström & Fredga (2004). No apparent reduction of gene flow in a hybrid zone between the West and North European karyotypic groups of the common shrew, Sorex araneus. Molecular Ecology 13(5): 1205-1215.
* Wójcik, Ratkiewicz & Searle (2002). Evolution of the common shrew Sorex araneus: chromosomal and molecular aspects. Acta Theriologica 47 (suppl. 1): 139-167.
Of course it should be noted that I did not say this was the norm among mammals (simply that it occurs; there are also more known examples than the above + quite certainly several that still haven't been discovered, as the chromosomes of most mammals still haven't been analysed in detail), and if speaking about the mammal you mentioned, the human, major changes in the chromosomes are generally either debilitating or lethal (at least all the ones I know).