CHROMOSOME-BASED SPECIATION DISCUSSION
INDEX

There have been several email exchanges between some ENSI-using teachers and ENSI-Co-Director Craig Nelson, attempting to clarify how significant chromosomal changes can be sustained in a population, much less lead to new species. Part of this discussion has referenced examples of hybrid production in animals, e.g., the products of horse-donkey matings (mules and hinnies) and the fact that most of these hybrids are sterile. Horses have 64 chromosomes, donkeys have 62, and their hybrid offspring have 63. What’s perhaps amazing is that healthy hybrids, with either a chromosome missing, or an extra chromosome, depending on which parent it’s compared with, are produced at all. These discussions, bringing results of actual studies, help bring clarity to the issue.

CHROMOSOMAL SPECIATION MODELS
This first treatment is with a question posed by teacher Tom, and Dr. Nelson’s reply and a few published studies on the subject. The essential question: “How does the first chromosome alteration get passed to future generations?” Dr. Nelson first points out how different the issue is with plants and animals. He shares these studies:
Chromosome speciation: Humans, Drosophila, and mosquitoes, (by Francisco Ayala et al, PNAS, 2005).
Chromosomes, Conflict, and Epigenetics: Chromosomal Speciation Revisited, (by Judith Brown, et al, Genomics and Human Genetics, Annual Review, 2010).
Chromosomal speciation of humans and chimpanzees revisited: studies of DNA divergence within inverted regions, (by JM SZamalek, et al, in Cytogenetic and Genome Research, 2007).
CHROMOSOMAL SPECIATION Cascading Chromosomal Speciation and the Paradoxical Role of Contact Hybridization as a Sink for Gene Flow (by William P. Hall, Dept. of EPO Biology, U. of Colorado, Boulder, CO, 1979).

SURVIVAL of CHROMOSOMAL CHANGES
Excerpts from the informative blog by Robert Williams (with links to published academic sources.

CHROMOSOMAL SPECIATION: Further Explanations
Dr. Nelson’s responds to another teacher’s query about what Dr. Nelson said in his earlier email: “I hope that the number of fairly severe assumptions in that model make it clear why this form of speciation [chromosomally caused] is not the commonest form for animals.” Specifically, this teacher (Rebecca) asks “why isn’t it a common form of speciation?” If not, “then what IS the most common form of speciation in mammals?” “how did they get different?” And, “why [were] flowers used as an example since they don’t have the reproductive barrier.” Craig’s response is followed by an extended post by Tom (the original posting), with further comments and interesting added information.