The correct pairing of the homologous chromosomes and their correct segregation (a chromosome for each daughter cell) are key to the proper development of the new cells, and specially of the new individual if we are talking about the early stages of embryogenesis. If there is an error during the splitting of chromosomes, and one cell gets a double set and the other misses a chromosome, numerical chromosomal abnormalities occur. These numerical abnormalities can happen on autosomes or sexual chromosomes, and common examples are as Down Syndrome (trisomy 21), Patau Syndrome (trisomy 13), Turner Syndrome (45,X) and others.
The number of the trisomy indicates the chromosome pair that suffered the adding or the subtraction of its chromosome.
This eventual process of speciation by natural selection is illustrated by a sketch ... The integration of genetics with models of natural selection shed tremendous ... processes which cause adaptation within species also tend to create new species. ... Darwin, C. On The Origin of Species by Means of Natural Selection,