DNA and chromosome counts differ widely between different organisms. This
dissimilarity contradicts the similarity we expect from common descent.
Chromosome counts should be either the same because the different forms of
life descended from a common ancestor (Pathlights n.d.), or more complex
as organisms get more complex (Thompson and Butt 2001). Neither is the
case. For example, humans have 46 chromosomes, some ferns have 512, and
some gulls have 12.
Chromosome counts are poor indications of similarity; they can vary
widely within a single genus or even a single species. The plant genus
Clarkia, for example, has species with chromosome counts of n = 5,
6,
7, 8, 9, 12, 14, 17, 18, and 26 (Lewis 1993). Chromosome counts in
the house mouse species (Mus domesticus) range from 2n = 22 to 40
(Nachman et al. 1994).
Chromosomes can split or join with little effect on the genes
themselves. One human chromosome, for example, is very similar to two
chimpanzee chromosomes laid end to end; it likely formed from the
joining of two chromosomes (Yunis and Prakash 1982). Because the
genes can still align, a change in chromosome number does not prevent
reproduction. Chromosome counts can also change through polyploidy,
where the entire genome is duplicated. Polyploidy, in fact, is a
common mechanism of speciation in plants.
References:
Lewis, Harlan, 1993. "Clarkia", In: The Jepson Manual: Higher Plants
of California, J. C. Hickman, ed., Berkeley: University of
California Press, pp. 786-793.
Nachman, M. W., S. N. Boyer, J. B. Searle and C. F. Aquadro, 1994.
Mitochondrial DNA variation and the evolution of Robertsonian
chromosomal races of house mice, Mus domesticus. Genetics
136(3):
1105-1120.
Yunis, Jorge, and Om Prakash, 1982. The origin of man: A chromosomal
pictorial legacy. Science 215: 1525-1530. See
http://www.indiana.edu/~ensiweb/lessons/chr.bk1.html
for Fig. 2a: Human
and chimpanzee chromosomes 1-4.
