Do the constraints of human speciation cause expression of the same set of genes in brain, testis, and placenta?

Evolution appears to be especially rapid during speciation, and the genes involved in speciation should be evident in species such as humans that have recently speciated or are presently in the process of speciation. Haldane's rule is that when one sex is sterile or inviable in interspecific F(1) hybrids, it is usually the heterogametic sex. For mammals, this implicates genes on the X chromosome as those particularly responsible for speciation. A preponderance of sex- and reproduction-related genes on the X chromosome has been shown repeatedly, but also mental retardation genes are more frequent on the X chromosome. We argue that brain, testis, and placenta are those organs most responsible for human speciation. Furthermore, the high degree of complexity of the vertebrate genome demands coordinate evolution of new characters. This coordination is best attained when the same set of genes is redeployed for these new characters in the brain, testis, and placenta. Copyright 2001 S. Karger AG, Basel.