Two interesting additional points: sympatric species show greater prezygotic isolation than allopatric species pairs. This pattern is consistent with the reinforcement hypothesis and suggest that reinforcement can act (see figs. 16.14 - 16.16, pg. 454-455). A second observation: less genetic distance between species pairs that produce sterile or inviable males than between species pairs that produce sterile or inviable females (D (A-B)sterile males < D (A-B)sterile females ) . This observation confirmed a well documented pattern known as Haldane's Rule (see table 15.2, pg. 406) stating that when hybrid crosses produce sterile or inviable offspring, the sex that exhibits this is most likely the heterogametic sex (the sex with two different sex chromosomes, e.g. X and Y in male humans and Drosophila; in birds and butterflies the female is heterogametic with Z and W). Another "rule" of speciation is that genes affecting reproductive isolation are typically found on the X chromosome (where X is the "female" chromosome; see another paper by Coyne
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