Table 1.

Levels of prezygotic isolation compared across three dichotomies (intrinsic postzygotic isolation, host plant divergence, and “small” vs. “large” X chromosome size). Our comparisons are for “young” (D_Nei ≤ 0.5) species contrasts after phylogenetic correction. The entries are mean values of the Coyne and Orr (1989a) index I_pre of prezygotic isolation, the number of comparisons (N), and the average values of D_Nei. Statistical significance is assessed with Wilcoxon tests. P values in parentheses are for D_Nei. Following C&O, the criterion for “sympatry” is any range overlap. The data analyzed are from Yukilevich’s web site.

Dichotomies		Less (N; DNei)	More (N; DNei)	P across dichotomy (for DNei)
Intrinsic postzygotic isolation		Ipost = 0	Ipost > 0
	sympatric	0.77 (13; 0.21)	0.81 (20; 0.27)	0.80 (0.34)
	allopatric	0.42 (17; 0.23)	0.43 (18; 0.23)	0.88 (0.89)
	P for sympatric vs. allopatric	0.005 (0.529)	< 0.001 (0.43)
Host plant differences		Ihost = 0	Ihost > 0
	sympatric	0.79 (11; 0.18)	0.76 (12; 0.27)	0.88 (0.12)
	allopatric	0.4 (9; 0.18)	0.6 (6; 0.25)	0.14 (0.14)
X-linked loci		small X	large X
	sympatric	0.73 (18; 0.25)	0.84 (7; 0.17)	0.98 (0.14)
	allopatric	0.53 (15; 0.29)	0.23 (8; 0.12)	< 0.001 (0.01)