Secondary Intergradation between Ipomopsis aggregata candida and I. a. collina (Polemoniaceae) in Colorado

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Abstract
Ipomopsis aggregata subspp. collina and candida have overlapping ranges and neighboring sympatry along the eastern slopes of the Front and Sangre de Cristo Mountains in Colorado and northern New Mexico. In some areas of neighboring sympatry I. a. collina and candida remain distinct, while in others they hybridize, and the two taxa are thus semispecies. Intrapopulation variation was investigated in three areas where hybridization occurs and was compared with that in areas where I. a. collina and candida are allopatric and nonintrogressive. Hybridized populations exhibit greatly increased variation for diagnostic characters differentiating the parental semispecies. Intergradation takes the form of shifts in the means, modes, and ranges of these characters from one population to the next on a transect. Two segregating characters may be significantly correlated in one population but not in another. Also, in a given population, one character pair may be correlated, whereas another character pair is not. Thus, character coherence is sometimes present and sometimes absent. Hybridization between I. a. collina and candida is the main cause of individual variation in these two semispecies.