Linanthus parryae is a diminutive desert annual native to southern California, especially the Mojave Desert. Flowers are commonly either blue or white and range from 1.5-2.0 cm across. Though floral color polymorphisms are found in other Polemoniaceae they are generally of rare occurrence, especially in the same taxon. Because of the unusually common occurrence of this polymorphism L. parryae has served as a model study system for population geneticists seeking to understand the processes of natural selection and genetic drift. The gene(s) responsible for blue flowers in dominant, although white is the most common color form (Epling et al. 1960).
Flowers are self-incompatible and pollinated by the beetle genus Trichochrous (Epling et al. 1960).
Seeds may lie dormant in the soil for years (up to at least 7) until the right weather conditions arrive. Consequently, populations are subject to large annual fluxes in size (Epling et al. 1960). At any given locality in any given year plants may form dense carpets or be absent (Epling et al. 1960).
Epling and Dobzhansky (1942) argued that because the proportion of a given color morph was more similar the closer populations were to each other and that “no relation between the composition of populations of Linanthus parryae and the environment in which these populations live is detectable,” the distribution of color forms in any given population was due to genetic drift. This hypothesis was reaffirmed by Wright (1943) .
Epling et al. (1960) after nearly 20 years of further research, suggested that the color polymorphisms were due to natural selection. They showed dramatic fluctuations in population size between years, though the proportion of each color type at any given locality remained constant over the 20 year period, ruling out genetic drift. They ascribe differences in population size to different annual weather conditions, with slightly different condition each year resulting in the germination of more or less seeds. The disproportionate number of a particular color morph between sites suggests natural selection. The slight local segregation of color morphs, in addition to the fact that some white plants can grow in close proximity to blue plants indicates that selection for corolla color is weak.
The work of Epling et al. (1960) sparked heated debate over the importance of selection versus drift in the population dynamics of L. parryae. Recent studies (Schemske and Bierzychudek 2001, 2007) suggest that natural selection is acting on the color morphs, although the direction of selection varies considerably over time. These studies demonstrate 1) differential seed production between years, 2) seed production is determined by spring precipitation; white flowered plants produce more seed in years of high precipitation, 3) different soil types favor different color morphs, 4) pollinators do not prefer one color morph over the other, and 5) white flowered plants appear to accumulate more cations than blue flowered ones.
It is proposed that in years of low precipitation, cations, while generally useful, could add up to toxic levels and so selection favors blue flowered plants. In times of high precipitation, blue flowered plants are less able to accumulate these cations and selection favors white flowered plants. Flower color is a result of pleiotropy. Polymorphic populations are maintained by a combination of a long-lived seed bank and variable annual precipitation. At a microhabitat scale, different soil types may have different moisture-retaining properties.
Epling, C. and T. Dobzhansky. 1942 Genetics of natural populations. VI. microgeographical races in Linanthus parryae. Genetics 27: 317-332.
Epling, C., H. Lewis, and F. M. Ball. 1960. The breeding group and seed storage: a study in population dynamics. Evolution 14: 238-255.
Schemske, D. W. and P. Bierzychudek. 2001. Perspective: evolution of flower color in the desert annual Linanthus parryae: Wright revisited. Evolution 55: 1269-1282.
Schemske, D. W. and P. Bierzychudek. 2007. Spatial differentiation for flower color in the desert annual Linanthus parryae: was Wright right? Evolution 61: 2528-2543.
Wright, S. 1943. An analysis of local variability of flower color in Linanthus parryae. Genetics 28:139-156.