Plant population dynamics and life cycle characteristics are presented for two genetically related, yet ecologically distinct, chromosomal races of Machaeranthera gracilis (Nutt.) Shinners. Seedling mortality was very high for a field population of the desert race (n=2) as only 8% of the maximum number of seedlings produced did survive to the period of peak reproduction. Lower mortality occurred in the foothills race (n=4) as 15% of the maximum number of seedlings produced did survive until the period of peak reproduction. The desert race also exhibited the capacity to assume the perennial growth habit. Perennial plants of the desert race produced 4.4 heads/plant (116 heads/m2) versus 0.2 heads/plant (4 heads/m2) for annual plants of the desert race under field conditions. Annual plants of the foothills race produced 2.7 heads/plant (75 heads/m2). The perennial habit did not occur in field populations of the foothills race. When grown in identical glasshouse conditions the desert race reached anthesis of the first four flower heads ca. 25 days prior to the foothills race. This acceleration to anthesis was accompanied by a greater potential reproductive allocation. The latter response was due in part to greater biomass allocation to reproductive structures at the expense of vegetative structures, and the uncoupling of net photosynthesis from phenological events.