The ecology of a recently evolved species, Mimulus cupriphilus, was compared with its presumed progenitor, M. guttatus. Both species live sympatrically as annuals on two small copper mines in Calaveras county, California, USA. There are almost no other species present in the areas where Mimulus dominates. The production of seed by the two species was compared. The relationship between capsule number and plant height differs, with M. cupriphilus having the steeper slope, so that at any particular height it produces more capsules. Mean seed number per capsule, and the relationship between seed number and size of plant, also varies between the two species. M. cupriphilus is normally sell-fertilizing, and thus has a higher probability that a capsule will he fertilized than M. guttatus, which relies more on fertilization by insects. The various parameters can be combined to produce a predicted relationship between size of plant and seed production. When plants are small, M. cupriphilus will produce more seeds, but at heights greater than about V cm M. guttatus will be fitter. The overall resource allocation pattern also reveals that M. cupriphilus devotes a greater proportion of its dry weight to seed and capsule production. These results show that in the habitat where the two species co-exist, arid deserted copper mines, the derived species is better adapted.