The following species of Prosopis were studied in the Pampa del Tamarugal (1st region) and Quillagua (2nd region): P. Tamarugo (tamarugoa), P. strombulifera (foryuna), P. burkartii (churqui) and two species of algarrobo. In all species three floral stages were recognized: !st, blosson stage, 2nd stage of enclosed flower and 3rd stage of full open flower. Protoginy is observed from the 2nd stage as the stigma is exposed in the closed blosson. The intrasexual selection with addition of fitness in the male and female function, independently one from the other, has favoured an evolution of dicogamy in these species. The presence of two species of algarrobo was, noted: one is called Prosopis flexuosa and the other, Prosopis alba in the paper. A large production of flowers with a great energy demand, but with a low production of fruits, can present at least 3 evolution trends: a.- with respect to the male sex, 1. an increase of male gametes, both in time and amount, due to the greater number of stamens exposed and to the gradual maturation of flowers starting from the base to the apical zone of the inflorescence. 2. bigger atraction of pollinators, due to a major visibility or to a greater reward in nectar. This allows for a greater amount of pollen being carried about and c an also better warranty the male fitness, in terms of a great number of fertilizations. B.- with respect to the female sex, 3. production of a far great number of flowers than it is possible to obtain fruit from. This originates a strong intrasexual pressure with an increase in fitness in the case of the first flowers to be fertilized. These, in turn, take up a lot of space and inhibit the coming zygotes, thus favouring the protoginic system