The mechanisms of species dispersal and colonization that drive the process of range expansion have always been in the focus of fundamental ecology. Normally, species ranges change slowly, but human activities through global processes (landscape transformations, climate warming, and biological invasions) have speeded up changes in species distributions, thus providing opportunities to observe and study range shifts in real time. In Kalmykia, southern European Russia, human-induced landscape transformations from desert to steppe and vice versa, allowed us to study the colonization process as a result of range expansion in the desert-dwelling rodent, Midday gerbil (Meriones meridianus). We studied the population dynamics, demographic characteristics of gerbils, their physical conditions, and infestation with fleas on the wave of colonization compared to the source population. We found that, during recent years, the species range has been rapidly expanding to the west, where gerbils have formed new colonies. The colonists differed significantly from the residents of the source population in a lower average body weight, this being associated not with the poorer physical condition, but with age structure: the fertility rate of female colonists was much higher, and the population of colonists was much younger, compared to the source population. In addition, colonists were much less infested with fleas. Our findings indicate that the population of colonists on the wave of colonization does not experience the negative effects of low numbers, whereas the age structure and high reproductive rate of the younger population accounts for its rapid growth and expansion.This research was supported by the Russian Science Foundation (project number 22-14-00223, https://rscf.ru/project/22-14-00223/).