The success of non-native species exposed to environmental conditions may depend on how the species adapt to new conditions. For this reason, non-native species offer the opportunity to understand evolutionary mechanisms such as natural selection that can promote adaptive evolution in new conditions, and also to investigate whether intraspecific admixture may serve as a stimulus for invasion by increasing fitness or a cost to fitness due to outbreeding depression. In addition, high performance of introduced species may be accomplished by a combination of multiple reproductive strategies (e.g., vegetative and sexual reproduction) that can contribute to dispersal and colonization ability. In this research, the herbaceous Mimulus guttatus native to North America and naturalized in United Kingdom (UK) is used to investigate: (1) the level of genotypic (clonal) diversity and genetic variation in non-native populations; (2) the effect of resource availability on the relative investment of sexual and clonal reproduction; (3) the level of phenotypic variation among non-native populations; and (4) patterns of natural selection in its introduced range, and evidence of outbreeding depression in admixed experimental populations.
The genotyping study using single nucleotide polymorphisms reveals that non-native populations show a wide variation of genotypic diversity and that the largest percentage of genetic variation is within populations either in native or introduced ranges. A common garden glasshouse experiment with non-native populations indicates that limited space intensifies the trade-off between sexual and clonal lateral spread, and suggests that populations under limited space conditions (e.g., high-density population) may have to invest less in sexual reproduction than in clonal lateral expansion. A survey of natural M. guttatus populations in UK indicates that production of flowers is favoured in places with low precipitation and high temperatures where production of stolons is limited in M. guttatus. The field experiment with F2 individuals from three crosses between introduced and native populations shows that admixed individuals from introduced populations have higher population growth rate due to increased survival, clonality, and seed production than admixed individuals from introduced and native populations, consistent with outbreeding depression. Selection through sexual fitness favours large floral displays, large vegetative traits, clonal spread, and early flowering in the non-native range. The results presented in this thesis indicate that clonal and sexual reproduction are integrated strategies that contribute to population growth rate, and the alternative investment in both traits in different environments may contribute to the colonization of the species in different habitats. Natural selection has an important role in the naturalization of a highly diverse species such as M. guttatus, and intraspecific admixture is not always beneficial in the introduced range as it may result in outbreeding depression, which further suggests the potential of non-native populations to evolve by adaptation.