Response to natural selection in Senecio lautus

Abstract

Natural selection is the established force driving evolutionary change in populations, however the link between selection and the formation of new species is much less concrete. There is a growing body of evidence that differences in ecological conditions within a species range can lead to adaptive changes in both appearance and underlying genetic background that can lead to the formation of discrete and divergent populations. When these differences accumulate to a degree that prevents genetic exchange, these populations may satisfy many of the categories defining distinct species. In the works described in this thesis, I examine the mechanisms of natural selection driving evolutionary divergence between three adjacent populations of Senecio lautus, a native Australian plant. First, I describe the phenotypic differences that have accumulated between the three populations and use empirical experiments to show that these changes are most likely the result of adaptation to divergent ecological conditions. In the second chapter I examine whether this adaptation to local conditions can influence the rate of gene exchange between populations by quantifying a range of potential ecological and genetic mechanisms that may influence the ability of populations to breed with one another. Results from this experiment suggest that intrinsic genetic differences between populations do not affect gene exchange and that the main barrier to gene flow between populations is associated with different patterns of adaptation preventing immigrant plants from establishing. These findings are consistent with the theories of ecological speciation, where speciation occurs primarily as a result of environmental factors, and I specifically test this hypothesis in chapter IV. The prediction of ecologically dependent hybrid fitness is a unique prediction of ecological speciation, and I use field based reciprocal transplant experiments to determine whether this pattern is observed between S. lautus ecotypes. Together, these chapters provide evidence that natural selection based on ecological variation drives phenotypic divergence between populations, is the major factor preventing gene exchange between populations, and leads to patterns of hybrid fitness that are consistent with the theoretical predictions of ecological speciation.3