Expanding populations are subject to strong selective pressures, leading to fast evolutionary responses at the expansion front. Apart from this rapid evolution, expanding populations also show typically specific demographic conditions, with large population sizes at the core, and low densities at the expansion front, which can lead to the occurrence of mate-finding Allee effects. In my PhD, I aim to research experimentally, genomically and theoretically how the presence of abiotic gradients, the mode of reproduction and gene flow affect evolution during expansion, using the protist Tetrahymena thermophila.