Abstract
Genome size (GS) variation is a fundamental biological characteristic; however, its evolutionary causes and consequences are the topic of ongoing debate. Whether GS is a neutral trait or one subject to selective pressures, and how strong these selective pressures are, may remain open questions. Fundamentally, the genomic sequences responsible for this variation directly impact the potential evolutionary outcomes and, equally, are the targets of different evolutionary pressures. For example, duplications and deletions of genic regions (large or small) can have immediate and drastic phenotypic effects, while an expansion or contraction of non-coding DNA is less likely to cause catastrophic phenotypic effects. However, in the long term, the accumulation or deletion of ncDNA is likely to have larger effects. Modern sequencing technologies are allowing for the dissection of these proximate causes, but a combination of these new technologies with more traditional evolutionary experiments and approaches could revolutionize this debate and potentially resolve many of these arguments. Here, I discuss an ambitious way forward for GS research, putting it in context of historical debates, theories and sometimes contradictory evidence, and highlighting the promise of combining new sequencing technologies and analytical developments with more traditional experimental evolution approaches.