Comparative genomic studies have identified noncoding regions of the genome which are often more highly conserved between species than protein-coding sequences. One possible explanation for this conservation of non-coding sequences is some form of selective constraint since sequence conservation at great evolutionary depths is a preliminary indication of functional constraint. Here, I consider nearly 2500 putative regulatory elements, termed Conserved Noncoding Elements (CNEs), that are conserved across seven vertebrate species (human, macaque, mouse, chicken, frog, zebrafish and fugu). I distinguish between CNEs that show accelerated rates of evolution and those that have remained more constrained throughout evolution, and identify CNEs that show higher than expected substitution rates in the human lineage that may be potential candidates of adaptive evolution. However, it is not trivial to demonstrate the action of selection on such sequences. It is relatively easier in the case of protein-coding DNA, since selection would be predicted to result in different rates of substitution for synonymous and non-synonymous sites. Hence, I use the same seven species to define phylogenetically invariant positions in CNEs in contrast to those that have at least one substitution and analyse them independently to determine if there is a positive correlation between evolutionary conservation and the strength of purifying selection at individual sites. In the 1000 Genomes, but not the HapMap, data I find a significant excess of rare derived alleles in CNEs relative to coding sequences. This excess of rare alleles can be best explained if selection is relatively consistent across sites, with most mutations resulting in a similar reduction in fitness. Finally, I explore patterns of variation in the allele-frequencies within human populations, however do not detect any significant differences in the underlying distribution of negatively selected variants among human populations.

High levels of primary sequence conservation are observed in many noncoding regions of eukaryotic genomes. These conserved noncoding elements (CNEs) have shown to be robust indicators of functionally constrained elements. Nevertheless, the function of only a small fraction of such CNEs is known and their role in genome biology remains largely a mystery. Comparative genomics analysis in model organisms can shed light on CNE function and evolution of noncoding DNA in general. Recently, it has been reported that short CNEs in the Drosophila genome are typically very AT-rich but have unusually high levels of GC content in a much larger (~500 bp) window around them. To understand whether these "side effects" are dependent on their CNE definition or are a more general feature of the Drosophila genome, we analysed base composition of CNEs from two different CNE detection methods. We found side effects are real, but are restricted to a subset of CNEs in the genome. An alternative hypothesis to explain the existence of CNEs is the mutational cold spot hypothesis. Previous work using SNPs was shown evidence that CNEs are not mutational cold spots. Here, non-reference transposable elements (TEs) were used to test cold spot hypothesis. A significant reduction in levels of non-reference TEs was found in intronic and intergenic CNEs compared to the expected number of insertions. TEs in intergenic CNEs were also found at lower allele frequencies than TEs in intergenic spacers. Furthermore, we used simulation to explore the effects of insertion/deletion (indel) evolution on noncoding DNA sequences with and without constrained noncoding elements. We assessed several indel-capable simulators to test expected outcomes with no selectively constrained elements. Simulations with constrained elements show that sequences grow in length even when the deletion rate is exactly the same as the insertion rate. This result can be interpreted as being due to purifying selection on CNEs acting to remove an excess of deletion over insertions. Together, the results presented here provide insights into the evolution of noncoding DNA in one of the most important model organisms.