[File:A genome alignment of eight Yersinia isolates.png|thumb|380px|Whole genome
alignment is a typical method in comparative genomics. This alignment of eight
Yersinia bacteria genomes reveals 78 locally collinear blocks
conserved among all eight
taxa. Each chromosome has been laid out horizontally and
homologous blocks in each genome are shown as identically colored regions linked across genomes. Regions that are inverted relative to
Y. pestis KIM are shifted below a genome's center axis.]]
Comparative genomics is a field of
biological research in which the
genomic features of different
organisms are compared. The genomic features may include the
DNA sequence,
genes,
gene order,
regulatory sequences, and other genomic structural landmarks. In this branch of
genomics, whole or large parts of genomes resulting from
genome projects are compared to study basic biological similarities and differences as well as
evolutionary relationships between organisms. The major principle of comparative genomics is that common features of two organisms will often be encoded within the
DNA that is evolutionarily
conserved between them. Therefore, comparative genomic approaches start with making some form of
alignment of genome sequences and looking for
orthologous sequences (sequences that share a
common ancestry) in the aligned genomes and checking to what extent those sequences are conserved. Based on these,
genome and
molecular evolution are inferred and this may in turn be put in the context of, for example,
phenotypic evolution or
population genetics.