In a phylogram, the length of a branch reflects the number of genetic changes that have taken place in a particular DNA sequence in that lineage.

This phylogram implies that more genetic changes have occurred in the Drosophila lineage than in the bird and mammal lineages since they diverged.

This phylogram was constructed by comparing homologous Sonic hedgehog homolog (SHH) genes involved in embryonic development; mutations in Drosophila cause embryos to be covered with small pointy projections,like a hedgehog.