The phylogenetic tree in the figure below was constructed by comparing sequences of homologs of a gene that plays a role in development; drosophila was used as an outgroup. The branch lengths are proportional to the amount of genetic change in each lineage; varying branch lengths indicate that the gene has evolved at different rates in different lineages. The relative lengths of the zebrafish and chicken branches in the phylogenetic tree indicate that
the homolog evolved more slowly in the zebrafish's lineage
the homolog evolved more rapidly in the zebrafish's lineage
zebrafishes evolved before chickens
chickens evolved before zebrafishes