The diversity of animal species is overwhelming, and an important goal of biology is to make sense of this diversity by arranging animals into groups. Classification can be done by grouping like with like, for example, by looking for similarities and differences in anatomical structure or gene sequences. This is a phenetic approach, with all similarities weighed equally regardless of their history. Alternatively, animals can be arranged into an evolutionary (phylogenetic) family tree by grouping species according to their ancestry. This approach depends on theories about the time at which a particular trait arose in evolutionary history.