The organisms that are most closely related are those with the most recent common ancestor, in terms of the number of generations that have passed since that ancestor. For instance, two siblings are more closely related than two cousins. Relationships between species follow a similar pattern.
The classification of organisms according to common ancestry is known as cladistics. Cladistics is one of the ways of examining taxonomy, or the classification of organisms. It uses several tools to classify organisms, including comparison of genetics, proteins and physical traits. Because it classifies organisms into what are known as clades based on their most recent common ancestry, cladistics can lead to some surprising classifications.
For instance, while salmon and lungfish would commonly be thought of as closer relatives than lungfish and cows, scientists think that the lineages of lungfish and cows diverged more recently than those of salmon and lungfish. Thus, lungfish and cows could be placed into a clade that excludes salmon. A clade that includes salmon and lungfish, however, could not exclude cows because the division between fish and cow occurred after the division of salmon and lungfish. Both lungfish and mammals are thought to be descended from the common ancestor of all mammals and other terrestrial vertebrates.