If different species share common ancestors, we would expect living things to be related to one another in what scientists refer to as nested hierarchies — rather like nested boxes. This is indeed what we observe in the living world and in the fossil record. When we study their traits, species naturally cluster into groups based on suites of similar, inherited traits (i.e., homologies). And they don’t just cluster into groups. They cluster into groups within groups within groups…etc. This observation makes most sense if we understand that, over time, lineages have split and formed new species, which then split and formed even more species, and so on, passing down traits and generating the branching structure that is the Tree of Life. An example is illustrated below.
In this evolutionary tree, sunflowers and orchids nest together because they share many homologies, including flowers. Similarly, pine trees and cypress trees share many homologies, including cones. These two groups (flowering plants and cone-bearing plants) nest together because they all share homologies such as seeds. And then seed-bearing plants nest with ferns because they all share homologies like stomata (pores for gas exchange). And so on … across the whole Tree of Life.
It doesn’t have to be this way. It could be that each species has a random assortment of traits and that there is no obvious way to group them. But that’s not what we observe. In fact, it’s fairly easy to group most organisms into a nested hierarchy. This was recognized by scientists like Linnaeus long before Darwin came along and proposed an explanation for the pattern: that organisms on the Tree of Life are descended with modification from common ancestors. Common ancestry is conspicuous when we examine the distribution of traits in living things.
Reviewed and updated, June 2020.