The theory of historical contingency is that repeated occurrences of an evolutionary event result in dramatically different outcomes. This theory was tested on the same four types of anolis species (ecomorphs) on four different islands in the Greater Antilles. The similar morphologies and niches of the lizards among the islands can be explained by one of two hypotheses: all ecomorphs originated from a common ancestor on one island and then dispersed to different islands, or that there was independent evolution of each set of ecomorphs on each island. The researchers found the latter to be true, indicating that adaptive radiation is a stronger driving force than historical contingency in producing evolutionary outcomes across islands.
The theory of historical contingency is that repeated occurrences of an evolutionary event result in dramatically different outcomes. For instance, fauna that have evolved in similar environments exhibit more differences than similarities, and these differences stem in large part from unique historical events and subtle environmental differences in different areas. Such historical factors, however, do not always lead to disparate outcomes.
Six morphometric characteristics that are closely linked to habitat use for members of each ecomorph class were investigated. The investigation revealed that members of an ecomorph class are more similar to members of that class on different islands than they are to members of a different class on the same island. Phylogenetic analysis based on mitochondrial DNA sequences indicated most members of the same ecomorph class from different islands are not closely related. Figure 1B depicts the most parsimonious tree derived from the DNA data, and the frequent transitions among the ecomorph classes reveal that ecomorphs on one island aren’t closely related to their counterparts in other islands. Statistical analysis revealed that at least 17 evolutionary transitions among ecomorph classes have occurred. Figure 1C indicates that within each island, there are different topologies of the four ecomorphs. In other words, the ecomorphs evolved in a different order on each island, which is a reflection of different historical events.
Based on the data the researchers came up with, each set of ecomorphs evolved independently on each island. Also, members of an ecomorph class are more similar to members of that class on different islands than they are to members of a different class on the same island, meaning that, at least across islands, historical contingency is not an evolutionary driving force, and that adaptive radiation is. Within each island, however, the topologies of the ecomorphs are different, and this is the exact result of different historical events that occurred on each island. Therefore, within islands, historical contingency is a major evolutionary driving force.